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Part 5. Historical Aspects and Crop Evolution


Genetic Evidence on the Origin of Triticum aestivum L. - J. Dvorák, M.-C. Luo and Z.-L. Yang
Introgression of Durum into Wild Emmer and the Agricultural Origin Question - M.A. Blumler
The Variation of Grain Characters in Diploid and Tetraploid Hulled Wheats and its Relevance for the Archaeological Record - K. Hammer and C.-E. Specht
Utilization of Ancient Tetraploid Wheat Species for Drought Tolerance in Durum Wheat (Triticum durum Desf.) - A. Al Hakimi and P. Monneveux
Archaeobotanical Evidence for Evolution of Cultivated Wheat and Barley in Armenia - P.A. Gandilian
Extinction Threat of Wild African Gossypium species in their Center of Diversity - V. Holubec

Genetic Evidence on the Origin of Triticum aestivum L. - J. Dvorák, M.-C. Luo and Z.-L. Yang

Introduction

Because of the great economic significance of wheat, its evolution has received a great deal of attention over the past 80 years. It was recognized early that wheat species fall into three natural groups according to their ploidy (Kihara 1924). Six biological species are recognized today, two at each ploidy level (Fig. 1). Of the six species, the most important economically is Triticum aestivum L., because it includes bread wheat. Triticum aestivum comprises a number of subspecies or other taxa which are interfertile and which differ from each other by a single or a few major genes (Mac Key 1966). In some of these subspecies, such as subsp. aestivum (bread wheat), subsp. compactum (Host) Thell. (club wheat), subsp. sphaerococcum (shot wheat) (Perc.) Mac Key, T. petropavlovskyi Udach. & Migush. (rice wheat) and subsp. tibetanum Shao (Tibetan wheat), glumes do not adhere to kernels and they are free-threshing. In contrast, subsp. spelta (L.) Thell. (spelt), subsp. macha (Dekapr. & Menabde) Mac Key, subsp. vavilovii (Jakubz.) A. Love and subsp. yunnanense King are hulled.

The fact that T. aestivum originated from hybridization of tetraploid T. turgidum L. (Fig. 1) with diploid Aegilops tauschii Coss. (Kihara 1944; McFadden and Sears 1946) is well documented by genetic evidence. Hexaploid wheat has been resynthesized by hybridization of T. turgidum with Ae. tauschii (McFadden and Sears 1946). The synthetic wheats resemble spelt and are invariably hulled (McFadden and Sears 1946; Kihara et al. 1965; Kerber and Rowland 1974). On the basis of this and other lines of evidence, it has been argued that spelt is ancestral to the free-threshing hexaploid wheat (McFadden and Sears 1946; Kuckuck 1959; Andrews 1964). Yet, some genetic and archaeological findings suggest that spelt may have been actually derived by introgressive hybridization between free-threshing T. aestivum and hulled T. turgidum subsp. dicoccon (Schrank.) Thell. (Tsunewaki 1968; Liu and Tsunewaki 1991; Nesbitt and Samuel 1996).

Whether the various forms of T. aestivum are monophyletic or polyphyletic is also open to debate. This uncertainty is true not only for subspecies but also for varieties within subspecies, such as the various types of spelt. Originally, spelt was known only from Europe which made it a poor candidate as an ancestral form of hexaploid wheat because Ae. tauschii does not grow anywhere nearby (Schiemann 1951). Later, spelt was discovered in Iran (Kuckuck and Schiemann 1957; Kuckuck 1959) and other locations in Asia. Jaaska (1978) concluded from isozyme variation in the A genome that the European spelt originated independently of the Asian spelt. The possibility that some of the subspecies and types of the Asian hulled wheats may be polyphyletic and may or may not be directly related to the evolution of free-threshing wheat must be seriously considered (Swaminathan 1966; Tsunewaki 1968; Johnson 1972).

A question related to the above dilemma is the exact source of the wheat D genome and the geographic place of the origin of T. aestivum. Aegilops tauschii encompasses several morphological varieties which are broadly grouped into two subspecies, tauschii and strangulata (Eig) Tzvelev. The former is distributed from eastern Turkey to China and Pakistan, whereas the latter occurs in two disjoined regions, southeastern Caspian Iran and Transcaucasia (Kihara et al. 1965; Jaaska 1995). Jaaska (1978, 1981) identified polymorphisms at the aspartate aminotransferase and aromatic alcohol dehydrogenase loci between the Ae. tauschii subspecies, and in both cases the subsp. strangulata allele corresponded to that in the T. aestivum D genome. He also pointed out that the profile of esterase alleles characterizing the D genome of T. aestivum fits subsp. strangulata better than subsp. tauschii (Jaaska 1980). The spectrum of alleles at the a-amylase loci in T. aestivum also corresponds better to the allele spectrum in subsp. strangulata than to that in subsp. tauschii (Nishikawa 1974; Nishikawa et al. 1980).

Fig. 1. Phylogeny of polyploid species of Triticum inferred from variation in repeated nucleotide sequences (Dvorák et al. 1988, 1993; Dvorák and Zhang 1990, 1992). Genomes of each species are indicated by capital letters.

Variation in the lengths of restriction fragments (RFLP) at a large number of single-copy loci and the rRNA gene nontranscribed spacer at the Nor3 locus was used to investigate genetic relationships among accessions of Ae. tauschii grouped according to their geographic origin and subspecies (Dvorák et al. 1998). Accessions from Turkey and western Iran were more closely related to those from Afghanistan, Turkmenistan and China than to those from the neighboring north-central Iran and southwestern Caspian Iran (Dvorák et al. 1998). A similar observation was made earlier by Lubbers et al. (1991). Dvorák et al. (1998) suggested that these findings reflect gene migration between subsp. strangulata and subsp. tauschii in Iran and proposed that Ae. tauschii is composed of two genepools, the strangulata and tauschii genepools, which do not correspond to the morphological delimitation of the subspecies. The strangulata genepool is wider than appears on the basis of morphology and comprises subsp. tauschii in north-central Iran and Caspian Iran. This study illustrated that morphology is of dubious value in assessing intraspecific genetic relationships.

In Transcaucasia, where the strangulata and tauschii genepools overlap (Jaaska 1995), genetic distances of individual accessions to subsp. strangulata and subsp. tauschii outgroups revealed that numerous accessions were allocated to an erroneous genepool on the basis of morphology (Dvorák et al. 1998). There is less introgression between subsp. strangulata and subsp. tauschii in Transcaucasia than in Iran. Only about 5% of all Transcaucasian accessions showed intermediate genetic distances to the outgroups; the rest of the accessions showed genetic relationships characteristic of either subsp. strangulata or subsp. tauschii. This contrasts with Caspian and north-central Iran where no accessions showing genetic relationships characteristic of true subsp. tauschii were found. Reallocation of Transcaucasian accessions to the respective genepools according to genetic relationships provided strong evidence that the T. aestivum D genome was contributed by the strangulata genepool (Dvorák et al. 1980).

Jaaska (1980) considered Transcaucasia to be the center of the distribution of subsp. strangulata and hence placed the origin of T. aestivum to Transcaucasia. Nishikawa et al. (1980) found that the a-amylase isozyme profile present in T. aestivum is most prevalent in southeastern Caspian Iran, not in Transcaucasia, and therefore suggested that the most likely birthplace of T. aestivum is southeastern Caspian Iran. Tsunewaki (1966) considered mountainous Azerbaijan as the place of the origin of T. aestivum because of the distribution of the waxy-bloom allele. Nakai (1979) placed the birthplace of T. aestivum in southwestern Caspian Iran and Transcaucasia on the basis of the distribution of esterase alleles.

In spite of the existence of polymorphism at isozyme loci in Ae. tauschii, no polymorphism has been found in the T. aestivum D genome that is shared with Ae. tauschii. Even at the highly polymorphic nontranscribed spacer at the Nor3 locus encoding the 18S-5.8S-26S rRNA species (rDNA NTS), only a single Ae. tauschii allele has been detected in wheat (Clarke et al. 1989; Lagudah et al. 1991; Dvorák et al. 1998). The only potential exception to this may be the high-molecular-weight (HMW) glutenin locus Glu1. The locus is composed of two genes: x, encoding HMW-weight glutenin subunits with a slower electrophoretic mobility, and y, encoding HMW-weight glutenin subunits with a faster electrophoretic mobility. In the wheat D genome, the Glu1a haplotype encodes HMW-glutenin subunits 2 (x subunit) + 12 (y subunit). This haplotype has been detected in Ae. tauschii (Lagudah and Halloran 1988). Another, less common, haplotype in the wheat D genome is Glu1d which encodes subunits 5 (x subunit) + 10 (y subunit). A haplotype encoding similar subunits exists in Ae. tauschii (Lagudah and Halloran 1988). Two chromosomes ID carrying this haplotype have been substituted to T. aestivum, one from subsp. strangulata and one from subsp. tauschii (Jones et al. 1990, 1991). A close examination of the electrophoretic mobility of the subunits encoded by the Ae. tauschii haplotype revealed that the mobility of the HMW-glutein subunit 5 is identical in wheat and Ae. tauschii. The mobility of the wheat subunit 10 was found to be slightly slower under some electrophoretic conditions (Jones 1991). Lagudah and Halloran (1988) called this Ae. tauschii subunit 10 whereas Jones (1991) called it T5. Although Lagudah and Halloran (1988) could not distinguish the two forms of subunit 10 by electrophoretic mobility, they found that they differ in the isoelectric point. While the Glu1 locus constitutes the only case in which more than a single Ae. tauschii allele at a locus may have been detected in wheat, because of the differences between subunits 10 and T5, the evidence is hardly conclusive, as pointed out by Lagudah and Halloran (1988).

The question of whether wheat originated by single or multiple hybridization events has ramifications for all of the problems discussed earlier. It is evident that if modern T. aestivum originated by multiple hybridization events, as speculated, e.g. by Jakubziner (1958), Dekaprelevich (1961), Kuckuck (1964), Morris and Sears (1967) and Yen et al. (1983), then the source of the T. aestivum D genome and the geographic place of its origin would be intrinsically uncertain. If wheat originated by a single hybridization event, as has been concluded by Jaaska (1980) or tacitly assumed by other authors (Liu and Tsunewaki 1991; Lubbers et al. 1991), all variation in the T. aestivum D genome would have originated since the origin of T. aestivum, which archaeologists place in the 7th millenium BC (for recent review, see Nesbitt and Samuel 1996). If, however, T. aestivum originated recurrently, some of the variation of Ae. tauschii would have been introgressed into wheat. These alternatives lead to different views of the wheat genetic system and wheat evolution.

To gain insight into these questions, polymorphisms of restriction fragment lengths (RFLP) at 53 single-copy loci, rDNA NTS and the HMW-glutenin locus Glu1 were investigated with the objective of assessing parallel polymorphisms between T. aestivum and Ae. tauschii and the evolution of T. aestivum. Additionally, the electrophoretic mobility of HMW-glutenin subunits was investigated in Ae. tauschii and the T. aestivum D genome with the objective of determining whether the widespread D-genome haplotypes Glu1a and Glu1d were contributed by Ae. tauschii.

Material and methods

Plant material

RFLP was investigated in DNAs of 172 accessions of Ae. tauschii, 178 accessions of subsp. aestivum, subsp. compactum and Chinese endemic wheats (Tibetan wheat, Yunnan wheat and rice wheat), 64 accessions of spelt, 10 accessions of T. aestivum subsp. macha and 2 accessions of T. aestivum subsp. vavilovii. The spelt accessions consisted of 52 accessions of the European spelt, 1 accession of spelt from Azerbaijan, 5 accessions of spelt from Iran, 2 accessions of spelt from Tadjikistan and 4 accessions of spelt from Afghanistan (the term spelt will be used only in reference to subsp. spelta, not to all hulled hexaploid wheats). The accessions and their sources have been described earlier (Dvorák et al. 1998). Triticum aestivum accessions were grouped according to subspecies and geographic region of origin (western and eastern). Ae. tauschii accessions were grouped by subspecies and the geographic region of the origin into 11 populations (Fig. 2).

RFLP

Nuclear DNA was isolated from leaves of a single plant per accession (Dvorák et al. 1988) and digested with DraI or XbaI. Restriction endonuclease-digestion, blotting, hybridization and DNA probes have been described previously (Dvorák et al. 1998). The position of each locus and allele in the wheat genomes was confirmed by synteny mapping using nullisomic-tetrasomic stocks (Sears 1966) and in some cases disomic substitution lines harboring single chromosome pairs of Lophopyrum elongatum (Host) A. Love substituted individually for wheat homoeologous chromosome pairs (Dvorák 1980; Dvorák and Chen 1984).

Variation at 55 loci was investigated (Dvorák et al. 1998). In Ae. tauschii, RFLPs at 53 loci were investigated in DraI-digested DNAs. For the Nor3 locus, TaqI digests were employed, and for the XGlu1 locus, XbaI digests were used. For 15 of the 53 single-copy loci, RFLP was also investigated in XbaI-digested DNAs. Thus, a total of 70 enzyme x probe combinations were used for Ae. tauschii. RFLP at the same 70 enzyme x probe combinations was also investigated in all accessions of T. aestivum subsp. macha and subsp. vavilovii, a limited sample of European and Asian spelta, and reference cv. Chinese Spring. Since some loci were monomorphic in Ae. tauschii or not informative in wheat, a subpopulation of 27 informative loci was selected for the investigation of the relationships between Ae. tauschii populations and those of T. aestivum. RFLP at 21 loci was investigated with DraI, two loci with XbaI, two loci with both DraI and XbaI, one locus with TaqI, and one locus with EcoRV. Thus, a total of 29 enzyme x probe combinations were used for the 27 loci.

Fig. 2. Approximate geographic location of Ae. tauschii populations used in this study: (1) Turkey, (2) Transcaucasia subsp. strangulata, (3) Transcaucasia subsp. tauschii, (4) western Iran, (5) southwestern Caspian Iran, (6) north-central Iran, (7) southeastern Caspian Iran subsp. strangulata, (8) southeastern Caspian Iran subsp. tauschii, (9) Turkmenistan, (10) Afghanistan and (11) China.

Shared polymorphism between Ae. tauschii and T. aestivum

Dvorák et al. (1998) reported polymorphisms in the T. aestivum D genome shared with Ae. tauschii at five loci. To scrutinize further the correspondence of these shared polymorphisms, several accessions of Ae. tauschii and T. aestivum representing each shared DraI or XbaI haplotype were selected and digested with either all or a subset of the following restriction endonucleases; ApaI, BglII, EcoRV, KpnI, SstI and XbaI. Southern blots were hybridized with clones detecting DNA fragments from these five loci. DNAs of relevant Chinese Spring nullisomic-tetrasomics and L. elongatum disomic substitution lines in the Chinese Spring genetic background were included in the blots. Restriction fragments belonging to the D-genome were identified in each restriction digest by comparing the profile of Chinese Spring with those of nulli-tetrasomics and disomic substitution lines.

HMW-glutenins

The endosperms of 222 accessions of Ae. tauschii and 25 accessions of T. aestivum were crushed, proteins were extracted and electrophoretically fractionated in denaturing polyacrylamide gels as described by Cole et al. (1981) and stained in 0.025% Coomassie brilliant blue R250 in 12% trichloracetic acid (TCA) for 24 hours and destained in 12% TCA for 2 days.

Results

Polymorphism sharing between wheat and Ae. Tauschii

Polymorphisms were encountered in the D genome of T. aestivum at 14 of the 70 enzyme x probe combinations representing 55 loci (Table 1). At five loci - Xpsr899, Xpsr928, Xpsr666, Xpsr901 and Glu1 - two Ae. tauschii haplotypes were found (Table 1). At 10 loci, haplotypes were found in T. aestivum which were not found in Ae. tauschii; a total of 18 haplotypes not encountered in Ae. tauschii were found in T. aestivum (Table 1). The possibility that some of these 18 T. aestivum-specific haplotypes are actually shared with Ae. tauschii but were not present among the Ae. tauschii accession investigated here cannot be discounted.

The Xpsr899 locus was investigated with four restriction endonucleses. DNAs of T. aestivum and Ae. tauschii accessions having DraI haplotype a shared restriction fragments in all digests (Fig. 3). The same was true for accessions sharing haplotype b (Fig. 3). In all four digests, the fragments of the a haplotypes differed from those of the b haplotypes (Fig. 3), indicating that haplotype a differs from haplotype b in a number of restriction sites in both species. By these criteria, the T. aestivum haplotype a appears to be identical to haplotype a in Ae. tauschii, and T. aestivum haplotype b appears to be identical to haplotype b in Ae. tauschii.

