Wild genetic diversity preservation in a small-sized first generation breeding population of <Emphasis Type="Italic">Allanblackia floribunda</Emphasis> (Clusiaceae)

Founder group size is of prime importance in tree breeding programs. We determined whether sampling 20-plus trees for breeding in Allanblackia floribunda, a tropical forest tree species that has been recently enrolled in tree improvement program for fruit and seed production, would affect neutral genetic diversity and inbreeding level in both breeding and production populations. Using eight informative microsatellite loci, we: (a) assessed the nuclear genetic diversity of ten natural populations, and of the breeding population in the humid forest zone of Cameroon; (b) investigated temporal effective-size fluctuations in A. floribunda natural populations, with a view to identifying the role of past demographic events in the genetic structure of the studied species; and (c) tested the hypothesis that genetic diversity in a founder group of 20 individuals is not different from that existing in the wild. The eight loci were variable. High levels of genetic diversity (A = 4.96; H _E = 0.59) and moderate differentiation (R _ST = 0.061) were found within and among populations in wild stands. High genetic distances existed between populations ( \textaverage chord distance = 0.\text2769 ±0.00\text554 ) \left( {{\text{average chord distance}} = 0.{\text{2769}} \pm 0.00{\text{554}}} \right) . Eight of the ten surveyed populations showed signs of deviation from mutation-drift equilibrium, suggesting Pleistocene population bottlenecks and fluctuations in effective population size. Mantel tests did not reveal any relationships between genetic and geographic distances. A neighbor-joining dendrogram showed a population structure that could be explained by historical factors. The hypothesis tested has been accepted. However, a slight increase in inbreeding was observed in the breeding population.     

Genetic variability and population differentiation in captive baird's Tapirs (Tapirus bairdii)

The objectives of this study were to assess the level of genetic variability and population differentiation within captive populations of an endangered large mammal, Baird's tapir (Tapirus bairdii). We genotyped 37 captive animals from North American (NA) and Central American (CA) zoos and conservation ranches using six polymorphic microsatellite loci. Standard indices of genetic variability (allelic richness and diversity, and heterozygosity) were estimated and compared between captive populations, and between captive and wild population samples. In addition, we evaluated levels of population differentiation using Weir and Cockerham's version of Wright's F-statistics. The results indicate that the NA and CA captive populations of Baird's tapirs have retained levels of genetic variability similar to that measured in a wild population. However, inbreeding coefficients estimated from the molecular data indicate that the CA captive population is at increased risk of losing genetic variability due to inbreeding. Despite this, estimated levels of population differentiation indicate limited divergence of the CA captive population from the wild population. Careful management appears to have kept inbreeding coefficients low in the NA captive population; however, population differentiation levels indicate that the NA population has experienced increased divergence from wild populations due to a founder effect and isolation. Based on these results, we conclude that intermittent exchanges of Baird's tapirs between the NA and CA captive populations will benefit both populations by increasing genetic variability and effective population size, while reducing inbreeding and divergence from wild populations. Zoo Biol 23:521–531, 2004. © 2004 Wiley-Liss, Inc.     

A draft genome of sweet cherry (Prunus avium L.) reveals genome‐wide and local effects of domestication

Sweet cherry (Prunus avium L.) trees are both economically important fruit crops but also important components of natural forest ecosystems in Europe, Asia and Africa. Wild and domesticated trees currently coexist in the same geographic areas with important questions arising on their historical relationships. Little is known about the effects of the domestication process on the evolution of the sweet cherry genome. We assembled and annotated the genome of the cultivated variety “Big Star*” and assessed the genetic diversity among 97 sweet cherry accessions representing three different stages in the domestication and breeding process (wild trees, landraces and modern varieties). The genetic diversity analysis revealed significant genome‐wide losses of variation among the three stages and supports a clear distinction between wild and domesticated trees, with only limited gene flow being detected between wild trees and domesticated landraces. We identified 11 domestication sweeps and five breeding sweeps covering, respectively, 11.0 and 2.4 Mb of the P. avium genome. A considerable fraction of the domestication sweeps overlaps with those detected in the related species, Prunus persica (peach), indicating that artificial selection during domestication may have acted independently on the same regions and genes in the two species. We detected 104 candidate genes in sweep regions involved in different processes, such as the determination of fruit texture, the regulation of flowering and fruit ripening and the resistance to pathogens. The signatures of selection identified will enable future evolutionary studies and provide a valuable resource for genetic improvement and conservation programs in sweet cherry.     

