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Wild fruit trees genetic resources conservation strategy
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Wild fruit trees (Prunus avium,
Malus sylvestris and Pyrus pyraster) genetic resources
conservation strategy
Jochen Kleinschmit1,
Richard Stephan2 and Iris Wagner1
1 Niedersächsische
Forstliche Versuchsanstalt, Abt. Forstpflanzenzüchtung, 34355
Staufenberg - Escherode, Germany
2 Bundesforschungsanstalt
für Forst- und Holzwirtschaft, Institut für Forstgenetik, 22927
Großhansdorf, Germany
Distribution
Wild cherry (Prunus avium [L.]
L.), wild apple (Malus sylvestris [L.] Mill.), and wild pear (Pyrus
pyraster [L.] Burgsd.) belong to the plant family Rosaceae. These
insect-pollinated species are autochthonous in nearly all European
countries, but with a very scattered occurrence. They are generally
rare species in mixed hardwood forests. Prunus avium still
exists in natural populations of limited size, e.g. in France, Italy
and Germany, although most of the trees occur as single individuals
or in small groups. Owing to their weak competitive ability, M.
sylvestris and P. pyraster exist mostly at the edge
of forests, in hedges on farmland or on very extreme sites where the
stronger competitors do not survive either. Even there they occur
only as single individuals or very few trees in small groups.
All three wild fruit tree species
are native to central, western and southern Europe. Pyrus
pyraster does not occur naturally only in the north European
countries. Malus sylvestris and P. avium were
introduced into new areas where they can be grown. Nevertheless, the
knowledge about autochthonous origin is often insufficient.
Therefore, it is necessary to prepare a survey of the natural range
in Europe and to improve the information about the occurrence of
autochthonous stands, groups or individual trees of the pure species
not contaminated by domesticated cultivars or originating from
those. This should be done in close cooperation between research
institutions. The results of such surveys can be shown in maps as,
for example, the occurrence of M. sylvestris (Fig. 1) or of P. pyraster
(Fig. 2) in northern Germany. Each occurrence was registered by its
geographical data and additional information was assessed.
In comparison with other tree
species, one can also assume that these tree species migrated into
southern refugia during the different glacial periods, e.g. into
areas south of the Alps. There are also several closely related
species, e.g. Pyrus nivalis in southeastern Europe or Pyrus
amygdaliformis, which occurs only in southern Europe (e.g. Malta
and Slovenia). Malus sylvestris has a great intraspecific
variability where several varieties can be differentiated. The
intraspecific variability should be maintained by suitable
conservation methods.
Forestry
The three minor Noble Hardwood
species are important from an ecological as well as from an
economical point of view. They are growing in mixed hardwood
forests, often at the margins of forest stands due to competition
for light. In mixed hardwood stands with beech as a major component
thinning usually favours the minor Noble Hardwood species in order
to prevent heavy competition and to reach sufficient size for an
economically interesting utilization.
Prunus avium has been planted
extensively in the past in many European countries. Pyrus
pyraster and M. sylvestris only survived occasionally on
very wet or very dry sites due to the competition with beech.
Plantation of these species was very rare and only inreased in
recent years slightly due to more public concern about their
endangered status. However, seed from suitable sources is mostly
missing due to the lack of sufficiently large breeding populations.
Both Prunus avium and Pyrus
pyraster can reach considerable size and diameter of high
quality stems. Their wood is highly valued on the market.
These two species are also very interesting for afforestation of
marginal farmland for the production of valuable timber in mean
(50-70 years) rotation time.
Fig. 1. Conservation of Malus
sylvestris genetic resources in Lower Saxony and
Schleswig-Holstein.
Genetic knowledge
The variability and genetic
structure of P. avium, M. sylvestris and P. pyraster
is not yet known in detail and should be investigated intensively.
All three species show great phenotypical variation. One can assume
that various ecological types can be distinguished due to the large
natural distribution area, and the fact that populations or
individual trees of the same species are growing under different
environmental conditions, e.g. on calcareous or siliceous soils.
Fig. 2. Conservation of Pyrus
pyraster genetic resources in Lower Saxony and
Schleswig-Holstein.
Rangewide provenance experiments
on P. avium do not yet exist. Only regional
provenance/progeny and clonal tests have been performed. They show
considerable variation in morphological and phenological traits
(Weiser 1996; Meier-Dinkel et al. 1997; Santi, pers. comm.;
Kleinschmit, unpublished). Santi and Lemoine (1990a) studied the
inheritance and linkage of isoenzyme loci and described variation in
isoenzyme patterns. They use these patterns to discriminate between Prunus
cerasus and Prunus cerasus x P. avium
(Santi and Lemoine 1990b).