The Xpsr928 locus was investigated with six restriction endonucleases. Iranian and Azerbijan spelt and two accessions of Ae. tauschii represented the DraI haplotype b and Chinese Spring and two accessions of Ae. tauschii represented the DraI haplotype a (Fig. 4). Identical restriction fragments were shared by DNAs of the T. aestivum and Ae. tauschii accessions representing the DraI haplotype b in all six digests. However, DNAs of the T. aestivum and Ae. tauschii accessions representing haplotype a shared the same fragments only in the KpnI and DraI digests (Fig. 4) suggesting that T. aestivum haplotype a is related but not identical to haplotype a in Ae. tauschii accessions KU2103 and KU2824. Actually, the two Ae. tauschii accessions also did not have identical haplotypes, as evidenced by polymorphism between them in the BglII and KpnI profiles (Fig. 4).

The Xpsr666 locus was investigated with a total of seven restriction endonuclases. Chinese Spring and Ae. tauschii accessions KU2103 and KU2824 represented the XbaI haplotype a and Tadjikistan spelt VIR56569 and Ae. tauschii accessions KU2103 and KU2824 represented the XbaI haplotype b. No fragment belonging to the D genome was seen in the SstI digest and no polymorphism between haplotypes a and b was observed in the BglII and EcoRV digests. In the remaining four digests, haplotype a differed from haplotype b and the differences were the same in both species.

The Xpsr901 locus was investigated with six restriction endonucleases. The D-genome restriction profiles in Chinese Spring and spelt PI367199 were identical to those in Ae. tauschii accessions KU2104 and KU2103 representing DraI haplotype a in all digests. The D-genome profiles in spelt PI90962 and PI347926 and Ae. tauschii accessions KU2112 and KU2151 representing DraI haplotype b shared the same DNA fragments in the DraI and SstI digests but not in the ApaI and BglII digests. In four digests - ApaI, BglII, DraI and SstI - T. aestivum a and b haplotypes differed/They were identical in the EcoRV and KpnI digests but the Ae. tauschii accessions representing haplotypes a and b differed. Because of the incomplete agreement between the DraI b haplotypes in wheat and Ae. tauschii accessions KU2112 and KU2151, all 85 Ae. tauschii having DraI haplotype b were digested with ApaI and BglII and Southern blots were hybridized with PSR901. Of the 85 Ae. tauschii accessions, only two, AL10/80-1 and AL10/80-2 collected in Nakhichewan by V. Jaaska (pers. comm.), had the same haplotype as spelt accessions PI90962 and PI347926. Both accessions belong to the tauschii genepool (Dvorák et al. 1998). Thus, there is a complete agreement between the DraI a haplotypes in T. aestivum and Ae. tauschii and between the DraI b haplotypes in T. aestivum and Ae. tauschii in all investigated restriction sites.

Table 1. Multiple allelisms in the D-genome of T. aestivum and the genome of Ae. tauschii and polymorphism sharing between T. aestivum and Ae. tauschii in DNAs digested with DraI or a restriction endonuclease indicated in parentheses.

Groups of accessions

Xmwg2031

Xcdo 1400

Xpsr371-6D

XGlu1 (Xbal)

Xpsr899

Xbcd 1302

XEsi3

Xpsr666 (Xbal)

Xpsr102

Xpsr 901-2D

Xpsr 928

Xwg 644

XGsp

Xcdo 749

Ae. tauschii

a,b

b,c,d,e

a,b,c

a,b,c,d

a,b,c,d,e,f,g

b,c,d,e,f

b,c,d

a,b,c

a,b

a,b

a,b

a,b,c

b,c,e,f

a,b,c,d

wheat west reg.

a

a,c

a

a,b

a,f,h,i

a,g

d

a

a,c

a,b

a,b

a

a

a

wheat east reg.

a

a,c

a,d,e

-

a,f

a,c

a,d

a,b

a,d

a,b

a,b

a

a

a

subsp. macha

a

c

a

a,b

a,f,j,k

a

a,d

a

a,d

a,b

a,b

a

a

a,e

spelt (Asia)

a,c

c

a

a

a,f

a,c

d

a,b

a,d

a

a,b

a

a

a

spelt (Europe)

a

c,f

a

a

a,f

a

d

a

a,c,d

a,b

a

a,b

a

a

subsp. vavilovii

a

c

a

a

a,f

a

a,d

a

d

a

a

a

a

a

T. aestivum haplotypes which were not found in Ae. tauschii are in bold.
Pairs of Ae. tauschii haplotypes shared with T. aestivum are underlined.
Fig. 3. Autoradiograms of Southern blots of DNAs of Ae. tauschii and T. aestivum accessions and Chinese Spring cytogenetic stocks digested with BglII (left) and DraI (right) restriction endonucleases and hybridized with pPSR899. Accessions sharing haplotype a and those sharing haplotype b are indicated at the bottom. Genome allocation of major bands is indicated. Note the absence of the Chinese Spring fragments assigned to chromosome 2D in the disomic substitution line 6E(6D). Note that the D-genome DNA fragments of the Ae. tauschii and T. aestivum accessions with haplotype a differ from those shared by Ae. tauschii and T. aestivum accessions with haplotype b in both digests.

The Glu1 locus was investigated with five restriction endonucleases. Triticum aestivum was polymorphic in the XbaI and EcoRV digests. The same DNA fragments were observed in Ae. tauschii (Table 1). In Ae. tauschii, the XbaI haplotypes a and b had frequencies 0.21 and 0.07, respectively, and the EcoRV haplotypes a and b had frequencies 0.94 and 0.06, respectively (not shown) indicating that the b haplotype is rare in Ae. tauschii. Of 172 Ae. tauschii accessions, XbaI haplotype b was found in 12 accessions and EcoRV haplotype b was found in 10 accessions (Table 2). Of these, eight shared the two haplotypes, indicating that the XbaI and EcoRV b haplotypes are in a strong linkage disequilibrium in Ae. tauschii.

Fig.4. Autoradiograms of Southern blots of DNAs of Ae. tauschii and T. aestivum accessions and Chinese Spring cytogenetic stocks digested with DraI (left) and KpnI (right) restriction endonucleases and hybridized with pPSR928. Accessions sharing haplotype a and those sharing haplotype b are indicated at the bottom. Genome allocation of major bands is indicated. Note the absence of the Chinese Spring fragments assigned to chromosome 2D in the disomic substitution line 2E(2D). Note also that the D-genome DNA fragments of the Ae. tauschii and T. aestivum accessions with haplotype a differ from those shared by Ae. tauschii and T. aestivum accessions with haplotype b in both digests.

Seven types of the x subunit of HMW-glutenin and eight types of the y subunit were found with SDS-PAGE (not shown). Most of the Ae. tauschii accessions with the DNA haplotype b had the HMW-glutenin x subunit 5 (Table 2, Fig. 5). The haplotype encoding subunit 5 was found to be rare in Ae. tauschii, frequency (f) = 0.06 (Table 3).

None of the accessions with the b DNA haplotype had the x subunit 2 or any other x-type subunit frequent in Ae. tauschii. Thus, the XbaI and EcoRV haplotypes b and the Glu1x allele encoding subunit 5 usually occur together and must be in a strong linkage disequilibrium in Ae. tauschii.

The same linkage disequilibrium exists in T. aestivum. Seed storage proteins were extracted from seeds of 25 accessions of the Asian and European spelt, subsp. macha, subsp. vavilovii and bread wheat cultivars of both western and eastern origin and fractionated by SDS-PAGE. Eight accessions had HMW-glutenin subunits 5 + 10 (haplotype Glu1d) and 17 had HMW-glutenin subunits 2 + 12 (haplotype Glu1a). All Glu1d accessions had EcoRV haplotype b and all Glu1a had EcoRV haplotype a (XbaI digests were not investigated because it was difficult to score the XbaI fragment in the wheat genetic background). These parallels between the T. aestivum D genome and Ae. tauschii showed that T. aestivum Glu1a and Glu1d haplotypes encoding HMW-glutenin subunits 2 + 12 and 5 + 10, respectively, were both contributed to T. aestivum by Ae. tauschii.

In Ae. tauschii, the Glu1a haplotype encoding HMW-glutenin subunits 2 + 12 occurs in Transcaucasia, the southeastern Caspian region and north-central Iran. The Glu1a haplotype was observed only in the strangulata genepool (Table 3). The haplotype encoding the pair of subunits 5 + T5, which corresponds to the wheat haplotype Glu1d (Fig. 5), was also observed only in the strangulata genepool; its highest frequency was in southwestern Caspian Iran (Table 3). Likewise, the x gene encoding subunit 5 occurred in the highest frequency in southwestern Caspian Iran (Table 3).

Discussion

Multiple contributions of Ae. tauschii to the wheat D genome

Pairs of haplotypes shared by Ae. tauschii and the T. aestivum D genome were found at five of the 55 investigated loci. Within each pair, haplotypes differed in multiple restriction sites. If these were merely restriction site differences or small independent insertions or deletions, it would be extremely unlikely that these parallel polymorphisms originated by reverse mutations in wheat. Several reverse mutations would have to occur in each case to convert one haplotype into the other, which is very unlikely. If, however, each of the polymorphisms is due to a single large DNA insertion that occurred in Ae. tauschii, an excision of the inserted DNA during wheat evolution could convert a haplotype into the ancestral one and, thus, generate a shared haplotype pair between wheat and Ae. tauschii.

This scenario can be discounted for the shared polymorphism at the Glu1 locus because the Glu1a and Glu1d haplotypes differ by several independent mutations. Should the difference between the wheat Glu1a and Glu1d haplotypes be due to reverse mutations, a minimum of three independent reverse mutations, HMW-glutenin subunit 5 to 2 (or vice versa), HMW-glutenin subunit 10 to 12 (or vice versa) and EcoRV haplotype b to a (or vice versa) would have to have occurred during wheat evolution. This is extremely unlikely.

Three of the five loci in which polymorphisms are shared between T. aestivum and Ae. tauschii are linked on chromosome 2D. Two, Xpsr666 and Xpsr928, are on the short arm and one, Xpsr901, is closely linked to them on the long arm (for map positions of these loci see Dubcovsky et al. 1996). Linkage among the polymorphic loci would hardly be expected if the polymorphisms were caused by reverse mutations, since these should be random. An intriguing coincidence is that chromosome 2D played a critical role in the evolution of the free-threshing T. aestivum. The free-threshing character is based on two mutations, the dominant mutation of q to Q on the long arm of chromosome 5A, and a recessive mutation of Tg to tg at the end of the short arm of chromosome 2D (Kerber and Rowland 1974). Chromosome 2D was substituted from four accessions of Ae. tauschii into bread wheat cv. Chinese Spring, which is QQtgtg. All four disomic substitution lines had adhering glumes in spite of having Q allele (J. Dvorák, unpublished). Clearly, Q itself is insufficient to cause a free-threshing habit. Therefore, the evolution of the free-threshing habit required fixation of a recessive tg mutation in T. aestivum. One can imagine a scenario in which a Tg to tg mutation happened more than once. Selection for the tg alleles could maintain hitchhiking linked polymorphisms on chromosome 2D in the wheat genepool.

Table 2. Lists of Ae. tauschii accessions (out of 172 investigated) having the rare XbaI and EcoRV haplotypes b at the Glu1 locus and the HMW-glutenin subunits encoded at the Glu1 locus in these accessions.

Xbal

EcoRV

HMW-glutenin subunits

KU20-10

KU20-10

5 + T5

KU2104

KU2104

5 + 12

KU2090

KU2090

5 + T5


KU2106

5 + null

KU2160

KU2160

5 + T5

KU2110

KU2110

5 + 12


KU2001

3 + 12

AL8/78-2

AL8/78-2

T6 + T8

AL9/78-3, -4, -6, -7


T6 + T5

AL10/80-1, -2

AL10/80-1, -2

5 + T9


Fig. 5. SDS-PAGE profiles in indicated Ae. tauschii accessions (KU), disomic substitution line in which the Cheyenne 1D chromosome was substituted for 1D of Chinese Spring (DSCnn1D), and Chinese Spring (CS). Subunits encoded by Glu1 haplotypes are indicated in parentheses. The x HMW-glutenin subunits (2 and 5) and y HMW-glutenin subunits (10, T5, and 12) are indicated by arrowheads. Chinese Spring profile shows subunits 2 + 12 (haplotype Glu1a), the profile of DSCnn1D shows subunits 5 + 10 (haplotype Glu1d), the profile of Ae. tauschii accessions KU2122 shows subunits 5 + T5, the profile of Ae. tauschii KU2121 shows subunits 2 + T5, and that of Ae. tauschii KU2110 shows subunits 5 + 12. Note that the mobility of subunit 5 in wheat and Ae. tauschii is identical. Likewise the mobility of subunits 10 and T5 is indistinguishable from each other in this gel.

Fig. 6. Phenograms produced by the neighbor-joining method using genetic distances based on 55 loci (top) and selected 27 loci (bottom). The magnitude of divergence between groups was computed as Nei's genetic distance D (Nei 1978) with the GDA program (Lewis and Zaykin 1997). The phenograms were constructed with the GDA program. A scale showing a Nei's genetic distance is shown. The phenogram based on 27 loci is longer than that based on 55 loci because it involves only highly informative loci (from Dvorák et al. 1998). In each phenogram, T = subsp. tauschii and S = subsp. strangulata.

Table 3. Frequencies of haplotypes encoding the indicated pairs of HMW-glutenin subunits in Ae. tauschii accessions grouped by geographic region and botanical subspecies (the frequencies were zero in the remaining regions).

Geographic region

Subspecies

2 + 12

5 + T5

5 + any subunit

Transcaucasia

tauschii

0.00

0.10

0.10

Transcaucasia

strangulata

0.29

0.00

0.03

Southwest Caspian Iran

tauschii

0.00

0.14

0.36

Southeast Caspian Iran

tauschii

0.00

0.00

0.00

Southeast Caspian Iran

strangulata

0.33

0.04

0.10

North-central Iran

tauschii

0.33

0.00

0.00


Sources of the wheat D genome and the geographic origin of T. Aestivum

Most of the evidence accumulated thus far suggests that the Triticum aestivum D genome is more related to the strangulata genepool than to the tauschii genepool (Nishikawa 1974; Jaaska 1978, 1980, 1981; Nakai 1979; Nishikawa et al. 1980; Lagudah et al. 1991, Dvorák et al. 1998). On a geographic region basis, the D genomes of all forms of T. aestivum were found to be most closely related to accessions of the strangulata genepool collected in Transcaucasia and southwestern Caspian Iran (Dvorák et al. 1998; see Figure 6). On the morphological basis, the former accessions belong to subsp. strangulata but the latter, collected by Kihara et al. (1965) in the coastal southwestern Caspian Iran, belong to subsp. tauschii. The latter population is composed of morphological varieties meyeri and typica (Kihara et al. 1965). On the genetic basis, however, there is no appreciable genetic distinction between the two varieties in this region and both belong to the strangulata genepool (Dvorák et al. 1998). Therefore, characteristics attributed to var. meyeri are pertinent to the entire southwestern Caspian population. Lagudah et al. (1991) failed to find the Nor3a restriction pattern, for which wheat is monomorphic (Clarke et al. 1989; Lagudah et al. 1991; Dvorák et al. 1998), and the Glu1a haplotype in var. meyeri. While the Nor3a restriction pattern does occur, and in a high frequency, in southwestern Caspian Iran (Dvorák et al. 1998), the Glu1a haplotype was indeed not found in that region (present data). The Glu1a haplotype is present in the Transcaucasian and southeastern Caspian subsp. strangulata. The Glu1x gene encoding the HMW-glutenin subunit 5 occurs with a high frequency in Transcaucasia and southwestern Caspian Iran and the Glu1 haplotype encoding HMW-glutenin subunits 5 + T5, which is ancestral to wheat haplotype Glu1d, occurs with the highest frequency in southwestern Caspian Iran. Thus, variation at the Nor3 and Glu1 loci is consistent with inferences based on genetic distances (Dvorák et al. 1998) indicating that the strangulata genepool in Trancaucasia and southwestern Caspian is the most likely source of the T. aestivum D genome. Earlier, Tsunewaki (1966) and Nakai (1979) placed the origin of T. aestivum to southwestern Caspian Iran and the neighboring mountainous Azerbaijan on the basis of the distribution of the waxy bloom and esterase alleles and Jaaska (1981) to Transcaucasia on the basis of the distribution of aspartate aminotranferese and aromatic alcohol dehydrogenase alleles.

