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A.Pliūra
Lithuanian
Forest Research Institute, Department of Forest Genetics
and Reforestation
Introduction
In Europe, there are four different Fraxinus
species growing naturally, the common ash (Fraxinus
excelsior L.) and narrow-leaved ash (Fraxinus
angustifolia Vahl.) being important commercially.
Common ash is the most important forestry species in many
European countries. This species is intensively used for
the production of timber of high value. Therefore,
interest for silviculture, breeding and gene conservation
activities of common ash in many countries has risen
recently. However, so far only few European countries have
developed gene conservation programmes or strategies for Fraxinus.
Most conventional concepts of conservation of biodiversity
as a whole, and genetic diversity in particular builds on
a misconception that maximum fitness have already been
obtained in nature. As a consequence of that the present
genetic constitution is identified as the prime objective
of conservation. That
results in static character of conservation activities
where no active silvicultural measures are taken. Eriksson
et al (1993) strongly emphasised that the present-day
genetic structure is transient. It should, therefore, not
be regarded as the objective of gene conservation but as
the starting point. The conventional programmes for
conservation of forest genetic resources are not able to
cope with extensive expansion of human into forest lands,
rapid global and local environmental changes which are
caused by human activity, and with stochastic
perturbations in the natural ecosystems that are heavily
disturbed by man. The limitations put on management in
strictly-protected areas or gene conservation populations
are not able to compensate for the negative impacts of
these environmental changes and increased magnitudes of
the stochastic perturbations. An active specially aimed
management is the only way to counteract these impacts and
enhance the diversity and adaptadness of forest tree
species. The understanding that to cope with present and
future pressure on forests the sound forest tree gene
conservation should be based on an evolutionary approach
and be dynamic is increasing (Namkoong 1984, Hattemer and
Gregorius 1990, Finkeldey and Hattemer 1993, Eriksson et
al 1993, Hattemer 1995, Pliūra 1997, Pliūra and
Eriksson 1997, Koski et al 1997). The evolutionary
approach means that the prime objective of gene
conservation is not just to conserve an existing genetic
variation but create good conditions for increasing
adaptation and future evolution of the species. Therefore,
the gene conservation must be dynamic and based
on evolutionary genetics. Defining a long-term gene
conservation strategy and subsequently developing
programmes on gene conservation requires the knowledge on
natural range of species, pattern of distribution,
population structure, ratio of within- and
among-population variation, role and stage in the
ecosystem, pollen and seed disseminators (vectors),
particular biologic features, present status and
tendencies of forest and silviculture, current and future
demands for all kinds of forest utilities, changes of
climate and environment.
Current status of Fraxinus
species and some particular features
The
common ash is the largest forest tree in the genus Fraxinus
typical of European fertile, multispecies, broadleaved and
mixed deciduous forests. The natural range of the common
ash covers nearly whole Europe with the exclusion of the
most northern and most southern part from the shores of
the Atlantic in the west to the Volga river in the east (Fig.
1). The most northern limit of its natural
range is in Norway at about 64o Lat. N. The
southern margins reaches the latitude of 37o N
in Iran. In the mountains common ash reaches its maximum
elevation in the Pyrenees at 1750-1800 m a.s.l., in Swiss
Alps at 1630 m a.s.l. (Hegi 1927). It can be found at much
higher elevations in Asia - up to 2200 m a.s.l. in Iran (Browicz
1990). The natural range of taxonomically more complicated
narrow-leaved ash is shifted more to the south and it
covers the Mediterranean region and is spread in northern
Africa and in West and Central Asia.
Fig. 1
The centre of natural occurrence of common ash in most
part of Europe is mainly in floodplain forests. However
common ash grows also along the water runs even in higher
vegetation zones. Especially in Southern Europe the
natural occurrence of common ash is shifted to the higher
elevations. Besides floodplain wet soil sites the common
ash represents a typical species of slopes and ravines,
growing there in mixture with some other characteristic
species like maple, lime and elm. In such conditions ash
can grow from the oak altitudinal vegetation zone to the
beech-spruce zone of sub-mountain and mountain regions,
sometimes even at top areas, mainly on basal soils. Due to
difference in sites occupied and stand types, some
publications refer to the existence of different ecotypes
of common ash – the ecotype from the floodplains,
ecotype from hilly-side growing along smaller water runs
in higher forest zones up to beech-spruce stand class, the
scree ecotype growing at slopes and limestone ecotypes.