Studies conducted at seven
isoenzyme systems in 14 Italian P. avium populations showed a
higher intrapopulation variability in the northern areas of the
natural range, where the distance between populations is low. The
interpopulation variability was higher in the Tuscany region and in
the central southern Apennines, where the distance between
populations is large. The lowest degree of poly-morphism was found
in isolated populations (Ducci and Proietti 1997).
Treutter and Feucht (1985) showed
differences in phenolic composition of P. avium clones.
For fruit juice colour and
albinism, Watkin and Brown (1956) found simply inherited
dominant-recessive pairs of alleles. The gametophytic
incompatibility 5 locus is polymorphic with at least 6 alleles
(Berger 1963).
Many of the data concerning
genetic traits in fruit trees are from cultivars. It is supposed
that there is no difference between the technique to separate
isoenzymes from cultivated or wild fruit trees (Chevreau, pers.
comm.).
Many analyses have been carried
out on apple, mainly in domesticated varieties. Malus is a
genus of the northern temperate zone with 25-35 species that are
difficult to circumscribe due to lack of distinguishing traits.
Reasons for taxonomic confusions may be:
In the apple industry, a reliable
verification of the cultivar being grown is also of great
importance, particularly for nurserymen and growers. Isoenzyme
analysis offers a possible method for cultivar identification which
has been successfully applied to several other crops. Previous
isoenzyme studies in Malus focused on:
-
Identifying apple cultivars
and rootstocks: Weeden and Lamb (1985) characterized 54 apple (Malus
domestica Borkh.) cultivars and found out that intercultivar
polymorphism at 6 isoenzyme systems was sufficient to permit
reliable and unambiguous identification of nearly every cultivar
-
Identifying genetic markers
linked with horticultural traits: no correlation could be
established between the inheritance of an isoenzyme and a
resistant (apple scab, cedar-apple rust, fire blight) phenotype
of the populations investigated (Weeden and Lamb 1987)
-
Isoenzyme inheritance
(Chevreau and Laurens 1987; Manganaris and Alston 1988)
-
Extraction techniques (Korban
and Bournival 1987)
-
Grouping Malus x
domestica accessions by using isoenzyme markers
-
Using phenotypes to identify
closely related forms.
Isoenzyme analyses at the species
level in Malus are limited. Recent investigations deal with
the isoenzyme diversity in North American Malus species
(Dickson et al. 1991) and in wild populations of Malus
sieversii (Ledeb.) M.J. Roem. (Lamboy et al.
1996).
Malus sieversii, native to
Central Asia, is thought to be the primary progenitor of the
cultivated apple M. domestica Borkh. and therefore important
in apple breeding, genetics, evolution and germplasm conservation.
Until now the method has failed
to distinguish M. sylvestris, M. pumila, M.
orientalis, M. asiatica and M. sieversii from each
other or these closely related wild species from M. domestica
using isoenzyme polymorphisms. The level of genetic diversity is
very high in all these species and is very similar to that found in
the domesticated apple. So there is one hypothesis that
interspecific hybridization is so rampant among these forms that
they actually constitute one panmictic species. The mentioned
species are perhaps not distinct, but form one large panmictic
population with an extension from western China to Europe. Analyses
of many more samples of the other close relatives of apple (others
than M. sieversii), especially of M. sylvestris, could
result in a rejection of that hypothesis as long as species-specific
alleles can be identified.
Genetic analyses based on DNA
markers may provide better information regarding the immediate
ancestors of cultivated apple and pear. So there is a second chance
to at least partially reject the hypothesis, but the experiments for
using DNA markers have yet to be performed (Weeden, pers. comm.).
There is little information about
pear. To work with isoenzymes in pear is much more difficult than
working with apple. In Europe experiences with pear do exist at INRA
Angers.
Objectives of gene conservation
The genetic resources of P.
avium, M. sylvestris and P. pyraster are seriously
endangered, mainly for the following reasons:
-
Extensive felling for
commercial purposes. Rare occurrence and a narrow genetic base
cause genetic drift due to small numbers of mother trees and
large distances between adult, reproducing trees.
-
Natural regeneration is not
guaranteed and, if it occurs, it is endangered by grazing.
Hybridization with cultivated forms of cherry, apple or pear is
a main obstacle. The identification of the wild fruit tree
species is difficult, especially for M. sylvestris and P.
pyraster, but suitable identification keys have been
developed (Wagner 1995; Müller and Litschauer 1996).
-
Uncontrolled seed transfer.
In EU countries P. avium, M. sylvestris and P. pyraster
are not included under national legislation for forest
reproductive material. Therefore, seed of unknown origin (even
residuals from distilleries) is used for plantations or for
afforestation purposes in the landscape and along highways. Also
clonal plantations are established through vegetative
propagation of selected material from unknown origin.