To investigate individual regions of Transcaucasia, Transcaucasian accessions were divided into four geographic regions: Georgia, Armenia, Nakhitshevan and Azerbaijan (Dvorák et al. 1998). The D genome of T. aestivum appeared to be most closely related to the strangulata genepool in Armenia (Dvorák et al. 1998). Most of the Armenian accessions were collected in the vicinity of Jerevan and along the Razdan River (Kihara et al. 1965; V. Jaaska, pers. comm.). Although genetic distances point to Armenia, other areas in Transcaucasia and potentially western coastal Caspian Iran may have played a role in the evolution of the T. aestivum D genome as well (Dvorák et al. 1998). First, differences among genetic distances between wheat and strangulata genepool accessions grouped by geographic regions in Transcaucasia and southwestern Caspian Iran are not large. Second, it was shown here that multiple Ae. tauschii sources contributed to the evolution of the T. aestivum D genome. There is no reason to suppose that these multiple Ae. tauschii sources were from a single geographic area or, in fact, that they all were from only the strangulata genepool. One of the five loci at which wheat and Ae. tauschii share polymorphism is the Xpsr901 locus. Only two of the 172 investigated Ae. tauschii accessions matched the restriction sites of the T. aestivum haplotype b. Both came from a single population from Nakhichean and both belonged to the tauschii genepool. Third, it is not known to what extent the distribution and genetic structure of the present-day populations of Ae. tauschii were affected by agriculture, particularly the cultivation of wheat in Transcaucasia and the nearby Iranian regions.

Genetic distances calculated from RFLP in the D genome suggest a very close relationship of both western and eastern accessions of bread wheat with the Asian spelt (Dvorák et al. 1998). These distances are about a tenth of the genetic distances between bread wheat and the European spelt (Dvorák et al. 1998). This finding agrees with Jaaska's (1978) conclusion that the European spelt differs from the Asian spelt. The D genome of the European spelt appears, in turn, more related to bread wheat than is that of subsp. macha and subsp. vavilovii (Dvorák et al. 1998). Although these findings could be interpreted to mean that bread wheat evolved from the Asian spelt, they could be equally well interpreted to mean that the Asian spelt was derived recently from bread wheat by mutation or hybridization of free-threshing bread wheat with hulled tetraploid wheats (see Introduction). Additional work is therefore needed to resolve this dilemma. However, monophylesis of all T. aestivum groups in the dendogram based on variation in the D genome (Fig. 6) and the observation that many polymorphisms unique to T. aestivum are shared among the various forms of T. aestivum (Table 1) strongly suggests that all forms of T. aestivum likely have evolved from a common hexaploid genepool (Dvorák et al. 1998).

Agriculture appears in Transcaucasia in the 6th millenium (Mellaart 1975). Findings of free-threshing hexaploid wheat in archaeological sites in Anatolia between 6000 and 7000 BC (Hillman 1978; de Moulins 1993) require rapid evolution of hexaploid wheat upon the arrival of wheat cultivation to Transcaucasia. If this timetable is correct, the antiquity of this hexaploid genepool would conceptually provide sufficient time for the evolution of modern free-threshing and hulled forms of T. aestivum by mutation or hybridization from this ancient genepool.

There are two basic scenarios by which the D-genome genepool could have been formed. Several amphiploids could have originated and their hybridization and recombination could have led to the evolution of a single genepool. Alternatively, a single amphiploid could have originated and founded a hexaploid population. Plants in this hexaploid population could have hybridized with Ae. tauschii and the hybrids could have been a bridge for geneflow from Ae. tauschii to T. aestivum. This scenario seems unlikely. While hybridization between tetraploid wheat and Ae. tauschii is easy and fertile amphiploids are produced by self-pollination of triploid hybrids owing to high production of unreduced gametes (Kihara et al. 1950), hybridization between hexaploid wheat and Ae. tauschii is difficult and production of hybrid plants usually requires embryo rescue. Therefore, the recurrent appearance of hexaploid amphiploids in the fields of tetraploid wheat or mixed tetraploid/hexaploid wheat was a likely source of geneflow from Ae. tauschii to the hexaploid genepool.

If multiple amphiploids contributed to the formation of the genepool ancestral to all present-day forms of T. aestivum, it is puzzling why shared polymorphisms between T. aestivum and Ae. tauschii are rare. An obvious example is rRNA gene locus Nor3 which is highly polymorphic in Ae. tauschii but appears to be monomorphic in T. aestivum (Lagudah et al. 1991; Dvorák et al. 1998). Since the Nor3a haplotype is rare in Ae. tauschii it is unlikely that this haplotype was present in every amphiploid that was involved in the formation of T. aestivum. This contradiction can be accounted for if it is assumed that the D-genome genepool was subjected to significant evolution prior to the differentiation of the modern forms of T. aestivum. The dates of the appearance of free-threshing wheat in archaeological sites provide ample time for this phase of T. aestivum evolution. During this phase, some polymorphic loci may have become monomorphic. Once the hexaploid genepool became large, most alleles contributed to the genepool by subsequent amphiploids would have a general tendency to be lost, unless they were selected for or hitchhiked with selected alleles. New amphiploids would be particularly disadvantaged in the fields of free-threshing wheat because their adhering glumes would tend to eliminate them during threshing.

Acknowledgments

This project is a contribution to the International Triticeae Mapping Initiative (ITMI). The authors acknowledge a grant received from the US Department of Agriculture -National Research Initiative Competitive Grant Program No. 96-35300-3822 to J. Dvorák and multi-investigator grant 92-37310-7664 from DOE/NSF/USDA Joint Program on Collaborative Research in Plant Biology, and a gift from Anheuser-Busch Co.

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Introgression of Durum into Wild Emmer and the Agricultural Origin Question - M.A. Blumler

Introduction

The transition from hunting/gathering to farming was a crucial watershed in human history, with enormous ramifications continuing right down to the present day (Diamond 1997a, 1997b). In recent decades, a highly productive burst of archaeological investigations into the origins of agriculture, complemented by equally fruitful cytogenetic studies of crop phylogeny, has shed considerable light on this crucial time period. Nonetheless, our understanding of the reasons for the transition to agriculture remains limited, and subject to legitimate debate (Blumler and Byrne 1991). A major problem is that archaeologists have found it extremely difficult to locate sites that display the transition from the use of wild plants and animals to the raising of domesticated species (Blumler 1992a). Most sites show hunting/gathering only, farming only, or hunting/gathering followed abruptly by an agricultural economy that is clearly at least in part intrusive from elsewhere (e.g. Hillman et al. 1989). For instance, Pickersgill and Heiser (1977:829) pointed out that the famous site of Tehuacan, Mexico, displays “domesticates but not domestication.” That is, domesticated crops apparently filtered into Tehuacan from some other place, where their wild progenitors previously had been taken under cultivation and domesticated. This is true not only of Tehuacan, but of all or almost all other archaeological sites as well (Smith's 1997 re-analysis of the Oaxacan squash remains may be an exception). Thus, we have a great deal of information about the spread of agriculture, but not about its inception. Until archaeologists locate the sites where domestication actually occurred, where hunter-gatherers made the decision to farm, independently (i.e. free from the influence of already existing farmers), it will be difficult to move beyond speculation concerning causes.

This difficulty was largely unanticipated, because archaeologists (and a few influential crop geneticists) had assumed that most crops were domesticated independently many times by many different small human populations. Consequently, it should not be so difficult to locate a site showing the complete transition from wild to domesticated organism. For instance, Harlan (1975) asserted that emmer wheat (Triticum dicoccum) no doubt was domesticated many times in various parts of the Fertile Crescent. This assumption rested upon a theoretical stance that Harlan, and most others concerned with agricultural origins, adopted in the 1950s in the context of a highly polarized debate over the importance of diffusion (the people on the 'other side' in this debate were geographers, notably Carl Sauer). It is obvious from Harlan's writings on the subject that this was one of those debates that generated far more heat than light, with scurrilous charges of racism being bandied about in attempts to discredit opposing viewpoints. Sadly, as often happens in such circumstances, scholars became locked into extreme and inflexible viewpoints (Blumler 1993, 1996). Only recently, as a profusion of new, cytogenetic and distributional evidence concerning the wild progenitors of crop species has become available, has it become possible to re-examine the issue dispassionately (Zohary 1989; Blumler 1992a). This new evidence demonstrates very clearly that, in most regions where agriculture began, primary crops were domesticated only once or a very few times (Table 1). Moreover, while Sauer's (1952) specific hypothesis concerning agricultural origins is probably invalid, his underlying premise that diffusion is far more important than independent invention is undeniably correct (Blumler 1996).

On the one hand, that so many primary crops were domesticated only once or twice makes the archaeologist's search for the agricultural transition analogous to attempting to find a needle in a haystack. On the other hand, it also means that it should be possible to narrow down the location of domestication by cytogenetic analysis, whereupon archaeological exploration might become more successful. With a single origin, only a small subset of the wild genepool is involved in domestication. If the progenitor genotype(s) is geographically restricted, as is often the case, a plausible center of origin can be delineated. For instance, Gepts (1990, this volume) has concluded that the common bean (Phaseolus vulgaris) was domesticated in Jalisco, Mexico and in northwestern Argentina. Since several additional crops also seem to have arisen in these two geographical areas, archaeological exploration is clearly called for (Blumler 1992a). Similarly, Heun et al. (1997) identified forms of wild einkorn (T. boeoticum) from Mt. Karacadag in southeastern Turkey as the nearest genetic relatives of einkorn (Triticum monococcum). Several archaeological sites in the vicinity of Karacadag provide evidence that einkorn was a very early domesticate there.

While this approach is promising, several limitations need to be kept in mind: (1) it assumes that the important ancestral genotypes are still extant, (2) it assumes no change in wild genotype distribution in the 10,000 years since domestication, despite considerable evidence for dramatic climatic changes during this time period, and (3) introgression from cultivars can cloud the picture. I focus on the latter problem in this paper. In brief, the difficulty is that presumed 'wild' individuals may be genetically similar to the domesticate because of geneflow from the crop into wild populations, and hence be mistaken for the progenitor. Therefore, it is important to recognize introgression where it occurs, and to determine what the genetic composition of the introgressed individuals would have been before they received genes from the domesticate, prior to carrying out the phylogenetic analysis (Blumler 1994).

Table 1. Most likely number of domestication events for putative primary crops and animals (Blumler 1992a, 1996), illustrating that most species apparently were domesticated only once in any given hearth region.

Domesticate

Most likely number of domestications

Near East



Emmer

1

Einkorn

1

Barley

2

Pea

1

Lentil

1

Chickpea

1

Broad bean

1

Bitter vetch

2

Flax

1

Sheep

1

Africa



Finger millet

1

Pearl millet

1

Yam bean

2

Andes



 

Common bean

>3

Potato

1

Mexico



Maize

1

Common bean

1

Tepary bean

1

Scarlet runner

2

Peppers

1

Amaranths

1

Squash (Cucurbita pepo)

2

Other



Sunflower

1

Rice

2

Sweet potato

1

Chicken

1

Cattle

2

Dog

2

Includes possible eastern US domestication event.
In practice this is difficult to do, although the best cytogenetic investigations into crop origins, such as the two mentioned above, have attempted to deal with the issue. Gepts (1990) and Gepts et al. (1986) accepted the criteria laid down by the International Center for Tropical Agriculture (CIAT) for identifying possibly introgressed individuals of wild common bean, and discounted such individuals. Heun et al. (1997) also devised a small set of morphological indicators of introgression in wild einkorn, and showed that the Karacadag populations do not possess these indicators. The problem, however, is that the criteria for identifying introgressed individuals are somewhat open to question. Some traits, such as lack of a seed-dispersal mechanism, seem clearly maladaptive in the wild but adaptive in the cultivated field, and consequently can be taken as likely indicators of introgression (Harlan et al. 1973; Blumler and Byrne 1991). On the other hand, populations sometimes are assumed to be introgressed merely because they tend to grow in and around cultivated fields (so-called 'weedy' forms), or because they produce large seeds, another common domesticated trait. But weedy tendencies or the ability to manufacture large seeds do not necessarily mean that introgression has occurred. 'Weedy' plants may be adapted to fertile soil conditions, and such genotypes may have been those initially cultivated by incipient agriculturalists. Similarly, the first farmers may have cultivated large-seeded genotypes of the ancestral wild species. Certainly, they tended to domesticate large-seeded species (Blumler 1992b, 1994; Diamond 1997a). In fact, Harlan and Zohary (1966) suggested that a large-seeded race of wild emmer wheat (T. dicoccoides), from the vicinity of the Upper Jordan Valley, is the likely progenitor of cultivated emmer and its successors, durum (T. durum) and bread wheat (T. aestivum), because of the wild race's similarity to domesticates in seed size and other morphological characters. In short, an improved ability to discriminate introgressed from truly wild individuals should foster a fine-tuning of the phylogenetic analysis of crop origins, and increase confidence in the results. I use the example of wild emmer below to illustrate some of the complexities involved, and to point the way toward such fine-tuning.

Wild emmer wheat

Wild emmer, a tetraploid, is the ancestor of most wheat cultivated today. It is distributed in the Fertile Crescent, from Palestine and Jordan to southeastern Turkey, northern Iraq and western Iran. It is more common in the western portion of this arc, while another, morphologically almost identical, wild tetraploid species (Triticum araraticum) is common in the eastern portion. When Harlan and Zohary (1966) proposed that emmer had been domesticated in or near the Upper Jordan Valley, it was not yet recognized that T. dicoccoides occurs in the eastern and northern portions of the Fertile Crescent. This fact became known in time for Zohary (1969) to modify his position, though in a note added in proof to his paper. Although the paper is widely read, many archaeologists have perhaps (understandably) overlooked the key footnote in which he pointed out that the species may have been domesticated almost anywhere in the Fertile Crescent, and not necessarily in the Upper Jordan Valley. Consequently, there appears to be some confusion about the geneticists' conclusions, with some scholars interested in agricultural origins continuing to believe that the Upper Jordan Valley has been identified as the most likely locus of domestication.

Indeed, many wild individuals from this region do look a lot like domesticated wheat. Especially in that portion of the valley just north of the Sea of Galilee, plants resemble domesticates in being robust, large-seeded, early maturing, and also in such specific traits as lack of anthocyanin pigment. Moreover, electrophoretic studies demonstrate that the wild emmer populations in this sector of the valley are sharply differentiated genetically from all other investigated Palestinian populations, so much so that a case could be made for classifying these populations as a separate species (Nevo et al. 1982; Golenberg 1986); here, it is categorized as the Upper Jordan Valley (UJV) race of wild emmer. This race disappears rapidly as one travels from UJV to the east or west, and is replaced by populations that are similar in their electrophoretic alleles to other Palestinian populations (Golenberg 1989; Blumler 1994).

In UJV, wild emmer is reported to be extremely abundant, especially where grazing is light, suggesting that it is well adapted to truly wild conditions (Zohary 1969; Noy-Meir 1990). Moreover, wild emmer is known to be scarcely if ever weedy (Harlan and Zohary 1966; Blumler and Byrne 1991), and is reported to have only limited geneflow (Golenberg 1987), so that introgression would seem to be less likely than in other crop progenitors that are common in cultivated fields. On the other hand, hybrid swarms (including one of several thousand individuals) were securely identified in UJV, on the edge of formerly cultivated fields (Zohary and Brick 1961). Little wheat is grown in UJV today, but formerly the region was long a center of durum production. For instance, the Biblical parable of the loaves and fishes is situated in UJV. More recently, up until 1948, there were many Palestinian villages in UJV, the denizens of which would have cultivated durum. Most of the Palestinian villages were destroyed during the 1948 war although a Bedouin settlement was left intact (Falah 1996). The continuing existence of this village, with its emphasis on pastoralism, seems to have given some scholars the false impression that traditional land use in UJV was pastoral when in fact farming played a major role. Thus, domesticated individuals should have had opportunities to come in contact with wild plants, especially along rocky, untillable field margins. Scholarly opinion concerning the resulting frequency of hybridization varies greatly, with some believing that there has been essentially no introgression, and others asserting that there was a great deal.