However, the existence of ecotypes has not been proven in
experimental field tests and therefore postulated soil
races or ecotypes should be considered as being phenotypic
(Weiser 1995).
On the European scale, neither the common ash nor the
narrow-leaved ash is endangered as a forest tree species.
However, the natural range and areas of ash forests have
decreased all the time during the last 4000 years. A lot
of the common ash forests have been destroyed by humans in
the areas of present agricultural lands. Many of present
populations of ash occupy the scattered refuge areas that
were less suitable for agriculture. Taking all these
circumstances into account, most countries consider Fraxinus
to be threatened at the population level. Just recently
(during last 3-4 decades) silviculture has promoted
(supporting natural regeneration, planting, favouring
during thinning, etc.) common ash considering its high
economic value. Due to that common ash has increase in
proportion of younger forest stands in many countries.
A combination of two types of pollen and seed dispersal in
the species (anemochory and zoochory) provides the
powerful mechanism for successful natural regeneration
that can be observed in many stands or temporary
forest-absent habitats. The common ash exhibits properties
that are intermediate between typical pioneer tree species
and permanent forest component. Diversity of behaviour
depending upon ecological conditions which express itself
in effective use of many ways of achieving reproduction
success is the most important feature of the biology of Fraxinus
species (Falinski 1995). Its reproduction strategy could
be based either on vegetative regeneration or on
generative one or on combination of both. Abundant young
generation of ash occurring under the canopy grows very
slowly but small trees preserve the capacity to survive
for a long period of time. They can regain normal growth
rhythm after the thinning or clear cutting of stand (Lust
1972). In spite of the high regeneration potential of the
species the reproduction of some valuable autochtonous
populations is not ensured.
It is recognised (Lande 1988) that future conservation
plans should be based on both knowledge of species
demography and population genetics in assessing the
requirements for species survival. One of the most
important steps in defining the conservation strategy is
to identify factors threatening that species. The factors
leading to extinction can be subdivided into two
categories: systematic pressures and stochastic
perturbations (Shaffer 1981). The systematic pressures
lead to the deterministic extinction (Gilpin and Soule
1985). Among systematic pressures that have already
threatened, continue or will threaten a Faxinus
gene resources in future should be considered:
deforestation and loss of habitats due to glaciation and
human activity, natural climatic changes, climatic changes
due to global warming, different effects of air pollution,
long-term pressure on forests for all types of human
utility production, improper forest management practice,
competition by other species, damage by game, uncontrolled
transfer of reproduction material for artificial
regeneration. Gene conservation activities should be
designed to compensate for all these systematic pressures
threatening the species. All these threats can be
considered as having a stochasticity aspects too. Shaffer
(1981) have distinguished four types of stochastic
perturbations (variation, fluctuation) that contribute to
population extinction: demographic stochasticity, genetic
stochasticity, environment stochasticity, and
catastrophes. All these perturbations can be considered
effecting the populations of Fraxinus.
Therefore our task is very demanding – to compensate or
overcome both systematic pressures and stochastic ones.
Genetic knowledge
The knowledge on the genetic structure of Fraxinus
is still insufficient since there has been litle research
on the population genetics on Fraxinus.
Nevertheless, the differences among provenances have been
clearly identified in phenotype and have been repeatedly
proven in progeny tests and in practical forestry. (Nikolaeva
and Vorob'eva 1984, Smintina 1993, Giertych 1995a, 1995b,
Kleinschmit et al 1996). The similar geographical pattern
variation of different traits was found in studies on
American species of ash (Clausen et al 1981, Clausen 1984,
Raymond and Lindgren 1990, Roberds 1990, Schuler 1994,
etc.). Population structure and ratio of within- and
among-population variation are influenced by pattern of
species distribution, its role and stage in forest
ecosystem, pollen and seed vectors, particular biologic
features, etc. The present scattered distribution and
specific ecological requirements indicate that the
populations of Fraxinus
are probably more differentiated than ones of wind
pollinated species with continuous distribution. Results
from half-sib progeny trials have shown the existence of
significant within-population variation as well. In most
experiments a variation between single tree progenies
between families within provenances was as high as
variation between provenances.