-
Several diseases, especially
viral diseases, have contaminated the three wild fruit tree
species and may endanger their existence in some areas.
Present and suggested gene
conservation activities
Measures must be taken for the
conservation of genetic resources of the three rare Noble Hardwood
species. Activities concerning collection and study of the existing
material is already underway in some countries. But Europe-wide
activities or projects are still lacking. Regarding P. avium, M.
sylvestris and P. pyraster, the following overview
informs about the actual situation and possibilities of conservation
measures in European countries. Most of the information was
presented at the first meeting of the EUFORGEN Noble Hardwoods
Network (Turok et al. 1996).
Prunus avium: Conservation
measures of different intensity are carried out in 13 European
countries (Table 2). The main activities cover in situ
conservation of selected stands and single trees, as well as ex
situ conservation of clones or families in seed orchards and
clonal archives. Other ex situ conservation measures have
obviously had no importance until now. In some areas the trees are
contaminated by viral diseases. Nevertheless, wild cherry is used
for plantations or afforestations in several countries. Forestry
practice is now interested in planting wild cherry because of the
high value of its timber.
Malus sylvestris and Pyrus
pyraster: Although wild apple and wild pear are
autochthonous in several European countries, intensive conservation
measures are still lacking with the exception of Germany, where for
both species conservation programmes have been carried out by
federal-state institutes for several years. These programmes
concentrate on seed orchard establishment to restore breeding
populations of sufficient size. The necessity of a Europe-wide
cooperation programme for the conservation of genetic resources of
these two valuable wild fruit tree species needs to be emphasized.
1. In situ conservation
The natural situation of the rare
fruit tree species restricts the possibilities for implementing in
situ conservation strategies. In only a few cases, e.g. Prunus
avium, natural stands with a minimum number of 30 to 50
individual trees can still form in situ conservation
populations. If such stands exist, they should be designated for
this purpose and naturally regenerated. However, due to the high
value of the timber, it will be difficult to maintain such stands in
private ownership over a long time, especially since P. avium tends
to decay from an age of 80 years onwards. Therefore, the selection
of in situ conservation stands is probably restricted to
public ownership where the necessary measures for natural
regeneration can be taken. One has to take into account that P.
avium propagates vegetatively by adventitious shoots from roots.
Therefore, monoclonal stands occur.
2. Ex situ conservation
For all three species the
establishment of ex situ conservation seed orchards seems to
be the most suitable and efficient conservation measure. Grafting is
not difficult in any of the three species. Prunus avium can
also be propagated by tissue culture methods (Meier-Dinkel et al.
1997). Seed orchards can be relatively easily established. They
should be regionally structured due to the ecological conditions. A
minimum of 50 clones per seed orchard should be aimed at. Especially
in M. sylvestris and P. pyraster, new
interbreeding populations can be established when individual
specimens, scattered over a large, but ecologically similar area,
are collected and planted together in a seed orchard. Because
regular and sufficient seed crop is hardly possible in natural
populations, the establishment of seed orchards is also a necessity
for ensuring seed procurement.
Seedling seed orchards seem to be
less suitable owing to the danger of pollen contamination from
cultivated forms within the seedlings and the risk of inbreeding
when only one or few trees are left for seed harvest as a base for
the seedling seed orchard. For P. avium, however, seedling
seed orchards may be a solution in those cases where sufficiently
large populations are available.
As far as possible, seeds of the
three species should be stored. The methods for long-term storage
need to be improved or further developed.
Outlook
Investigations about naturally
occurring populations, groups or individuals of the three wild fruit
tree species should be started with joint forces. Significant
occurrences of the species, e.g. P. avium, should be
protected in situ as gene conservation stands. Small groups
or individuals should be grafted and planted in clone collections
for further use. Those materials should be tested to verify whether
or not they are contaminated by diseases (virus, bacteria, fungi).
Research about intraspecific
variation and genetic structure by using, among others, molecular
markers, as performed by Santi and Lemoine (1990a, 1990b) for P. avium,
must be intensified in cooperation between the European countries.
The establishment of clone
collections or archives and the exchange of those materials and
information between the Noble Hardwood Network members will
facilitate conservation and propagation of the three species.
Suitable clones can be used for the establishment of seed orchards,
from which one can expect genetically variable and valuable seed
material for plantations. Such plantations should guarantee a type
of "pseudo in situ conservation". The combination
of gene conser-vation and utilization is in the long term the only
viable and meaningful method of conservation, since without
utilization the conservation strategy would lead to a dead end.
References
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