Harlan and Zohary were not the first to propose Palestine as the place where wheat farming began. N.I. Vavilov traveled through the region in 1926; he noticed “a peculiar subspecies of wild wheat [that] accompanies cultivated hard wheat in Palestine” (Vavilov 1957:98), and concluded that it must be the wild progenitor because of its similarity to the domesticate. So distinct from other forms of wild emmer that he classified it as a subspecies, his description fits UJV. Yet he encountered his new subspecies to the south, at the edge of the Esdraelon Plain:

“At the base of the hills, from which flows the underground River Esdraelon, we observed large growths of wild wheat in mixture with wild double-rowed barley. This was an abandoned lot with soft fertile ground positioned right next to the field. The wheat here had a very different appearance from that which we collected in Hauran, Syria. Spikes were large, but with rough spikelets and large seeds. This was no longer an extreme of a xerophyte, but essentially a plant close to cultivated wheat.

“Investigating the fields of the Esdraelon Valley, we found wild wheat in large quantities on edges, along boundaries. No doubt it represents the closest wild source of cultivated wheat, especially hard wheat.” (Vavilov 1962:141)

Subsequent Russian taxonomists have maintained Vavilov's bipartite division of the species into a large-seeded Palestinian subspecies judaicum, and a small-seeded subspecies horanum, though their descriptions of judaicum are not entirely concordant (Jakubziner 1932; Poyarkova 1988). Jakubziner reported that judaicum occurs primarily in the Esdraelon but also is found in UJV, Mt. Hermon, Syria, Jordan and even in the Cilician Taurus. He also described some hybrids that had been collected in UJV. In addition, he noted that judaicum populations usually include some horanum individuals. Finally, both Jakubziner (1932) and Vavilov (1962) were struck by the great variability of judaicum in contrast to the ubiquitous horanum.

These observations, taken together, suggest introgression. The Esdraelon is a region that supported extensive durum cultivation in Vavilov's time, but today is given over to other crops. It appears that Esdraelon populations of wild emmer were located at the base of rocky, untillable slopes, at the edges of deep soil and cultivation -precisely where hybridization between a crop and a non-weedy species would be most likely to occur. One expected outcome of introgression would be high variability. Another would be possible reduction or elimination of the stands after cultivation ceased. Israeli geneticists have collected wild emmer intensively throughout their territory, but have never reported populations from the Esdraelon. Other judaicum specimens discussed by Jakubziner also seem to be from areas of intensive cultivation. For instance, he describes judaicum from Mt. Hermon, at Majdal es-Shams, a population center in a wheat-growing region.

Approaches to the identification of introgression

Several lines of investigation are proving fruitful in identifying introgression in wild emmer, and should be useful in other species as well (Box 1). First, while introgressed individuals should resemble domesticates, they should be more similar to modern varieties than to primitive ones. Progenitors, on the other hand, should resemble primitive forms of the domesticate more than later-evolving derivatives. For example, wild emmer gave rise initially to emmer, which is rarely cultivated today, and not at all in Palestine. Emmer later evolved into durum and several other tetraploid wheats, while bread wheat is an allopolyploid derivative. In recent centuries, durum has been the major wheat cultivated in Palestine, though there has been a shift toward more bread wheat since 1948. Introgression from the latter would be limited because of the difference in ploidy level, though it does occur (Dorofeev 1969). Thus, one would predict that UJV plants will resemble durum more than emmer if introgressed, while they will be more similar to emmer if they represent the genotypes originally taken under cultivation.

Second, the ecological conditions to which wild plants must adapt are very different from those which a domesticate faces, and this results in varying degrees of selection for morphological and other characteristics. For instance, indehiscence is strongly selected against in the wild, since the seed needs to disperse from the top of the plant and find its way to the soil in order to germinate. In contrast, indehiscence is usually selected under cultivation (Harlan et al. 1973; Blumler and Byrne 1991). The presence of indehiscent individuals in wild populations, therefore, suggests very recent introgression. Similarly, domesticated cereals characteristically exhibit uniform, high rates of germination, while wild synaptosperms such as wild emmer have pronounced dormancy polymorphisms. A synaptosperm is a species that disperses more than one seed in each diaspore; wild emmer spikelets typically have two seeds. If both seeds germinate at once, their seedlings will compete. Selection, then, favors a somatic polymorphism in which one seed is non-dormant, and the other is dormant for perhaps a year (Zohary 1969; Blumler 1991, 1992a). Selection against non-dormancy should not be as severe as against indehiscence in the wild, so the trait should persist for a longer time period after the introgression has ceased. Other domesticated traits, discussed below, probably are selected against to a still slighter degree, and persist still longer, but on the other hand become less reliable indicators of introgression for precisely the reason that selection does not work strongly against them. By considering a range of such traits, one should be able to test whether introgression has occurred in a given population.

Finally, introgressed plants should exhibit a predictable spatial pattern. They should be more common in and around current or former cultivated fields, especially where cultivation abuts on untillable land that is good habitat for the wild species. Introgression is more likely to be significant also where the amount of cultivation is great in comparison with the size of the wild populations: theoretically, then, geneflow from the crop might overwhelm selection for wild traits. Wild populations can be sampled along transects running away from cultivated land, to determine if the putative introgressed alleles decrease with distance. If so, introgression is likely, especially if there is no corresponding environmental gradient along the transect.

Comparing wild emmer with durum and emmer

Jakubziner's (1932) monograph remains the most authoritative description of the morphological variation in emmer and durum, and is useful also because of its treatment of Palestinian durum landraces, which would be the most likely parents in introgression events. Durum and emmer are different from each other, especially in spikelet characteristics. In addition to the characters described in Jakubziner, I added a few cryptic (i.e. physiological rather than morphological) traits investigated by others. To compare with wild emmer, I examined the herbarium specimens at important repositories such as Edinburgh and the Hebrew University, and also visited a few populations in the field. While typical (horanum) wild emmer is not at all similar to durum, UJV varies in the direction of durum in several respects (Blumler 1994). Grain shape, glume shape, first glume tooth, glume pubescence, spikelet width and glutenin A1-1 allele all are at least occasionally like durum, while there are no characters for which UJV is demonstrably more similar to emmer. In general, UJV is highly variable but intermediate between durum and horanum wild emmer, as one would expect if it were the product of hybridization (Blumler 1994).

Box 1. Approaches to identifying introgression.

1. Morphological comparisons

wild progenitor should be more similar to emmer than durum
2. Ecological considerations
certain traits are likely to be adaptive in the wild, others in the cultivated field
3. Spatial pattern
introgression is more likely in or around former cultivated fields

The glutenin data of Levy and Feldman (1988) are particularly informative. Glutenin A1-1 is present in domesticated emmer, but absent in durum. It is almost always present in wild emmer, but is generally absent in UJV, which suggests that introgression from durum may have been massive. Of the 19 wild populations outside UJV that Levy and Feldman examined, glutenin A1-1 was 100% present in all but two: a population just south of UJV, and Majdal es-Shams. As mentioned above, Jakubziner (1932) reported judaicum from Majdal es-Shams. Both populations are in agricultural areas, whereas most wild emmer populations are on rocky slopes where cultivation could only have been practised in a few select spots. Interestingly, the glutenin A1-2 locus presents a different pattern: absent in most wild emmer populations, absent in durum and emmer, but present in bread wheat and in some UJV plants. Since Levy and Feldman did not study Palestinian durum landraces, it would be interesting to determine if they also possess A1-2 alleles.

Ecological influences on introgression

As discussed above, a few domesticated traits such as indehiscence and lack of dormancy are selected against in the wild. On the basis of ecological research, I believe that several additional traits are selected against, though less strongly than indehiscence (Box 2). I believe, too, that a few domesticated traits are sometimes advantageous in wild environments (Blumler 1994). Most of these traits are spikelet characters, because of the importance of seed dispersal and self-implantation in the ecology of the wild plant. Zohary and Brick (1961) and Zohary (1969) described the 'arrow-shaped' spikelets of wild emmer, and pointed out that they are so structured as to enable them to drill into the soil where they are protected from predation until the fall rains initiate germination. The glumes tightly invest the two grains, which are elongate in order to fit into the arrowhead shape. I examined specimens that differ from typical forms in being more like domesticates. Any change to rounder grains, more grains per diaspore, or more divergent glumes makes the structure less streamlined and therefore, presumably, less able to drill into the soil.

I also investigated the drilling ability of the spikelets under various microenvironmental conditions (Blumler 1991). I found that the spikelets generally drill into soil cracks or adjacent to rocks or other obstructions where the vegetation has been mostly removed (e.g. under grazing); on the other hand, in more-or-less undisturbed conditions where the dead plants remain standing at season's end, the two awns of the spikelet tend to catch in the litter, and prevent the diaspore from penetrating all the way to the soil surface. Seedling establishment is nonetheless excellent under these circumstances, as the grains end up close enough to the surface that their roots can penetrate the soil after germination. Thus, it seems possible that the changes in spikelet morphology induced by introgression would be selected against where there is grazing, but might not be under undisturbed conditions. The somewhat more awkward spikelets of hybrids, with round grains or three grains, probably still can drop through litter to just above the soil surface, whereas they may be less efficient at drilling into cracks on bare ground. Further research is needed to test this hypothesis.

There are predictable effects of domestication on grain size and shape. As the spikelet is no longer needed to disperse the grains, the tightly investing glumes open up over the course of evolution, allowing the florets to expand outwards or to increase in number. Since grain size and shape reflect that of the floret (Millet 1986), the 'naked' grains such as durum are often round, whereas the glume wheats such as emmer remain elongate. Those species or varieties that increase the number of seeds per spikelet, such as bread wheat, produce relatively small grains, whereas those with fewer grains per spikelet produce large ones, such as some forms of durum. Thus, with introgression from durum, wild grain size is likely to increase.

But whether this will be selected against should depend on site conditions. In annual species, seed size is strongly correlated with site productivity, both among and within species (Blumler 1992b). While the within-species variation may be largely phenotypic, the variation between species is so great that it must be primarily genetic. On infertile sites, annuals have difficulty producing enough photosynthate to fill large seeds, whereas they should be able to produce small seeds relatively easily. Sinnott (1921) showed in cultivated annuals that seed size increases phenotypically as productivity (plant size) increases, eventually leveling off. I studied several Jerusalem wild emmer populations on hard limestone, where soil depth, and hence productivity, vary over short distances, providing a natural analogue to Sinnott's experiment (Blumler 1992b). Results were identical to his: grain size increased with plant height up to a point, and then appeared to level off (Fig. 1). However, some populations leveled off at a low mean grain weight (<20 mg), whereas others did not do so until they were considerably larger (>35 mg). The populations with low mean grain weight occurred on sites that were relatively low in productivity over most of the site, though with a few high-productivity pockets, whereas the larger-grained populations were on deeper, more productive soil with a few small areas of infertile substrate. When seeds from wild populations were planted in a uniform garden, however, there was no relationship, within populations, between weight of grain planted and grain produced. That is, the within-population variation was mostly phenotypic. The relevant implication is that the large grain size of most domesticated genotypes should be advantageous on fertile sites but not where productivity is low. Since most cultivated fields have deep soil, they are productive compared with natural stands of wild emmer. Thus, in the places where introgression is likely to occur, large grain size may not be disadvantageous, and may even be beneficial.

Box 2. Ecological aspects of introgression.

1. Dormancy

selection for polymorphism within spikelet
2. Drilling ability of spikelet
selection for elongate grains, parallel glumes, narrow spikelets, two seeds/spikelet, dehiscence
3. Productivity
selective and phenotypic effects on grain size
4. Pigment
selection for coleoptile pigment, glume pigment

Finally, it appears that there may be a selective advantage to anthocyanin and other pigments in wild plants, but not in domesticates. The ecology of anthocyanin is poorly understood and even less studied - one suggestion is that it makes plants less palatable to herbivores - but there is no question that loss of pigment is a feature not only of wheat domestication, but also of the domestication and evolution of many other crops. Exceptions occur when humans intentionally select pigmented forms for ornamental purposes (e.g. 'Sangre de Cristo' maize, amaranths). Most wild emmer populations are black glumed, with anthocyanin pigment present in various vegetative parts, often including the coleoptile. Most durum lacks pigment.

With these ecological considerations in mind, and also the morphological comparisons discussed in the previous section, we can construct a list of traits that are likely to be characteristic of domesticates but not of truly wild plants (Box 3). Included will be those characters controlled by genes linked to the traits of ecological concern. For instance, the rounding of the glumes that often occurs in durum because of the change from elongate to rounded grains typically is associated with specific changes in the glume teeth that seem likely to be of no selective value. Glume pubescence also is characteristic of durum, particularly the Palestinian landraces (Jakubziner 1932; Poiarkova and Blum 1983), and especially the varieties with rounded glumes, but it is difficult to understand how it could be adaptive. Wild emmer populations are mostly glabrous, but the sibling species T. araraticum is usually pubescent. It seems likely, therefore, that glume pubescence is a neutral or nearly neutral character. I noticed, during the course of my examination of herbarium specimens, that wild emmer with pubescent glumes occurs, without exception, in populations that show more reliable indicators of introgression such as rounded glumes or three-seeded spikelets. Consistent with this conclusion, Oppenheimer (1963) reported that pubescent glumed individuals of wild emmer are less dormant than those with glabrous glumes. Perhaps there may be pubescent individuals somewhere that are not the result of introgression but they have not been collected.

There are numerous additional traits, not listed in Box 3 because of insufficient data, that merit further investigation. For instance, glaucous glumes and absence of leaf sheath cilia both appear to be durum traits (Jakubziner 1932), but I have little information on their distribution in the wild. More speculatively, it seems possible that pale glumes, which are common in durum but infrequent though widespread in wild emmer, may be a reliable indicator of introgression. In my Jerusalem studies, for instance, I noticed that pale-glumed forms of wild emmer make larger grains and grow in more productive sites than populations that produce black glumes. I have yet to encounter a pale-glumed form of wild emmer that produces the 20-mg grains of some black-glumed types, except under stress.

Regardless, populations from UJV that I examined either in the field or the herbarium all exhibit more than one of the indicators listed near the top of Box 3. Possibly typical is the population at Migdal described by Jakubziner (1932), which included the varieties (with some peculiar variants) listed in Table 2. All of the peculiar variants are typical of durum. Of course, in this case there are no data on characters that he did not investigate, such as glutenin alleles.

Spatial analysis of UJV

As mentioned above, all UJV-type populations occur in close proximity to extensive areas of former durum cultivation. More precise spatial analysis of allele frequencies should allow a more rigorous test of the introgession hypothesis. Fortunately, this is possible because of the existence of a detailed data set from Ammiad, a kibbutz located at the former site of the Palestinian waystation of Jubb Yusuf. A multidisciplinary team has carried out detailed ecological and genetic studies of the population, located on karstic limestone hills that form the western boundary of UJV, as well as the adjacent UJV alluvium (Anikster and Noy-Meir 1991). The site is located only about 100 meters from the large hybrid swarm described by Zohary and Brick (1961) and D. Zohary (pers. comm.), but the Ammiad team (with the exception of Felsenburg et al. 1988) assumed that no plants in the population are introgressed. Plants were gathered along transects, and analyzed genetically, while ecologists characterized the four primary microenvironments occurring there. If UJV plants are truly a locally evolved race, adapted to the basalt and alluvium of UJV itself, and not to other environments, then UJV alleles should occur at Ammiad where transects extend into the alluvium, but only incidentally on the adjacent slopes, in microsites that mimic UJV. On the other hand, if UJV is the result of introgression, UJV alleles should decline with distance from the alluvium regardless of microenvironment. The full analysis will be reported in a separate paper. Here, I will discuss only the most dramatic evidence.

This evidence comes from Transect 'C', which is in extreme karstic terrain. Noy-Meir et al. (1991) concluded that the microenvironment along Transect C differs greatly from the microenvironment on the UJV alluvium. In fact, according to Noy-Meir et al. these are the two most differentiated environments at the site. However, the base of Transect C is geographically close to the alluvium of a wadi that runs into UJV, and cultivation would have been practised in the wadi in the past (D. Zohary, pers. comm.). Thus, on ecological grounds one would predict C to have few if any UJV-type plants, while it might have many such plants if the introgression hypothesis is correct.

In fact, and in contrast to the remainder of the population, the section of C nearest to the alluvium (accessions 160-180) is laden with plants that have many UJV alleles (Table 3). As one goes up the transect away from the wadi, the UJV alleles suddenly drop out at accession 160, which is half way up a limestone cliff. The accessions on the transect above the cliff are almost completely devoid of UJV alleles, suggesting that the cliff forms a topographic barrier to their dispersal upslope from the wadi. Gene frequencies in lower C are very similar to the two UJV populations that have been intensively studied, Yehudiyya and Tabgha (Table 3). Lower C plants also resemble UJV morphologically. In contrast, the remainder of the Ammiad population exhibits gene frequencies that are very similar to non-UJV populations. Given that C is ecologically so dramatically different from UJV, it is difficult to explain this pattern as the result of natural selection. Moreover, if selection favors UJV alleles on this particular karstic site, then UJV plants should turn up on extreme karstic limestone elsewhere as well. Karstic limestone is extremely widespread in Palestine, and commonly supports wild emmer populations, but as far as is known, does not support UJV plants elsewhere.