Gene conservation objectives
A sound
gene conservation strategy should be based on sound
objectives. The prime objective of gene conservation is to ensure a continuous
survival and adaptability of the species over unlimited
number of generations in continuously changing environment
through evolution. Gene conservation should be based
on methods that are able to cope with all types of
systematic pressures and stochastic perturbations. One of
the general prerequisite for successful evolution is that
the gene resource population is regenerated. Thus, active
measures should be foreseen where there are difficulties
in maintenance of the designated gene resource population
over generations.
In a long term perspective there will with high
probability be an increased pressure to raise the
production of timber. The increased demand in future
(Anonymous 1994) raises the importance of tree breeding.
Gene conservation ought to be carried out jointly with
tree breeding in order to save costs. Such combined
breeding/gene conservation has also to be dynamic in order
to cope with the uncertain future.
Gene conservation approach
In order
that gene conservation to be successful, attention needs
to be given to the links and interdependencies of
conservation at the ecosystem, species and interspecific
levels. Conservation of genetic diversity is one of the
three key issues in sustainable conservation of
biodiversity. To be successful the conservation of
biodiversity should be built on gene conservation as its
elementary component. Therefore, gene conservation should
be considered as integral part of nature conservation and
be integrated into nature conservation programmes.
To reach the main objective of gene conservation it is
needed to promote the maintenance of a broad genetic variation and to
create good conditions for fast adaptation of species. To
be able to compensate negative impact of environment
changes and increased magnitudes of stochastic
perturbations gene conservation should be dynamic and
based on evolutionary approach with continuously
increasing adaptability by means of especially aimed
management.
Conservation of
genetic diversity in forest ecosystems can be achieved
through a diversity of approaches combining strictly
protected areas with forest intensively managed for the
production of timber or other utilities (Kemp 1992,
Palmberg-Lerche 1997, Ouedraogo 1997). Two different
strategies of gene conservation can be used: a) specific
active gene conservation measures, and b) sustainable
forest management and nature conservation. They should be
complementary to each other. If specific conservation
programmes are only successful in designated gene
conservation areas in the future just there may be just
“an oasis of flourishing genetic diversity in the desert
of vast landscape”. The ideal situation would be to
integrate silviculture in the commercial forest stands
similar to the areas of dynamic gene conservation.
Therefore, main elements of the strategy that is
designated for gene conservation in especially aimed areas
should be strongly recommended for common commercial
forestry too. Gene conservation activities has to be
integrated into management plans. The strategy for
promoting and gene conservation of Noble Hardwoods by
sustainable forest management (silviculture) is presented
in the paper of Rotach (1998, this issue). We scrutinise
the strategy for specific active gene conservation
measures in designated gene conservation areas.
Genetic variation is a function of allele frequencies and
allelic effects. The species carries a low number of
common alleles at intermediate frequencies and a very
large number of low-frequency alleles resulting from
mutations (Fig.
2). Two variables could be distinguished to
describe the distribution of alleles: first, alleles can
be divided into those which are common (>0.05) and
those which are rare, second, the alleles can be divided
as to whether they are widespread over populations or
localized to few populations (Adams 1981, Yang and Yeh
1992). That classification results in 4 types of alleles:
common widespread, common localized, rare widespread, and
rare localized. The main contributors to existing
genotypic variation are the ‘common’ alleles. These
alleles are of main interest for diversity of reproductive
material. Additive variance is a prerequisite for progress
by natural or artificial selection. However, genes at low
frequencies (< 0.01)
as well as genes at high frequencies (>0.99) do
not contribute much to the additive variance. Therefore,
neither natural selection nor breeders will be able to
raise low frequency genes. Less
common alleles contribute to potential variation (Danell
1993a). Therefore, rarer alleles should be conserved for
long-term tree breeding and evolutionary needs (Fig. 2).
At present there is a general consideration that gene
conservation should aim at conserving alleles of
frequencies above 0.01 (=1% of alleles).
Fig. 2
Multiple
population approach
Both in situ and ex situ gene
conservation of Faxinus spp.
should be based on multiple population approach when gene resource
population is split into small subpopulations located over a range
of environments and thus exposed to natural selection and in turn to
evolution in a variety of directions.