Fig. 1. Relationship between plant height and grain weight in a population of wild emmer in Jerusalem (Blumler 1992b). Seed weight appears to increase linearly up to a plant height of approximately 75 cm and a weight of 21 mg; it appears to be stable at greater plant heights.

Box 3. Some morphological and other indicators of introgression in wild emmer.

Traits are ranked according to the reliability with which they suggest that introgression has occurred. Indehiscent plants are almost certainly introgressed, whereas plants that produce large grains may not be.

Indehiscence

Poorly developed second glume tooth

Rounded grains

Absence of glutenin A1-1

Three-grained spikelets

Absence of pigment

Low grain dormancy

Short grain brush

Pubescent glumes

Large grains

Curved glume tooth



Table 2. Some varieties of wild emmer in a collection from Migdal in UJV (Jakubziner 1932).

Variety

Glumes

Peculiar variants

horanum




 

kotchii

white, glabrous

glaucous glume

palestinicum

white, pubescent


judaicum




 

arabicum

white, glabrous

glaucous glume with long curved tooth; glabrous leaf sheath

vavilovi

black, glabrous

long curved apical tooth

fulvovillosum

white, pubescent

large tooth


Although the C plants are very similar to those from UJV populations, they are not identical; for instance, compare allele frequencies at loci Pgi-A, Pgi-B and Glu-A1-1 in Table 3. Nor are Yehudiyya and Tabgha identical to each other. At most loci there is a characteristic allele that predominates throughout Palestine, and also at Ammiad, but not in UJV. Within UJV, on the other hand, the predominant allele at any given locus is highly variable from site to site, and alleles sometimes are common at one site but absent at others. This is as one would expect if UJV resulted from repeated introgression events with diverse durum landraces, but makes little sense if UJV is the result of natural selection in a purely wild setting. UJV appears ecologically fairly uniform, in contrast to karstic limestone sites where soil depth, moisture and shading typically vary greatly over short distances.

Discussion

While these results are still preliminary, the convergence of several lines of evidence strongly indicates that massive introgression has occurred in UJV, giving rise to the UJV 'race' of wild emmer. Does this mean that UJV is not the locus of domestication? At present, we have no information that bears on that question. The evidence for massive introgression at UJV merely means that its populations are no more or less likely than other wild emmer populations to be the progenitor. UJV-type plants reported elsewhere, such as on Mt. Hermon, also seem to be introgressed. If pale glumes occur in the wild only after introgression, then many additional populations and subpopulations have been altered by interactions with domesticated wheat. Introgression appears to have been more massive in Palestine than elsewhere in wild emmer's range. This may reflect the abundance of low-elevation, warm-winter, highly productive habitats in Palestine, though it may also be an artifact of the greater density of exploration and scientific study in Palestine compared with other parts of the Fertile Crescent.

Stands of wild cereals in and around UJV are known to be especially productive (Harlan and Zohary 1966), so large grain size might be present naturally in wild populations, or alternatively, might be selected in the wild after introgression from domesticates. On the other hand, most wild emmer populations occur on much less productive sites, where introgression of genes for large seed size might be selected against. This is a possible explanation for the apparently massive introgression in UJV, and the smaller degree of introgression elsewhere. Moreover, the elimination of Palestinians from UJV in 1948 was followed by a long period of limited exploitation of the land. Settlers initially grazed the region very lightly because they were unsure how much pressure the vegetation could support. Characters such as large seed size and low dormancy should be favored under relatively undisturbed conditions when stands are dense. Recently, grazing pressure has been increasing, and wild emmer populations appear to be declining in consequence. Under these circumstances, selection against domesticated traits may be more severe.

That introgression apparently has been so massive in wild emmer, a non-weedy species, raises questions concerning the extent of introgression in other wild crop relatives. Considerable concern and debate exists about the likelihood that genetically engineered crops might pass the engineered genes to wild relatives, with subsequent effects on ecosystems. Ecologists tend to express serious concern (Tiedje et al. 1989), whereas geneticists sometimes dismiss the possibility, perhaps because of an underlying assumption that wild plants are already well-adapted to natural environments and therefore unlikely to be improved in fitness by geneflow from domesticates. But the example of wild emmer suggests that genes from cultivated plants can indeed affect the genetic make-up and perhaps the fitness of wild plants.

Geneticists have studied wild emmer to address theoretical issues, such as the comparative roles of natural selection and neutral mutation in evolution. Nevo et al. (1982, 1986) concluded that natural selection was at least partly responsible for allozyme distribution patterns, but Nevo's statistical methodology is known to be invalid (Heywood and Levin 1985). In a more rigorous series of experiments, Golenberg (1986, 1989) found no evidence for an adaptive basis for allozyme distribution in UJV. Neither Nevo nor Golenberg considered the possibility that durum genes might have introgressed into their populations. The evidence presented here for massive introgression suggests that the odd electrophoretic variants in UJV are contaminants from durum, currently hitchhiking on other traits upon which selection is working. Similarly, the Ammiad study site was selected in part because of its high genetic variability; but when the putative introgressed alleles are subtracted, genetic diversity becomes very low (Blumler, unpublished). Consequently, the assumption of the researchers that there has been local-scale adaptation to differing microenvironments at Ammiad (Nevo et al. 1986) is probably incorrect.

Conclusion

To return to the agricultural origin question, one implication of these results is that introgression may be more massive than realized in other wild progenitors, such as einkorn and bean, as well. Einkorn probably has not been cultivated near Karacadag for some time, and would have been rare for a still longer period, so any introgression that might have occurred presumably would have been much more ancient than the introgression in UJV. Assume for the sake of argument that massive introgression did occur, say, 3000 years ago. By now, the most obvious indicators of introgression, such as indehiscence, would have been selected out of the wild populations. Only more-or-less neutral introgressed alleles might still be present. But since the analysis of Heun et al. (1997) was of DNA that probably is more-or-less neutral, it does not seem possible to reject introgression entirely. I should emphasize that I am quite comfortable with the conclusions of Heun et al. (1997), and also with those of Gepts (this volume), because they conform to my own views on agricultural origins; for instance, Gepts' report of a northwest Argentinian origin for Andean common bean was anticipated by my paper (Blumler 1992a). But at the same time, given the amount of introgression I am finding in wild emmer, I cannot but feel that the introgression issue needs to be addressed more fully in all other crops as well.

Table 3. Allele frequencies in Triticum dicoccoides in Palestine.

Locus

 

Ammiad

Transect C 160-180

Yehudiyya (UJV)

Tabigha (UJV)

Other Palestine

Ammiad, exc. 160-180

N

Freq.

N

Freq.

N

Freq.

N

Freq.

N

Freq.

N

Freq.

6Pgd-2

191


21


96


39


354


170


M


0.890


0.524


-


0.333


0.958


0.935

S


0.110


0.476


1.000


0.667


0.006


0.065

Pept-3

146


8


96


0


49


138


M


0.918


0.500


0.016


-


1.000


0.942

S


0.082


0.500


0.984


-


-


0.058

Pgi-A

230


20


39


37


375


210


F


0.387


0.550


0.026


0.189


0.115


0.371

M


0.583


0.250


0.974


0.811


0.797


0.615

G


0.030


0.200


-


0.088


0.014



Pgi-B

230


20


39


40


378


210


M


0.252


0.650


-


-


-


0.214

S


0.748


0.350


1.000


1.000


1.000


0.786

b-Gluc-A

217


17


39


40


361


200


M


0.843


0.294


-




0.914


0.890

N


0.157


0.706


1.000


1.000


0.053


0.110

b-Gluc-B

216


17


96


0


49


199


F


0.194


0.765


1.000


-


0.020


0.145

M


0.806


0.235


-


-


0.980


0.855

Mdh-1A

224


19


96


40


369


205


F


0.085


0.316


1.000


0.950


0.014


0.064

M


0.915


0.684


-


0.050


0.986


0.936

Glu-A1-1

178


21


33


88


345


157


a


0.101


0.857


0.727


0.568


0.046


-

e


0.022


-


0.273


0.284


0.046


0.025

k


0.573


-


-


-


0.099


0.650

I


0.303


0.143


-


0.011


0.220


0.325

Glu-B1-1

178


21


33


88


345


157


g


0.129


0.857


0.000


0.148


0.058


0.032

Glu-B1 -2

178


21


33


88


345


157


l


0.129


0.857


0.000


0.125


0.043


0.032

Pept-1B

208


11


96


40


374


197


F


0.038


-


0.995


0.675


0.048


0.040

M


0.962


1.000


0.005


0.325


0.952


0.960

Aat-2a

231


21


39


40


378


210


F


0.009


-


0.385


0.050


0.021


0.010

M


0.991


1.000


0.615


0.950


0.979


0.990

Acph-3

231


21


39


40


378


210


F


0.022


0.048


0.590


0.975


0.003


0.019

M


0.957


0.952


0.410


0.025


0.947


0.957

S


0.022


-


-


-


0.050


0.024

From Nevo et al. (1982, 1986); Golenberg (1986); Levy and Feldman (1988).

Allele frequencies of HMW glutenin loci calculated as mean of S. Almagor, NW Almagor and S Ammiad (Levy and Feldman 1988).

There is a great deal of emphasis today on molecular approaches to phylogeny, and rightfully so, but at times this has had the unfortunate effect of causing a neglect of traditional morphological analysis. Attention to morphology was an essential component of the research discussed here, and must continue to be if we are to obtain a clearer picture of introgression in wild emmer. Also essential will be further ecological research, for instance, into the adaptive value of anthocyanin or the drilling abilities of spikelets of different morphologies. My own investigation into the latter (Blumler 1991) was merely synoptic. In fact, it is fair to state that ecological research is the weak link in agricultural origins studies. In comparison, both archaeological and genetic investigations are going great guns, despite funding problems, difficulties of access to crucial locations, and so on. Until we understand the ecology of wild progenitors better, it will be difficult to be certain where they grew, in what abundance, and how hunter-gatherers impacted them.

Acknowledgments

Serge Glushkoff translated the passage from Vavilov (1962). Conversations with Daniel Zohary clarified or confirmed my understanding of variation patterns in wild emmer.

References

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Blumler, M.A. 1992a. Independent inventionism and recent genetic evidence on plant domestication. Econ. Bot. 46:98-111.

Blumler, M.A. 1992b. Seed weight and environment in mediterranean-type grasslands in California and Israel. PhD Thesis. University of California, Berkeley, USA.

Blumler, M.A. 1993. On the tension between cultural geography and anthropology: commentary on Christine Rodrigue's 'Early animal domestication.' Prof. Geogr. 45:359-363.

Blumler, M.A. 1994. Evolutionary trends in the wheat group in relation to environment, Quaternary climate change and human impacts. Pages 253-269 in Environmental Change in Drylands (A. C. Millington and K. Pye, eds.). John Wiley & Sons, Chichester, UK.

Blumler, M.A. 1996. Ecology, evolutionary theory, and agricultural origins. Pages 25-50 in The Origins and Spread of Agriculture and Pastoralism in Eurasia (D.R. Harris, ed.). UCL Press, London, UK.

Blumler, M.A. and R. Byrne. 1991. The ecological genetics of domestication and the origins of agriculture. Curr. Anthropol 32:23-54.

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Dorofeev, V.F. 1969. Spontaneous hybridization in wheat populations of Transcaucasia. Euphytica 18:406-416.

Falah, G. 1996. The 1948 Israeli-Palestine war and its aftermath: the transformation and de-signification of Palestine's cultural landscape. Ann. Assoc. Am. Geogr. 86:256-285.

Felsenburg, T, A.A. Levy and M. Feldman. 1988. Variation in high molecular weight glutenin subunits of the endosperm. Pages 35-38 in The Biological Structure of Native Populations of Wild Emmer Wheat (Triticum turgidum var. dicoccoides) in Israel (Y. Anikster, ed.). USDA-ARS, Corvallis, Oregon, USA.

Gepts, P. 1990. Biochemical evidence bearing on the domestication of Phaseolus (Fabaceae) beans. Econ. Bot. 44(3S):28-38.

Gepts, P., T.C. Osborn, K. Rashka and F.A. Bliss. 1986. Phaseolin seed proteins variability in wild forms and landraces of the common bean, Phaseolus vulgaris: evidence for multiple centers of domestication. Econ. Bot. 40:451-468.

Golenberg, E.M. 1986. Multilocus structures in plant populations: population and genetic dynamics of Triticum dicoccoides. PhD Thesis. State University of New York-Stony Brook, USA.

Golenberg, E.M. 1987. Estimation of gene flow and genetic neighborhood size by indirect methods in a selfing annual, Triticum dicoccoides. Evolution 41:1326-1334.

Golenberg, E.M. 1989. Migration patterns and the development of multilocus associations in a selfing annual, Triticum dicoccoides. Evolution 43:595-606.

Harlan, J.R. 1975. Crops and Man. American Society of Agronomy, Madison, Wisconsin, USA.

Harlan, J.R. and D. Zohary. 1966. Distribution of wild wheats and barley. Science 153:1074-1080.

Harlan, J.R., J.M.J. de Wet and E.G. Price. 1973. Comparative evolution of cereals. Evolution 27:311-325.

Heun, M., R. Schäfer-Pregl, D. Klawan, R. Castagna, M. Accerbi, B. Borghi and F. Salamini. 1997. Site of einkorn wheat domestication identified by DNA fingerprinting. Science 278:1312-1314.

Heywood, J.S. and D.A. Levin. 1985. Associations between allozyme frequencies and soil characteristics in Gaillardia pulchella (Compositae). Evolution 39:1076-1086.

Hillman, G.C., S.M. Colledge and D.R. Harris. 1989. Plant-food economy during the Epipalaeolithic period at Tell Abu Hureyra, Syria: dietary diversity, seasonality, and modes of exploitation. Pages 240-268 in Foraging and Farming - The Evolution of Plant Exploitation (D.R. Harris and G.C. Hillman, eds.). Unwin Hyman, London, UK.

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The Variation of Grain Characters in Diploid and Tetraploid Hulled Wheats and its Relevance for the Archaeological Record - K. Hammer and C.-E. Specht

Introduction

Today, hulled wheats can be considered as underutilized or neglected crops (Padulosi et al. 1996). In past societies hulled wheats had a great importance as staple crops. The declining importance in connection with the preferred use of naked wheats and other influences led to a decline in hulled wheats which have been only relic crops or were considered extinct at the beginning of the 1980s. New accessions from the Mediterranean area proved that hulled wheats are still cultivated in remote places. The conservation of Triticum monococcum L. and T. dicoccon Schrank in field reserves of south Italy was proposed in 1984 (Perrino and Hammer 1984) and new interest for these traditional crops developed, leading to a certain increase in the growing area. Material was collected and preserved ex situ in genebanks. Large living collections were established, characterized and evaluated, which led to a better knowledge of the variation in hulled wheats.

Material and methods

The material for this study came from the Gatersleben genebank. Diploid (T. monococcum, T. boeoticum Boiss. and T. urartu Thum. ex Gandil.) and tetraploid (T. dicoccon, T. dicoccoides Körn. ex. Schweinf.) hulled wheats were selected for this study.

The material investigated is shown in Table 1. For all accessions the thousand-grain weight (TGW) was determined by weighing 100 grains three times and calculating the value for 1000 seeds. The husks were manually removed. Grain length and thickness were determined for selected accessions representing the extremes in TGW. The statistical calculations were done with a computer using the program PLABSTAT (Utz 1991).

Table 1. Material used for the studies.

Species

Number of accessions

Number of botanical varieties according to Dorofeev et al. (1979)

Triticum monococcum

125

11

Triticum boeoticum

29

16

Triticum urartu

8

4

Triticum dicoccon

139

28

Triticum dicoccoides

24

14


Results and discussion

There is a great variation in the material investigated. Triticum monococcum has a higher grain weight than its wild progenitor T. boeoticum because of domestication. The wild T. urartu, which also plays an important role in the evolution of the cultivated wheats, is comparable to T. boeoticum. On the tetraploid level we find similar relations with the cultivated T. dicoccon and its wild progenitor T. dicoccoides but the overlap is much higher (Table 2).

There is a tendency for the grains to be heavier in tetraploid wheats than in diploid ones. But, again, there is an overlap between the higher weight classes of T. monoccocum and the lower classes of T. dicoccon (Table 2, see also Fig. 1). Therefore, weight of grain, even when determined on a relatively large number of seeds, which is usually not available in archaeological excavations, may be useful only for lower seed weight classes in T. monococcum (about 0.025 g/seed) and higher seed weight classes in T. dicoccum (about 0.04 g/seed). The loss of weight during carbonization of this material could be easily determined experimentally.

On the basis of weight data, extremes have been selected for the determination of grain length and grain thickness which are easier to investigate in archaeological material. The reduction by carbonization can be seen by comparison of our data with those of Kroll (1992), e.g. carbonized einkorn grain length of 3.3-5.7 mm minimum and in our data, 6.1-9.0 mm (Table 3).

Of all species studied, T. dicoccoides had the longest seeds (Table 3). A general increase in length during domestication did not occur. On average, T. dicoccum is shorter-seeded than T. monococcum. Grain thickness is a better character for showing evolutionary tendencies, and accordingly T. dicoccum has thicker seeds than T. monococcum.

More difficult is the distinction between emmer and emmer-like einkorn. One of the first authors to compare the variations of recent and fossilized materials was E. Schiemann (1940). She came to the conclusion that there is a certain amount of two-grained spikes in recently collected einkorn material. In this case one or both of the grains can keep their typical einkorn shape. In 1996 a botanical variety of einkorn was described which is typically two-grained with well-developed einkorn kernels (var. clusii Szabó et Hammer from Transylvania, Szabó and Hammer 1996). Also the archaeological literature increasingly reports two-grained einkorns (van Zeist and Bakker-Heeres 1982; Kroll 1992). Interestingly these findings extend from the Balkans to the Levant with outposts (?) to more central European areas (Kreutz, pers. comm.). If we could assume that the wild two-grained einkorn (T. boeoticum subsp. thaoudar (Reut. et Hausskn. Grossh.) should be the progenitor of the two-grained cultivated einkorn, its more eastern distribution in comparison with the typical one-grained wild einkorn subsp. boeoticum (see Dorofeev et al. 1979) would be difficult to explain. More reliable is a positive selection within the seed trait which also intergrades in the wild progenitor (Harlan and Zohary 1966; Zohary 1969). Similar problems occur with one-grained spikelets of T. dicoccoides in comparison with typically one-grained T. boeoticum (Pasternak, pers. comm.).

Our data show tendencies but they cannot provide a definite distinction, especially in cases of two-grained einkorn as compared with emmer. Many recent einkorn spikes have been examined and show a marked tendency to two-grained forms. This tendency is especially high in the newly described T. monococcum var. clusii. Some indications for the distinction of einkorn and emmer can be obtained from the kernel thickness but also from more detailed studies of grain characters. According to Nesbitt and Samuel (1996), einkorn grains have a typical spindle shape with pointed apex and pronounced ventral convex curve as opposed to emmer grains with a blunter apex and straighter sides. Moreover, Kroll (1992) reports on a small depression in the ventral outline. Our data confirm the statement of Kroll (1992) that it is not possible to distinguish emmer-like einkorn grains from emmer grains by metrical means only and also it is impossible to separate them accurately by morphological features.

Table 2. Thousand-grain weight (TGW in g) for diploid and tetraploid hulled wheats.


x ± s

Min. - Max.

Number of accessions

Triticum monococcum

28.822 ± 3.401

19.90 - 38.40

125

Triticum boeoticum

15.931 ± 2.629

11.60 - 19.80

29

Triticum urartu

16.362 ± 2.462

10.90 - 18.90

8

Triticum dicoccum

41.260 ± 7.713

29.80 - 67.80

139

Triticum dicoccoides

31.450 ± 7.379

20.20 - 51.20

24


Table 3. Grain length (mm) and grain thickness (mm) for diploid and tetraploid hulled wheats.


Length (x ± s)

Min. - Max.

Thickness (x ± s)

Min. - Max.

No. of grains

Triticum monococcum

7.88 ± 0.72

6.1 - 9.0

2.25 ± 0.35

1.5 - 3.0

60

Triticum boeoticum

7.80 ± 0.82

6.9 - 10.0

1.68 ± 0.22

1.4 - 2.2

40

Triticum urartu

8.39 ± 0.68

7.0 - 10.0

1.53 ± 0.33

0.9 - 2.1

50

Triticum dicoccon

7.59 ± 0.86

6.0 - 9.0

2.95 ± 0.39

2.0 - 4.1

60

Triticum dicoccoides

9.24 ± 0.86

7.0 - 11.0

2.30 ± 0.48

1.5 - 3.8

40


Fig. 1. Variation and frequency of thousand grain weight (TGW) in Triticum monococcum and Triticum dicoccon. In the class 28.1-30 g, two accessions of Triticum monococcum are not visible.

References

Dorofeev, V.F., A. A. Filatenko, E.F. Migushova, R.A. Udachin and M.M. Jakubziner. 1979. Wheat. Flora of Cultivated Plants. Vol. 1. Kolos, Leningrad, USSR [in Russian].

Harlan, J.R. and D. Zohary. 1966. Distribution of wild wheats and barley. Science 153:1074-1080.

Kroll, H. 1992. Einkorn from Feudvar, Vojvodina, II. What is the difference between emmer-like two-seeded einkorn and emmer? Rev. Palaebot. Palynology 73:181-185.

Nesbitt, M. and D. Samuel 1996. From staple crop to extinction? The archaeology and history of the hulled wheats. Pages 41-100 in Hulled Wheats. Promoting the Conservation and Use of Underutilized and Neglected Crops. 4. Proc. of the First International Workshop on Hulled Wheats, 21-22 July 1995, Castelvecchio Pascoli, Tuscany, Italy (S. Padulosi, K. Hammer and J. Heller, eds.). IPGRI, Rome, Italy.

Padulosi, S., K. Hammer and J. Heller (eds.). 1996. Hulled Wheats. Promoting the Conservation and Use of Underutilized and Neglected Crops. 4. Proc. of the First International Workshop on Hulled Wheats, 21-22 July 1995, Castelvecchio Pascoli, Tuscany, Italy. IPGRI, Rome, Italy.

Perrino, P. and K. Hammer. 1984. The farro: further information on its cultivation in Italy, utilization and conservation. Genet. Agraria 38:303-311.

Schiemann, E. 1940. Die Getreidefunde der neolithischen Siedlung Trebus, Kr. Lebus, Mark. Ber. Dtsch. Bot. Ges. 58:446-459.

Szabó, A.T. and K. Hammer. 1996. Notes on the taxonomy of farro: Triticum monococcum, T. dicoccon and T. spelta. Pages 2-40 in Hulled Wheats. Promoting the Conservation and Use of Underutilized and Neglected Crops. 4. Proc. of the First International Workshop on Hulled Wheats, 21-22 July 1995, Castelvecchio Pascoli, Tuscany, Italy (S. Padulosi, K. Hammer and J. Heller, eds.). IPGRI, Rome, Italy.

Utz, H.F. 1991. PLABSTAT. Ein Computerprogramm zur statistischen Analyse von pflanzenzüchterischen Experimenten. Institut für Pflanzenzüchtung, Saatgutforschung und Populationsgenetik, Universität Hohenheim, Germany.

van Zeist, W. and J.A.H. Bakker-Heeres. 1982. Archaeobotanical studies in the Levant, 1: Neolithic sites in the Damascus Basin: Aswad, Ghoraifé and Ramad. Paleohistoria 24:165-256.

Zohary, D. 1969. The progenitors of wheat and barley in relation to domestication and agricultural dispersal in the old world. Pages 47-66 in The Domestication and Exploitation of Plants and Animals (P.J. Ucko and G.W. Dimbleby, eds.). Duckworth, London, UK.

Utilization of Ancient Tetraploid Wheat Species for Drought Tolerance in Durum Wheat (Triticum durum Desf.) - A. Al Hakimi and P. Monneveux

Introduction

The variability of drought tolerance related traits is rather limited in durum wheat and hence interspecific crosses would seem to be a good tool to introduce such traits into durum wheat. Interspecific crosses between durum wheat and the other tetraploid species of the Triticum genus represent a promising and more rapid path toward this objective. Arguments for extensive use of wild and cultivated tetraploid wheats in durum wheat breeding have been previously discussed (Grignac 1965; Joppa and Williams 1988; Damania et al. 1988; Al Hakimi et al. 1994).

The use of ancient and obsolete tetraploid Triticum species to incorporate some characteristics lacking in the modern commercial durum varieties has been suggested by several breeders (Damania et al. 1992). However, Clarke and Townley-Smith (1984) pointed out the need for backcrosses to recover the yield potential of the recurrent parent. Moreover, double or three-way crosses may also be useful for transferring desired traits to adopted genotypes.

Genetic variation for drought-resistance attributes has been observed in obsolete tetraploid wheat forms (T. dicoccum, T. carthlicum, T. timopheevi and T. polonicum). These include maintenance of leaf water potential (LWP), proline content (Pro), soluble sugar content (SSC), photochemical quenching of chlorophyll fluorescence (qQ), relative water content (RWC), chlorophyll content (chl), root growth and plant development or morphological attributes such as flag leaf area, leaf orientation, tiller number, yield and yield components (Damania et al. 1992; Al Hakimi and Monneveux 1993). Genetics of morphophysiological traits involved in resistance are less well known and complex interactions between these traits have to be further investigated (Blum 1993).

Yield improvement in durum wheat as in many other crops has been achieved through direct selection for absolute performance. However, critics of this approach have always pointed out that an understanding of the mechanisms (morphological and physiological) that might be correlated with or contribute to drought resistance is essential to the success of breeding varieties for areas where water is limited. These critics have also suggested that more gains in yield can be achieved rapidly and predictably if these morphophysiological mechanisms that contribute to drought resistance are better understood. Mechanisms that enable plants to adapt to moisture stress are multiple and a few of these are considered in this study.

Root architecture in response to water stress has been clearly established in cereals, especially in durum wheat (Benlaribi et al. 1990); the required root architecture can, however, vary in relation with the type of drought stress (Ali Dib and Monneveux 1992). Although selection and breeding for desirable root characteristics associated with drought resistance have been practised in wheat (Hurd 1974) and other cereal species (Taylor and Yamauchi 1991), the genetics of root characters have not been fully investigated. A high variability has been observed in the Triticum genus for several root characters, including the quantity of roots (Mac Key 1979; Robertson et al. 1985), but this variability has not been exploited systematically in wheat improvement.

Relative water content (RWC) is a physiological trait frequently proposed as a criterion in selection for drought resistance (Acevedo 1987; Schonfeld et al. 1988). RWC is influenced by osmotic adjustment (Morgan and Condon 1986) and by water absorption and transpiration (Schonfeld et al. 1988).

Theoretical considerations (Farquhar et al. 1982) as well as experimental data (Farquhar and Richards 1984; Hubick and Farquhar 1989; Acevedo 1993) have shown that variations in water-use efficiency should be associated with differences in the ability of plants to discriminate against 13C compared with 12C during CO2 diffusion and fixation. Genetic variations in carbon isotope discrimination (D) have been observed in bread and durum wheat (Farquhar and Richards 1984; Condon et al. 1990; Ehdaie et al. 1991; Gate et al. 1992; Araus et al. 1993), but information is scarce concerning related species.

The objectives of the present study were to: (1) evaluate the genetic variability and describe the behavior of some morphophysiological traits related to drought resistance in ancient and obsolete tetraploid wheat species, (2) introduce these traits into improved varieties by interspecific crosses, (3) determine the potential of indirect trait selection for improving drought resistance, grain and biological yield under stress, using divergent selection in F2 and F3, (4) determine the genetic control of these traits, broad and narrow sense heritability response to selection and realized heritability, and (5) investigate the possibility of using ancient and obsolete tetraploid species to improve abiotic stress as well as grain quality.

Material and methods

Some accessions of tetraploid species (Table 1), identified for their special characteristics in improvement of drought resistance, were used in a physiology-based crossing program. Crosses obtained were evaluated in F2 population by two methods: (1) indirect selection for various drought resistance related traits (divergent selection), and (2) direct selection for yield by using the 'F2 progeny' method proposed by Smith (1987).

Divergent selection was applied by selecting groups of plants with high and low value of the trait in F2 and F3. Traits measured were: (1) RWC evaluated as the method describe by Barrs (1968), under greenhouse conditions; three crosses were involved in this study; (2) chlorophyll fluorescence (qQ) after Havaux and Lannoye (1985), under greenhouse conditions in two crosses; (3) root attributes were assessed in hydroponic conditions after the method discussed by Al Hakimi (1995), under the greenhouse in one cross, and (4) kernel carbon isotope discrimination (A) as described by Hubick et al. (1986) for evaluating water-use efficiency (WUE). The ratio of carbon isotope (R = 13C/12C) was analyzed by using a multiple-collector mass spectrometer connected to an element-analyzer, which allowed the 'on-line' analysis of 50 samples per day.

Agronomic and morphological characters, such as grain yield (GY), biomass production (BP), plant height (PLH), harvest index (HI), growth vigor (GV), days to heading (DH), spike length (SL), spike shape (SSH), number of spikelets/spike (NSP/S) and number of grains per spike (NG/S), were also assessed. Heritability was calculated in the broad-sense (h2b) according to the formula:

hb = [s2F2 - (s2P1 + s2P2)/2)]/s2F2

and in the narrow sense (h2n) as parent-offspring regression (Smith and Kinman 1965). Heritability (h2r) was calculated according to Falconer (1989) as the ratio of selection in F3 to selection differential in F2, between the two divergent groups mentioned above.

h2r = (HF3 - LF3)/(HF2 - LF2)

where H and L refer to the mean values of the (trait +) and (trait -) in F2 and F3.

Table 1. Crosses used in direct selection for yield and indirect selection for physiological traits.

Crosses

Abbreviation

Traits

T. dicoccum norcicum x T. durum 'Cham 1'

Tdi1 x Ch1

RWC&qQ

T. dicoccum fuchsii x T. durum 'Om Rabi 5'

Tdi2 x OR5

Root

T. dicoccum semicanum x T. durum 'Om Rabi 5'

Tdi3 x OR5


T. polonicum pseudochrysospermum x 'Cham 1'

Tp9 x CM

RWC&qQ

T. polonicum pseudochrysospermum x 'Om Rabi 5'

Tp9 x OR5

D

T. polonicum Hadrache x T. durum 'Cham 1'

Tp11 x Ch1


T. carthlicum stramineum x T. durum 'Cham 1'

Tc12 x Ch1

RWC

T. durum leucomelan x T. polonicum 'Hadrache'

Td21 x Tp11


T. durum leucomelan x T. carthlicum fuligi

Td21 x Tc14


Direct selection for yield.

Results and discussion

Genetic variation for some morphophysiological traits related to drought resistance in ancient and obsolete tetraploid wheat species has been observed:

· T. dicoccum shows some interesting characteristics, including grain yield, biomass production, root growth, plant height, reduction of flag leaf area under water stress, high relative water content (RWC) and leaf water potential (LWP) which were recorded in some accessions under field and greenhouse conditions.

· T. polonicum has a high peduncle, high spike fertility, high 1000-kernel weight, good superficial rooting pattern; low variation for RWC, leaf area, chlorophyll content (chl), chlorophyll fluorescence (qQ), and high sugar accumulation which were registered under water stress; in addition T. polonicum is characterized by good grain quality.

· T. carthlicum is characterized by good tillering ability, good spike fertility, low leaf area; under water stress, high variation was observed for LWP but RWC, chl and qQ maintained low variation, and high level of SDS and protein content.

· T. timopheevi show erect and thin leaves, good root growth, high number of grains per spike, and high protein content.

· Durum wheat (T. durum) and bread wheat (T. aestivum) were characterized by high grain yield and harvest index; they maintain a high LWP, but RWC, chl and qQ are strongly affected by drought.

Genetic studies show that physiological traits (RWC, qQ, A) and root characteristics are under complex genetic control; broad sense heritability was found to be high but narrow sense heritability and response to selection were low in the case of root parameters and qQ (Table 2). Heritability figures obtained (which represent a good indicator of the selection efficiency) showed that RWC, A and root number could be used as selection criteria in plant breeding programs to improve water-use efficiency and to facilitate the development of cultivars adapted to arid environments.

Tetraploid wheats used in this study differed for kernel D (D value was 12% lower in the T. polonicum than in the T. durum accession). These results reflect the importance of using the ancient and obsolete tetraploid wheat with low D (high water-use efficiency) for improving drought resistance in high-yielding durum wheat varieties. Heritability was high for this trait (D) and involved dominance effects. However, narrow-sense heritability was found to be of intermediate value, which suggests that additive effects are involved in the expression of this trait. These results are in agreement with those of Ehdaie and Waines (1994).