The multiple population
breeding system (MPBS) concept first developed for breeding
(Namkoong 1976) and then extended to joint breeding and gene
conservation (Namkoong 1984) should be the core of gene
conservation. The MPBS means that the joint breeding/gene resource
population would consist of small sub-populations over range of
environments. The essence of dynamic gene conservation according to
MPBS concept is to promote adaptation by exposing the gene resource
population to natural selection and in turn to evolution in a
variety of directions (Eriksson et al. 1993) (Fig.
3). The main principles of
dynamic gene resources conservation by applying Multiple population
Breeding System (MPBS) concept are presented in
Figure 4.
Fig. 3
Fig. 4
A prerequisite for natural or artificial
selection to be operative is that the gene resource population is
large enough to capture high genetic (additive) variation and to
avoid genetic drift. According to MPBS concept, breeding/gene
resource conservation population of single species should consist of
approximately 10-20 subpopulations, each with an effective
population size (Ne) of
50 genetic entries making a total gene resource population of
500-1000 entries. These figures are based on the probability of
saving genes at frequencies above 0.01 and of avoiding severe
inbreeding in the subpopulations (Danell 1993a, Pliura and Eriksson
1997). With an effective population size of 50 individuals in a
subpopulation the rate of inbreeding will be 1% per generation (1/2Ne)
which might be regarded as satisfactorily low. This is also the rate
of loss of additive variance (Fig 5). In order to capture alleles of frequencies above 0.01
with 0.99 probability gene conservation population should consists
of about 750-1150 individuals (Fig. 6). If we consider capturing alleles of lower frequency,
the number of individuals needed increases very rapidly. Due to
risks of natural or man made disasters larger gene conservation
populations than 1000 trees would be meaningful. The relatively
small number of genotypes in subpopulations will help fixation of
new genes arisen from mutations and may speed up evolution.
Fig. 5
Fig. 6
At present, the advantages of MPBS concept is widely recognised
(Namkoong 1984, Barnes et al. 1984, Namkoong et al. 1988, Williams
et al 1995, Barnes 1995, Koski and Tigerstedt 1996).
The most intensive form of MPBS includes planting of regular
progeny trials (Danell 1993b, Pliûra and Eriksson 1997). Less
intensive forms may be utilised for species not included in
extensive breeding program (cf. Eriksson et al. 1993, Varela and
Eriksson 1995).
Choice of gene conservation method
Different patterns of genetic variation and therefore different types of
adaptability can be noticed both between species and within single
species in different parts of natural range (that is the case with
common ash as well). Therefore, gene conservation activities should
be different when the species is common over large areas
constituting large randomly mating populations or when a species is
rare along its margin and constitute small locally more or less
adapted populations with limited gene exchange. Each country should
decide upon what strategy should constitute the core of national
gene conservation of Fraxinus.
For
many European countries where larger populations of common ash are
found on a variety of sites, including optimum ones, in
situ methods are sufficient for conservation and can constitute
the core of national gene conservation programmes. The protected
areas for conventional in situ conservation (gene reserves, seed reserves, seed stands,
etc) can be used as a base for establishing the network of gene
resource subpopulations for in
situ dynamic gene conservation by applying the MPBS concept. As
common ash on many sites constitutes pure stands, the gene
conservation programmes can be designated just for conservation of
single species. However, ash could be jointly conserved with other species in situ in the ecosystem in which it exists as well.
That would be a low cost alternative for joint
in situ gene conservation of some species in mixed stands. To be
successful, the joint gene conservation of all species there should
be dynamic, evolutionary oriented, and based
on multiple population breeding system (MPBS) concept as
well.
Ex
situ conservation of gene resources in form of progeny trials as
the most active and effective gene conservation method (Eriksson et
al 1993) should be defined as the main conservation method for Fraxinus
as an option for countries that are facing difficulties to conserve in-situ (where populations are extremely threatened by industrial
air pollution or other kind of irreversible destruction of natural
habitat) or which consider Fraxinus
as economically important species and are going to have extensive
breeding programs (jointly with gene conservation). This type of
conservation provides for fast adaptation of a population by
combining natural processes with human management. The
ex situ method for conservation of gene resources in form of
progeny trials is rather close to the in
situ method due to it considers to establish plantations in
ecological conditions rather similar to ones that exists in
locations where the planting material is originating from.