Correlations between D and biomass or grain production vary as a function of climate. Low D (greater water-use efficiency) was associated with higher grain production (r = -0.26**) and increased biomass accumulation under water stress (season with low precipitation, 373 mm) which was contrary to the results (positive correlation, r = 0.34***) obtained during the season with high precipitation (632 mm) in F3. These results clearly indicate that climatic conditions and earliness due to high rainfall strongly affect the relationship between D and grain yield.

Evaluation of lines selected in F4 and F5 under field conditions in different locations for different morphological traits, yield and yield components shows that populations derived from interspecific crosses and selected for high RWC and low D had greater drought resistance, since they demonstrated early growth vigor, high harvest index, high grain yield and biomass production compared with those populations selected for low RWC and high D (results not presented here).

Direct selection for yield confirmed that early generation selection in F2 and F3 populations produced some lines which performed better than their parents under both water-stressed and optimal conditions. The evaluation for grain quality of F3 lines shows that high SDS sedimentation value, high protein content and high 1000-kernel weight were incorporated from ancient and obsolete tetraploids into durum wheat lines (Table 3). Results presented in Table 3 indicate that high protein content and SDS sedimentation values were obtained in the crosses between a durum wheat landrace from Iraq and a T. carthlicum accession, and in the crosses between T. dicoccum and the durum wheat variety 'Om Rabi 5'. Protein content was higher in all the tested interspecific progenies than in the durum wheat check ('Om Rabi 5'), and furthermore, some lines reached higher grain yield and 1000-kernel weight than this check.

Table 2. Broad and narrow sense heritability and realized heritability for different morphophysiological traits in durum x ancient/obsolete tetraploid wheat crosses.

Attribute

h2b

h2n

h2r

Relative water content (RWC)

0.73

0.59

0.91

Photochemical quenching (qQ)

0.54

0.12

0.28

Carbon isotope discrimination (D)

0.48

0.37

0.57

Root number

0.63

0.10

0.10

Root number more than 30 cm

0.91

0.49

0.43

Depth (cm)

0.66

0.12

0.23

Root volume (cm3)

0.75

0.10

0.11


Table 3. Number of F3 lines derived from interspecific crosses, evaluated for grain yield (G) and grain quality parameters (protein content, SDS sedimentation test, and 1000-kernel weight (TKW))

Varieties/crosses

No. of F3 lines

Protein content (%)

Sediment test (SDS)

Grain yield (g)

TKW (g/m2)

T. durum (Cham 1)

6

11.8 ± 0.3

21.0 ± 2.0

548 ± 82

41 ± 0.3

T. dicoccum 1

2

12.0 ± 0.4

19.0 ± 0.3

704 ± 102

41 ± 0.7


(Tdi1xCh1) F3

22

11.1 ± 0.7

19.2 ± 2.3

662 ± 150

46 ± 4.0

T. polonicum 9

2

13.1 ± 0.3

24.0 ± 0.1

481 ± 108

52 ± 0.9


(Tp9xCh1) F3

20

11.8 ± 0.8

24.1 ± 4.1

525 ± 151

47 ± 4.5

T. carthlicum 12

2

12.4 ± 1.0

40.0 ± 5.0

547 ± 91

31 ± 0.3


(Tc12xCh1) F3

11

12.8 ± 2.4

28.4 ± 6.3

328 ± 218

36 ± 3.9

T. polonicum 11

4

10.5 ± 0.5

24.3 ± 1.2

330 ± 102

42 ± 0.6


 

(Tp11xCh1) F3

13

9.6 ± 0.8

20.4 ± 3.0

422 ± 93

41 ± 4.3

(Td 21xTp11) F3

28

11.3 ± 1.4

32.0 ± 6.5

428 ± 112

41 ± 3.5

T. durum (Td21)

3

11.2 ± 1.2

36.0 ± 3.0

458 ± 69

48 ± 2.1


(Td21xTC14) F3

14

11.9 ± 1.2

31.6 ± 4.7

400 ± 88

37 ± 3.4

T. carthlicum 14

2

13.0 ± 1.1

29.0 ± 3.0

312 ± 98

30 ± 1.0

T. durum ('Om Rabi 5')

2

12.4 ± 0.3

25.5 ± 0.5

596 ± 126

44 ± 3.8


(Tdi3xOR5) F3

22

12.4 ± 1.1

24.6 ± 4.4

547 ± 87

43 ± 3.1

T. dicoccum 3

2

12.3 ± 0.2

25.0 ± 5.0

468 ± 44

34 ± 2.1


Conclusions

Evaluation of ancient and obsolete tetraploid wheats for drought resistance related traits indicates that many accessions are potentially useful for the improvement of drought resistance in durum wheat varieties. The ability to maintain the relative water content (RWC) under water stress, a criterion closely related with osmotic adjustment, and carbon isotope discrimination (D) appear to be of a polygenic nature and highly heritable. Moreover, positive transgressive effects have been observed in some interspecific crosses. All these results, as well as the possibility to introduce grain quality characteristics such as protein content into durum wheat through interspecific crosses, encourage the use of ancient and obsolete tetraploid wheats in durum wheat improvement.

Heritability and selection response of morphophysiological traits related to drought resistance were evaluated and the impact of the divergent selections on morphological and agronomical characters was studied under field conditions. Results indicate that selecting for the morphophysiological traits related to moisture stress, such as relative water content and carbon isotope discrimination, is possible, since they showed high heritability values. However, the effect of selection for these traits on yield stability has to be further investigated. Improvement of yield is possible, by both direct and indirect selection.

Populations selected from these studies were evaluated in preliminary yield trials in Yemen. Preliminary results indicated that the use of related species combined with a bulk modified breeding method is promising not only for increasing yields in durum wheat in drought-prone environments but also to improve durum wheat yield stability across a wide range of climatic conditions. However, further studies are needed in this regard to study the long-term behavior of progenies of these crosses.

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Archaeobotanical Evidence for Evolution of Cultivated Wheat and Barley in Armenia - P.A. Gandilian

Introduction

Geographically the territory of the Republic of Armenia is part of a larger region conventionally called the Armenian Upland (Abich 1882). In the past Armenia was the place of origin of the ancient Armenian nation (Urartu) that encompassed the whole upland. The Armenian Upland acts as a kind of wedge between the Iranian and the minor-Asiatic mountainous structure which is above these highlands by almost 500 m. That is why the Upland is sometimes also called the Mountain Island. Its area is more than 300,000 km2 and the mean altitude is 1700 m asl. The territory of the Republic of Armenia is 29 740 km2 (nearly 10% of the former historic Armenia realm). However, almost all of the Upland's natural environments, relief and climatic features as well as plant and animal diversity are replicated within the country.

The Armenian Upland is an integral part of the West Asia region which is widely known as one of the primary centers of human culture. Humans have been living here since the early Stone (lower Palaeolithic) Age. This land attracted people not only for its favorable environment (such as climate, suitability for building settlements, etc.), but also for its character and specific features of vegetation. It is known that primitive communities established themselves predominantly in areas rich in biodiversity of vegetation, including a wide range of edible plants. A large part of the Armenian Upland includes the Armeno-Iranian province which is characterized by rich and unique flora with high generic and specific endemism (Takhtajan 1978).

In the scriptures and ancient books Armenia with its regions is called a “fertile country” and “warehouse of the wheat and barley”, among other names. There is a mention in manuscripts of wheat and barley being taken from Armenia by Assyrian kings after their conquests during the 9th to 8th centuries BC. For instance, Assyrian King Sargon II (722-705 BC) conquered many countries and forced their people to pay taxes with domesticated animals. However, when Sargon II overran Urartu (ancient Armenia) he noticed large warehouses full of wheat and barley. “I opened the doors of these warehouses and fed my soldiers with a lot of grains,” said he (Thureau-Dangin 1912).

The archaeobotanical materials from Armenia are of great importance for the study of the origins of crops and domestication of plants in the Near East. There is strong evidence to suggest that wheat (Triticum L.) and barley (Hordeum L.) have been cultivated in Armenia from the earliest times. Charred remains of spikelets and grains of wheat and barley have been found in Armenia side by side with other archaeological finds related to human culture of that time.

Many scientists accept that the place of wild plant domestication and the transformation to cultivated forms must lie within the area of the plant's natural habitat. Helbaek (1959) wrote that the ancient crops of the Old World were wheat and barley. Wild progenitors of wheat, barley and rye are found in Armenia even today. According to the archaeologist Sardaryan (1967) wheat was found in Neolithic strata of settlements (6th millennium BC) and barley grains were found in clay pots, and also in mortars and grain-graters. During the Neolithic period the people of Armenia not only harvested wheat with the use of sickles but also pulled out entire plants or spikes by hand. According to archaeological data, wheat-barley mixtures are often found in Armenia. Even in the supposedly 'pure' barley material, wheat grains were found. Whether or not they were contaminants or were put there on purpose is not certain.

Tumanyan's research of archaeological materials at the beginning of the Bronze Age did not include separate remains of wheat and barley. But in material from the Urartu period (about 1000 years BC) their differentiation process was completed and pure plantings of both these crops appeared. However, it is necessary to stress that 'appeared' does not mean that wheat and barley were planted separately at all places. Tumanyan (1944) emphasized that grain material from only some regions of Karmir Blur (Araratian lowland) is characterized by purity and absence of weed seeds.

The residences of ancient Armenian (Urartu) kings were centers where substantial stocks of farm produce were concentrated or stored. The grains were cleaned before storage. In addition, 'pure' wheat and barley were planted on special fields of the kingdom. There is more information in the literature about these 'pure' plantings. For example, the chief scribe of Assyrian King Sargon II, Nabu-Schalim-Shunun, mentions that during their trip (713 BC) to the region of Hayots Dzor they saw a wheat crop of the best quality (about 300 ha). About this observation Adontz (1946) believed that it was a special field of the kingdom, signifying that particular care had been taken to keep the field free of contaminants.

Even today some farmers plant spring wheat and barley together (called 'kiardika') in mountainous zones of Armenia in order to guarantee food supply. If wheat suffered from unfavorable conditions (e.g. a disease or a drought) barley would give at least some yield. However, barley was also planted exclusively for making beer.

Most barley of the Bronze Age consists of round-grained awnless multi-row forms. At the beginning of the Urartu period they became rare as they made way for new forms with elliptical grains. Two-row forms began to prevail over multi-row ones during the process of subsequent climatic changes (Tumanyan 1948).

During the Greek military campaigns in West Asia (401-400 BC) 10,000 troops passed through the rich fertile regions of the Armenian Upland. It was noted that wheat, barley, vegetables and barley beer from Armenian settlements were made available to the soldiers (Ksenofont 1951). Ksenofont's information is confirmed by archaeological evidence of brewing beer in Armenia. A brewery and beer shop from the Urartu period were discovered during excavations at Karmir Blur (Minasyan 1961).

Through morphological investigations of plant remains from excavations of ancient Vagarshapat (beginning of the 1st millennium BC) Tamamshyan (1935) came to the conclusion that the material belonged to emmer wheat (Triticum dicoccum). Tumanyan (1944) found that in the Urartu period some varieties (cultivars) of naked wheat were grown in Armenia. He identified them as T. aestivum and T. spelta in archaeological materials excavated from that period.

Gulkanyan (1966) investigated materials from the excavation of the town of Argishtikhinly from the Urartu period. He divided wheat grains into two groups: round-grained and oblong-grained. Round-grained wheat was identified as T. sphaerococcum and oblong-grained as T. aestivum and/or T. persicum. He referred to some forms as T. compactum and others as T. dicoccum or T. araraticum.

Some general results of several investigations are given below. Characteristics of material studied are given as they were investigated (i.e. not dependent on the age of the excavation).

1. Carbonized remains of partially threshed plant mixtures from graveyards that belong to 2nd millennium BC, e.g. Gugavan village (S. Chilingaryan's excavation): The grains and seeds of the following plants were identified. (a) wheat kernels from round to elongated forms (similar to T. aestivum, T. compactum, T. sphaerococcum); (b) a spikelet of hulled two-grained wheat (probably T. dicoccum). There were two typical emmer grains in the spikelet ventrally oriented toward each other (as in emmer wheat); (c) kernels of multi-row barley (they are dominant over other barley types); (d) grains of different other species (Agropyron, Lolium, Bromus, etc.); (e) fruits and seeds of various weeds (Caucalis daucoides, Polygonum convolvulus, Galium, etc.), which can still be found growing in the same area; (f) carbonized remains of vegetative organs of other plants.

The materials found from the graves of Gugavan village are very interesting because they provide an opportunity to appreciate the planting conditions in economic and botanical terms. Threshed and semi-threshed grain material, apparently of barley, had been put in graves without chaff (straw).

2. During an excavation near the ancient fortress (Bronze Age) located on the hill to the southwest of Gegharkunik village, an ancient earthenware pot was accidently broken, disgorging cereal kernels. There were many barley kernels with small peduncles and some wheat grains too, particularly of short or round forms.

3. The archaeological material of ancient settlements of the town-fortress Argishtikhinly (Urartu period) was excavated by the archaeologist Martirosyan. Of 1160 analyzed grains only 13 were wheat kernels (about 1.3%), the rest were of barley. Wheat grains were small: length 4.5-5 mm; breadth 3-4 mm; width 2-4 mm (similar to T. sphaerococcum or T. compactum).

Barley grains found were 61% naked and 39% hulled. Also, there were bottle-shaped kernels. Spike bar segments were of different sizes (length 3.0-8.0 mm; breadth 1.0-2.0 mm;). Consequently, there were loose-ear and dense-ear forms. Some ear segments, usually short and narrow, had hair (there was comparatively dense hair around the edge of the ear bar). This could be identified as wild barley. Kernels and seeds of weeds also were observed.

4. The archaeological material from Lorut village of the Alaverdy region from the 3rd millennium BC (V. Avetisyan's excavations): Here only wheat grains were found. They were comparatively large but there were very small kernels also. Wheat grains were very similar to T. compactum, T. sphaerococcum, T. aestivum and some may belong to T. spelta since they had glumes. There were also germinated kernels. The presence of germinated kernels suggests that the material was conserved under moist conditions or was already wet before storage.

5. Archaeological material from Oshakan village of the Ashtarak region (4th and 6th centuries AD): Wheat grains were very similar to T. aestivum or T. compactum. Of 147 kernels, 120 were wheat, 9 rye and 18 sorghum.

6. The archaeological material from the ancient settlement of Metsamor in the Ararat Valley (excavations carried out by E. Khanzadian): These belong to the period of early Iron Age (end of 12th to 9th century BC). A mass of carbonized kernels of millet (Panicum miliaceum L.) was found, among which there were some emmer and barley kernels and wheat spikelets (T. dicoccum or T. persicum).

7. The archaeological material from Aigevan village of the Ararat region (the excavation of B.B. Piatrovskyi) is most interesting since it is represented by four samples from different layers (depths): (a) 12.5 m, (b) 10.5-11.0 m, (c) 8.7 m and (d) 8.0 m. Archaeologists estimated the sample ages as follows: (a) and (b) belong to the second half of the 3rd millennium BC, (c) to the end of the 3rd millennium BC, and (d) to the first half of the 2nd millennium BC. Barley prevailed over other crops in samples from all four layers. Wheat grains were present, once again, as mixtures (1-20%). Study of the archaeological materials suggests that during 1000 years an increase in grain size of both wheat and barley grains had taken place. Most of the more ancient grains have round forms. Naked barley prevailed in the samples of more ancient origin whereas hulled ones were more frequent in later periods.

The samples of the second half of the 3rd millennium BC are also interesting. Wheat kernels were naked but hulled ears were found too. There were two-grained spikelets. Kernels in the spikelets of adjacent flowers adjoined closely. Some kernels had an accrete upper tip of the second glume on the side of topknot making a 'heel' (see Pasternak, this volume).

Multi-row ears of barley were dominant in all samples, but there were also some two-row forms. The assumption that awned and two-row forms of barley were not found in the material from the Bronze Age is not confirmed. There were bottle-shaped barley kernels in all samples. Probably, this form had a 'horseshoe' at the base of the peduncle. There was also a specific dense-ear form with small grains which has since become extinct.

Some considerations on domestication of wheat and barley

About bottle-shaped barley

Such kernels have been observed in all samples of carbonized archaeological barley material from Armenia. Tumanyan identified two ancient species of barleys: round-grained Hordeum antiquorum sphaerococcum Thum. and the oblong elliptical kernels of H. urartu Thum. Judging from the picture he published (Tumanyan 1948) there are kernels with peduncles among both species. Peduncles are not formed under all spikelets, but only under the lateral ones. Therefore, it became obvious that H. antiquorum sphaerococcum and more so H. urartu correspond to H. lagunculiformae Bacht. (Bakhteyev 1956,1965). Tumanyan (1948), during his identification, did not consider peduncles. Regarding the kernel forms of H. lagunculiformae, Bachteyev (1956) emphasizes that they can be round-ellipsoid, ellipsoid and elongated-ellipsoid. The fossil samples sometimes have hair and a 'horseshoe' on the ear axis. It is a feature of wild H. lagunculiformae. The bottle-shaped barley can still be found in Armenia but only as a wild plant (Bakhteyev 1962). Ears of this barley were collected by this author in Turkmenistan in 1990.

About fossil hulless round-grained forms of wheat

In the opinion of specialists on phylogeny of polyploid wheat species, wheat evolved in the following way. Tetraploid wheat of T. dicoccoides-type arose as a result of a spontaneous cross between a wild one-grained wheat (with genomic formula AA) and a species of the section Sitopsis of the genus Aegilops L. (with the genome SS or BB). The round-grained wheat could not be among the tetraploid wheats since the parental forms are narrow-elongated. Tetraploid wheat (2n = 28) was crossed with another diploid species, Ae. tauschii, with the genomic formula DD to give rise to bread wheat (T. aestivum) with the genomic formula AABBDD.

Zohary (1969) observed that it was not until its cultivation reached Armenia, Transcaucasia and the Southern Caspian that plants of cultivated tetraploid wheat came into contact with the diploid donor of the D genome. Keeping in mind that this process of wheat hexaploidization took place a long time ago, scientists referred to short round-grained archaeological materials as bread wheat and, consequently, they determined that it is a hexaploid. For example, Heer (1878) referred to wheat samples of the Swiss Neolithic as Triticum vulgare antiquorum Heer. Flaksberger (1930) suggested that this variety be transferred to T. compactum Host, and named it T. compactum antiquorum. Kislev (1980) considered that short round-grained samples of the fossil wheat of the countries of the Near East are not hexaploids but tetraploids with a genomic formula AABB and he gave them a specific name - Triticum parvicoccum Kislev. In his scheme of classification the cultivated emmer T. dicoccum arose from wild tetraploid wheat T. dicoccoides (AABB) as a result of mutations and introgressive hybridization in the Mediterranean climate. Triticum parvicoccum with the compact ear and spherical grains arose from this species.

Conclusions

1. It is important to search for more remains of wheat and barley kernels grown in Armenia during the Stone Age.

2. The wheat-barley crop mixtures existed in Armenia in the past, though the relatively 'pure' plantings of wheat and barley were also present.

3. Both wheat and barley forms with small and round grains were found in more ancient samples. Subsequently, the oblong forms appeared and gradually dominated over other forms. This process was a result of climate changes on one hand and genetic changes on the other hand.

4. The gradual xeric changes in the climate led to the disappearance of round-grained wheat and round-grained as well as hulless barley from Armenia. The multi-row forms are grown now exclusively under the more moist conditions.

5. Bakhteyev named the 'bottle-like' cultivated barley (with a peduncle on the lateral spikelets) as Hordeum lagunculiformae Bakht. Tumanyan found such forms among the archaeobotanical materials in Armenia and named the round-grained forms H. antiquorum sphaerococcum Thum. and the forms with oblong, elliptical kernels, H. urartu Thum. (Tumanyan 1948). These forms now do not exist except in the wild.

6. The theory that barley of the Bronze Age was awnless should be further discussed. Ears with well-developed awns have been found.

7. The kernels and spikelets of hulless wheat similar to T. dicoccum (tetraploid) and T. spelta (hexaploid) have been found but their grains appear to be round.

8. The multi-embryonic barley and wheat with narrow ears similar to T. persicum have been found from the second half of the 3rd millennium BC.

9. It is necessary to preserve and document archaeobotanical material in special genebanks for further study on crop evolution.

References

Abich, H. 1882. Geologie des Armenischen Hochlandes. I Westhalfte, Vienna, Austria [in German].

Adontz, N. 1946. Histoire D'Armenie. Paris, France [in French].

Bakhteyev, F.Kh. 1956. The fossil form of cultured barley Hordeum lagunculiformae. DAN SSSR 110(1):153-155 [in Russian].

Bakhteyev, F.Kh. 1962. The new link in the wild species of barley. Bot. Zhurnal 47:844-847 [in Russian].

Bakhteyev, F.Kh. 1965. Hordeum lagunculiformae s. str from Swiss Neolithic sediments. Bot. Zhurnal 50:541-543 [in Russian].

Flaksberger, K.H. 1930. Triticum antiquorum (Heer). Jzv. Gl. Bot. Sada SSSR:72-88 [in Russian with German summary].

Gulkanyan, V.O. 1966. About some agricultural cultures from Argishtikhinils. Jforico Filologitcheskyi J. 4:103-116 [in Russian].

Heer, O. 1878. Die pflanzen der Pfanlbauten. 2nd edition. Pages 518-536 in Switzerland and Other Parts of Europe (J.E. Lee, transl). Neujbl. Naturf. Ges. Zurich, Switzerland [in German].

Helbaek, H. 1959. Domestication of food plants in the Old World. Science 130:365-372.

Kislev, M.E. 1980. Triticum parvicoccum. Sp. nov., the oldest naked wheat. Isr. J. Bot. 28:95-107.

Ksenofont. 1951. In Anabasis (H.G. Dakyns, transl.). MacMillan, London, UK.

Minasyan, A.K. 1961. Barley of Armenia. Yerevan, Armenia, USSR [in Russian].

Sardaryan, S.H. 1967. Primitive Society in Armenia. Yerevan, Armenia, USSR [in Armenian with English summary].

Takhtajan, A.L. 1978. The Florist Regions of Earth. Nauka, Leningrad, USSR [in Russian].

Tamamshyan, S. 1935. The Fossil Wheat from Arm SSR in the Excavations of Ancient Vagarshapat (A. Kalantar, transl.). Yerevan, Armenia, USSR [in Russian].

Thureau-Dangin. 1912. Une relation de la huitième campayne de Sargon. Paris, France [in French].

Tumanyan, M.G. 1944. Crop plants of Urartu period in Armenian SSR. Izv. Akad. Nauk. Armenia SSR, obshestv. nauki 1-2:73-82 [In Russian].

Tumanyan, M.G. 1948. The principal stages of barley in Armenia. Izv. Akad. Nauk. Armenia SSR 1-1:73-85 [in Russian].

Zohary, D. 1969. The progenitors of wheat and barley in relation to domestication and agricultural dispersal in the Old World. Pages 47-66 in The Domestication and Exploitation of Plants and Animals (P.J. Ucko and G.W. Dimbleby, eds.). Duckworth, London, UK.

Extinction Threat of Wild African Gossypium species in their Center of Diversity - V. Holubec

Introduction

Cotton is one of the most important commercial crops in the world. Out of the four cultivated species - G. arboreum, G. herbaceum, G. barbadense and G. hirsutum - only the last-mentioned represents the staple crop at present. Wild ancestors and related species found in the Americas and Australia were extensively studied but little information was available on species from Africa and Arabia. Africa has the highest diversity of cotton germplasm. Native species of at least four genomic groups (A, B, E and F) are spread throughout the whole continent including the Arabian Peninsula. Their distribution was poorly documented and so the threat to their habitat was not envisaged. The objective of the present study was to: (1) map the distribution of wild species and naturalized (escaped from cultivation) landraces, and (2) assess the need for collecting and conserving them so that priorities could be established.

Material and methods

The major herbaria possessing Gossypium plant material, established by the former colonial powers (England, France, Belgium and Portugal) were visited and all available data were recorded in a computerized database. The identification of species was revised. The collecting sites were ascertained and their geographical coordinates were provided. The species distribution was plotted on the maps in the Miller's oblate stereographic projection in the scale 1:50 million (Miller 1953). The Vegetation Map of Africa was used to match the species distribution with various mapping units (White 1983). The borders of the mapping phytochoria were digitized and drawn into the distribution maps. Sprintsky and Snyder (1986) provided the algorithm applicable for computer graphics. The program and maps were generated at the Computing Center of Texas A & M University.

Results and discussion

Taxonomy of African and Arabian cotton

The taxonomy of African and Arabian cotton species with a special emphasis on wild species was reviewed. Two genera, Gossypium and Gossypioides, are included here. The genus Gossypioides used to be confused with Gossypium herbaceum because of some morphological similarities, but its chromosome number is different. The taxonomic treatment of the genus Gossypium as proposed by Fryxell (1979) was used with the following changes: Gossypium herbaceum var. africanum was accepted at the subspecies level as proposed by Mauer (1954). A subdivision of G. somalense into species G. somalense, G. benadirense and G. bricchettii was adopted as suggested by Vollesen (1987). Gossypium trifurcatum described by Vollesen (1987) was included in the subgenus Gossypium.

Simple keys for distinguishing the genera Gossypium and Gossypioides and their species of Africa and Arabia were designed and are available from the author.

Distribution of Gossypium

Distribution maps of species of Gossypium are available from the author. The maps for wild species are much more exhaustive and more accurate than that for cultigens. Generally it is possible to conclude that wild species of Gossypium are restricted in their distribution to the tropical belt between latitudes 20°N and 25°S. They are confined to semi-desert regions so that their distribution is disjunct with a hiatus in the Zaire and North Zambezi basins. Gossypium longicalyx and partly G. herbaceum subsp. africanum tend to occupy slightly more mesophytic habitats. Cultivated species that grow wild and primitive landraces are distributed throughout the tropical belt except for the high mountains. They predominantly occupy more mesophytic and humid habitats.

Priority regions for collecting of cotton germplasm

Africa and southern Arabia should be the places of first priority for collecting of germplasm of cotton. They have the greatest diversity of cotton germplasm, because Africa is believed to be a center of origin of the genus Gossypium. However, this region has been explored the least.

It is desirable to outline the collecting activities throughout the region of distribution to get as much genetic variability of a particular taxon as possible. Exploration should start in places of former successful collection of herbarium specimens to confirm the present occurrence. As the collector becomes familiar with the habitat, further investigation of suitable habitats will be easier. Then some expedition time should be devoted to examining the regions where the occurrence of the taxon could be expected but the taxon has not been collected yet. It is important to examine the limits of the distribution since a higher genetic variability may be expected there.

It is difficult to set up the regional priorities in Africa since exploration is desirable nearly throughout the continent. Particular attention must be drawn to the wild species. They are endangered by subsequent desertification on their 'arid limits' and by human influences (agriculture, grazing) on their 'humid limits' of distribution. This is important especially in the Sahel, but also in the Kalahari and the Namib Deserts. A similar situation may exist on the border of the coastal desert on the Somali peninsula, but data are not available.

On the basis of data from herbaria and from floristic and collector's literature, suggestions for future collecting activity of cotton germplasm in Africa, Arabia and adjacent islands were made. Special emphasis was given to the wild species. Cultivated species and species escaped from cultivation were treated separately, because their distribution is not connected with a particular phytogeographic region.

From these suggested regions for collecting, places of the first priority importance were chosen. The choice was made to include populations endangered in their habitats and to include species with very limited distribution (Box 1).

The second list of locations (Box 2) was outlined for further collecting activity. It covers all other regions of distribution of the wild species. Suggested localities were chosen to be confined to the areas of the most frequent occurrence of the wild species, places with higher concentration of the different species including cultivated, and places that are more easily accessed.

Box 1. First priority places (regions where taxon is endangered or very limited in distribution).

1. Niger, Air Mountains, and generally northern Sahel

species: G. anomalum, G. somalense; endangered by desertification, only a few collections known; additional species: G. hirsutum.
2. Yemen, Aden region and SE coast
species: G. areysianum, G. incanum; very limited in distribution, may be endangered; probably also G. barbadense
3. Somalia, central part
species: G. benadirense, G. bricchettii, G. somalense, G. stocksii, G. incanum, G. trifurcatum; species not in cultivation and/or with limited distribution
4. Sudan, Meshra el Zerav
species: G. longicalyx; only one specimen last reported in 1910; additional species: G. arboreum, G. hirsutum
5. Somalia, Ethiopia, Kenya, frontier region, Upper Giuba
species: G. benadirense (not in cultivation), G. somalense
6. Cape Verde Islands, Sao Antao, Sao Tiago
species: G. capitis-viridis, distribution very limited, probably endangered, very rare; additional
species: G. hirsutum and probably G. barbadense


Box 2. Places with higher concentration of cotton germplasm.

1. Mali, inner Niger delta east to Gao

species: G. anomalum, G. herbaceum, G. arboreum, G. hirsutum, G. barbadense
2. Chad, east of Lake Chad to Lake Fittri and Ennedi Plateau
species: G. anomalum, G. somalense, G. herbaceum, G. arboreum, G. hirsutum
3. Mozambique: Maputo and South Africa: Transvaal
species: G. herbaceum subsp. africanum, G. herbaceum (cultivated), G. hirsutum
4. Angola/Namibia border, along the River Cunene, and Namibia: Windhoek Mountains
species: G. anomalum, G. triphyllum, G. herbaceum subsp. africanum and probably G. barbadense
5. Madagascar
species: G. arboreum, G. hirsutum, G. barbadense
6. Tropical West Africa: mainly Sierra Leone, Ghana, Benin, Nigeria
species: G. hirsutum, G. barbadense, G. herbaceum, G. arboreum
7. Zaire: lower River Zaire and Cabinda and Luanda of Angola
species: G. barbadense, G. hirsutum and the northern limit of G. anomalum

Conclusions

· Species distribution was corrected by plotting of the exact data

· Distribution of most species matches with certain phytochoria/mapping units of the Vegetation Map of Africa

· Wild species of Gossypium are confined to the tropical belt between latitudes 22°N and 25°S with hiatuses in the Zaire and North Zambezi basins

· The northern distribution of G. anomalum and G. somalense follows mostly the Sahel transition zone

· The southern distribution of G. anomalum follows the Karoo-Namib regional center of endemism and transitions to Kalahari and Zambezian regions north of 25° latitude.

· G. herbaceum subsp. africanum follows open Colophospermum mopane communities and adjacent undifferentiated woodlands in the valleys of the Limpopo and Cunene Rivers

· The southern distribution of the G. somalense complex is restricted to Somalia-Masai Acacia-Commiphora deciduous bushlands

· The African distribution of G. stocksii follows the Somalia-Masai semi-desert grassland

· The distribution of G. longicalyx is restricted to a small area in central Tanzania

· G. incanum occupies the coastal strip of S. Yemen; one specimen was found in the Nogal Valley, Somalia. This coincidence suggests the possibility of predicting their further distribution

· The distribution of wild species is restricted mostly to arid habitats which are generally endangered by desertification and overgrazing

· The distribution of restricted species is known from very few or only one locality

· G. incanum and G. longicalyx in germplasm collections are restricted to one sample only; other samples, if present, are duplications

· G. benadirense, G. bricchettii and G. trifurcatum are not available in germplasm collections

· There is a serious danger of extinction of all restricted species

· Collecting of all African and Arabian species should be accorded a high priority for ex situ conservation

· In situ conservation, at least of species which have a restricted habitat, should be considered on a priority basis

· A vocabulary of vernacular cotton names should make collecting and conservation more efficient.

Acknowledgments

This project was sponsored by the International Board for Plant Genetic Resources (IBPGR), Rome, Italy. The author would like to thank J.T. Williams, R.J. Kohel, F.A. Fryxell and A.E. Percival for support and guidance during the study.

References

Fryxell, P.A. 1979. The natural history of the cotton tribe. TAMU Press, College Station, Texas, USA.

Mauer, EM. 1954. Proiskhozdenie i sistematika khlopcatnika. Tashkent, USSR [in Russian].

Miller, O.M. 1953. A new conformal projection for Europe and Asia. Geogr. Rev. 43:405-409.

Sprintsky, W.H. and J.P. Snyder 1986. The Miller oblated stereographic projection for Africa, Europe, Asia and Australasia. Am. Cartographer 13(3):253-261.

Vollesen, K. 1987. The native species of Gossypium in Africa, Arabia and Pakistan. Kew. Bull. 42(2):337-349.

White, F. 1983. The Vegetation Map of Africa. Natural Resources Research 20. UNESCO, Paris, France.


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