The
establishing some of ex situ
gene conservation populations in the areas of historical natural
range of Fraxinus on
optimum sites should be encouraged as well in order to expand the
range of environment and in turn adaptation and genetic variation.
For
countries where ex situ
methods constitute a core of gene conservation programmes
in situ conservation should serve as supplemental method that
increases safety due to decreasing vulnerability of whole
conservation programme.
Conventional
ex situ gene conservation
that is based on clone archives and seed orchards is rather static
and does not promote the adaptation and evolution of the species.
For more rare and scatteredly distributed species, clone archives
and seed orchards provide an efficient instrument for conserving and
even increasing genetic variability. However, clone archives and
seed orchards can be considered just as temporal gene conservation
means aimed at generating progenies that would later on become the ex
situ gene conservation population. In case where the clone
archives are considered as core gene conservation means, that
postpones the launching of really dynamic gene conservation system
that is based on ex situ
gene conservation subpopulations in form of progeny test
plantations. The clone archives can serve as supplemental means
aimed at increasing safety of gene conservation programs through
supplying progenies in case where regeneration of gene conservation
population is not successful. Clone archives being established by
using full randomisation of clones and ramets, can be transformed
into conventional commercial seed orchards as soon as a selective
thinning is made.
Long term storage of seed or other kind of germaplasm should be used
for increasing safety of gene conservation programmes too.
Minimum requirements for
gene conservation of Fraxinus species from an European perspective
A sampling of 20
populations based on the existing genetic knowledge or the
ecoclimatic conditions in the area of distribution should be carried
out. Each population should have at least 50 trees. Whenever
possible this sampling can be done in conjunction with sampling of
other noble hardwoods. The sampled stands will constitute in
situ subpopulations in a dynamic MPBS type of gene conservation.
Sampling
The number of populations
sampled and number of individuals constituting gene conservation
populations must be large enough to include most of the genetic
variation that exists both within and between populations. In order
to capture as large as possible genetic (additive) variation which
is the main prerequisite for successful natural and artificial
selection and in turn for evolution of species, the sampling should
cover a whole range of species distribution both central and
marginal populations on specific habitats (sites). Different types
of populations are of importance for conservation and should be
sampled: a) populations representing the main regions of provenance
(forest eco-regions or breeding zones), b) marginal populations, c)
isolated populations, c) populations growing under specific
ecological conditions, d) endangered populations, e) populations
carrying rare features, f) populations valuable for breeding.
Sampling of different populations increases the probability of
capturing already existing adaptedness.
To
capture alleles of different type different sampling strategies are
needed. To capture localized alleles (both common and rare) the
sampling should cover more populations over range of environments at
the cost of fewer individuals per population.
Sampling
should be slightly different (modified) when: a) the species are
common over large areas and there are large populations, b) when a
species is rare and of scattered distribution along its margin. The
genotypes for each ex situ
synthetic subpopulation should be sampled in populations of one
region of provenance in order to capture adaptadness already
existing within that region, not destroy any co-adapted gene
complexes if such exist, and prevent the potential risk associated
with provenance hybridisation. The sampling of 10-20 stands within a
region of provenance (where the species is common) would provide a
representative sample for establishing of one gene conservation
subpopulation. If there are few populations in a given region (that
is the case along the margins of species natural range),
complementary to marginal populations the populations from the
neighbouring regions of provenance should be sampled.
Establishment
A network of gene
conservation subpopulations (both ex
situ and in-situ)
should be created with sufficient coverage of the species geographic
genetic variation and ecological variation within the species
distribution range. Present genetic structure is neither optimal nor
stable. However, even if fitness of populations to ecological
conditions of specific ecological regions or regions of provenance
not reached maximum, it would be meaningful to utilise already
existing adaptedness and promote them. Therefore, both sampling and
establishment of gene conservation subpopulations should be done on
ecoregional base.
To
conserve gene resources of a single region of provenance in situ, 1-3 stands of sizes 5-15 ha should be selected. These
stands preferably should be selected within gene reserves, seed
reserves or other types of conventional gene or nature conservation
areas that already exists in the country.
To
conserve ex situ, 1-3
progeny conservation/breeding plantations of sizes 2-4 ha each are
suggested. Ex situ dynamic
gene conservation system could be created on the basis of
conventional progeny test plantations that were established over a
range of environment for tree breeding or genetic study purpose. Two
subpopulations of different nature can be established side by side
to have a possible alternative for choosing a best way of gene
conservation in the future. A first one could be composed of
progenies from single trees randomly selected in 10-20 populations
within the region of provenance. The second one could be established
from progenies originating from plus trees selected within the same
region.
The
gene conservation of associated species can be done by creating
large gene conservation populations, up to 200 hectares, one in each
main region of provenance per country. Each large gene conservation
population can consist of some stands of different species
composition, age, and site. These populations should be managed to
create maximum habitat diversity (different, age, species
composition, etc.) within the gene resource population.
Management
Gene conservation
populations should be intensively managed to improve the adaptadness
of each subpopulation and to increase genetic differences between
them. The management should guarantee: a)sustainability of
populations during ontogenesis: a) the continuous regeneration of
population of target species, and c)protection against all types of
damage. Management of each gene resource subpopulation should be
done according to individual management plans. The continuous
monitoring of natural regeneration and health condition of
population is needed.
Regeneration
is a key aspect of gene conservation. To increase a speed of
evolution for its better synchronisation with fast changing
environment the turn over of generations should be accelerated.
In
case of in situ gene
conservation, subpopulations should be intensively managed to
support the natural regeneration of target species and prevent from
competition of other species, that may become dominant following the
rules of natural successions. If the subpopulation being conserved in
situ consists of even-aged mature stand, the parts of gene
conservation subpopulation should be opened (thinned or cut in
narrow strips or gaps) as soon as possible in order to create
conditions for natural regeneration (preferably next year following
the mast). If population consists of some stands of different age
but there is no regeneration, the oldest (but not necessarily
mature) part of a subpopulation should be cut as soon as mast years
will have produce sufficient seed yield or regeneration (1-st year
seedlings) under the canopy of stand or in areas being set aside and
aimed at growing of next generation. By increasing the number of
stands or demes (groups of trees) of different age that constitute
the subpopulation, the total period of regeneration expands.
Therefore, the larger the portion of trees involved in regeneration
the larger the within-population genetic variation. In case where
these regeneration support measures is not successful, the
artificial planting should be done using planting material that
originates from these stands. Bred material that originates from ex
situ gene conservation/breeding subpopulations of the same
region of provenance can be used as well. To secure the physical
sustainability of each subpopulation, a careful tending should be
carried out. Thinning should be made by standard silvicultural
practices of each country.
The
new generation of ex-situ
gene conservatin/breeding subpopulations should be created using
open pollination of the best individuals selected within each of the
families constituting subpopulations as soon as the progenies comes
to reproductive phase.
Integration
with tree breeding
The
ex situ dynamic gene
conservation system should be integrated with genetic research and
tree breeding. The selection criteria for tree breeding are based on
developing qualities beneficial for human use. However, the
improving of adaptive properties are as important as in gene
conservation. Increasing gene diversity will provide the possibility
for increasing the efficiency both for short and long-term tree
breeding. Artificial crossing could be used in combination with
breeding if there is ash breeding programme. Equal amount of parent
individuals from each family should be used for producing new
generation in order to keep a high effective population size.
Totally about 50 best adapted individuals should be founders of each
new gene conservation/breeding subpopulation (within given region of
provenance). The results of selection (tree breeding) within both in
situ and ex situ gene conservation subpopulations, could be utilised by
establishing seed orchards at the end of each cycle of
conservation/breeding–establishing a new generation using cuttings
of best individuals selected within each generation.
Supplementary
measures
In addition to the
specialized gene conservation activities, the following means for
reducing the pressures and erosion of genetic resources can be
recommended:
- genetic studies
and integrating them with demographic and ecological studies,
- establish and
adopt legal regulations on gene conservation,
- establish and
adopt of regulations on seed transfer and seed trade, control and
documentation of seed sources (OECD Scheme for the Certification of
Forest Reproductive Material),
- control of
pollution originating from industry, transport, agriculture, etc.
- education towards
gene conservation and public awareness, etc.
Acknowledgements
I wish to thank
professor Gösta
Eriksson, Uppsala, Sweden, for valuable comments and discussion, and
David Clapham and Jozef Turok for revision of the English text
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