Genetic structure of cherry fruit fly (Rhagoletis cingulata) populations across managed,unmanaged, and natural habitats

The cherry fruit fly (CFF), Rhagoletis cingulata Loew (Diptera: Tephritidae: Trypetini), is endemic to eastern North America and Mexico, where its primary native host is black cherry [Prunus serotina Ehrh. (Rosaceae)]. Cherry fruit fly is also a major economic pest of the fruit of cultivated sweet (Prunus avium L.) and tart (Prunus cerasus L.) cherries. Adult CFF that attack wild black cherry and introduced, domesticated cherries in commercial and abandoned orchards are active at different times of the summer, potentially generating allochronic isolation that could genetically differentiate native from sweet and tart CFF populations. Here, we test for host‐related genetic differences among CFF populations in Michigan attacking cherries in managed, unmanaged, and native habitats by scoring flies for 10 microsatellite loci. Little evidence for genetic differentiation was found across the three habitats or between the northern and southern Michigan CFF populations surveyed in the study. Local gene flow between native black cherry, commercial, and abandoned orchards may therefore be sufficient to overcome seasonal differences in adult CFF activity and prevent differentiation for microsatellites not directly associated with (tightly linked to) genes affecting eclosion time. The results do not support the existence of host‐associated races in CFF and imply that flies attacking native, managed, and unmanaged cherries should be considered to represent a single population for pest management purposes.     

Microsatellite variation in China’s Hainan Eld’s deer (<Emphasis Type="Italic">Cervus eldi hainanus</Emphasis>) and implications for their conservation

Hainan Eld’s deer (Cervus eldi hainanus) experienced a dramatic decline in the late 1960s through early 1970s and by 1976 only 26 deer remained in Datian of Hainan Island, China. Since then, conservation efforts have successfully rescued this deer from extinction. We employed 10 microsatellite DNA loci to index genetic variation in the one source (Datian) and two introduced populations (Bangxi and Ganshiling) and suggest implications for the conservation of the species. A total of 40 alleles at 10 loci were examined from 198 deer blood samples. The source population harbored all 40 alleles, while the Bangxi and Ganshiling translocated populations contained 24 and 26 alleles, respectively. The genetic variability was low (H _e ≈ 0.33) for each of the three populations. No significant difference in genetic variability between the three populations was detected (P > 0.05); yet significant differentiation was found among the three populations. Our results suggest that founder effects and genetic drift have affected the two translocated populations. For conservation we recommend the three populations be managed as a meta-population. When establishing future reintroductions, the founder population should have a size larger than the original 26 founders in Datian population or be composed of a cohort of over 20 same-age individuals with 1:1 sex ratio. Genetic monitoring for both the source and translocated populations should be continuously conducted in order to assess the effectiveness of deer conservation in the future.     

Genetic differences within natural and planted stands of <Emphasis Type="Italic">Quercus petraea</Emphasis>

Five sessile oak [Quercus petraea (Matt.) Liebl.] stands from the Czech Republic were studied to learn about the impact of different types of forest management regimes on the genetic differences among tree populations and on population structures. One population had not been markedly affected by human activity, two populations represented unplanted stands that were extensively managed for a long period of time using the coppice system, and two populations were planted stands. Approximately 100 trees from each stand were mapped and subsequently genotyped using 10 nuclear microsatellite loci. We determined the spatial genetic structure of each population and the genetic differentiation among the populations. We found that: (i) the populations were genetically differentiated, but the differences between the unplanted and planted stands were not markedly significant; (ii) the genetic differentiation among the populations depended on the geographical distribution of the populations; (iii) within unplanted stands, a strong spatial genetic structure was seen; and (iv) within planted stands, no spatial genetic structure was observed. Our findings implies that the analysis of spatial genetic structure of the sessile oak forest stand can help reveal and determine its origin.     

Microsatellite analysis of the genetic relationships between wild and cultivated giant grouper in the South China Sea

The giant grouper (Epinephelus lanceolatus) is a coral fish with high commercial value in Southeast Asia. In the present study, we isolated 11 microsatellite DNA markers, and analysed the genetic diversity and differentiation between cultured stocks and wild populations of the giant grouper originating from the South China Sea. A total of 390 alleles at 11 microsatellite loci were detected in 130 individuals from five different populations. The expected heterozygosity varied from 0.131 to 0.855 with a mean value of 0.623 and the observed heterozygosity varied from 0.145 to 0.869 with a mean value of 0.379. The allelic richness and heterozygosity studies revealed that the genetic diversity of the cultured population was significantly reduced when compared with that of the wild population. The F_is, pairwise F_st values, analysis of molecular variance (AMOVA), three-dimensional factorial correspondence analysis and structure analysis revealed significant population differentiation between the cultured stocks and the wild populations, among the three cultured populations and between the two wild populations. These differences may be caused by random genetic drift, the effects of artificial selection and founder effects. Our results will be useful in the management of cultured stocks and conservation of wild populations of the giant grouper.     

The effect of deforestation on the genetic diversity and structure in <Emphasis Type="Italic">Acer saccharum</Emphasis> (Marsh): Evidence for the loss and restructuring of genetic variation in a natural system

The level and distribution of genetic diversity can be influenced by species life history traits and demographic factors, including perturbations that might produce population bottlenecks. Deforestation and forest fragmentation are common sources of population disturbance in contemporary populations of forest ecosystems. Although the genetic effects of forest fragmentation and deforestation have been examined by assessing levels of genetic variation in forest fragments that remain after logging, few considerations have been made of the populations that re-colonize once-cleared areas. Here we examine the effects of human-mediated population bottlenecks on the level and distribution of genetic diversity in natural populations of the long-lived forest tree species, Acer saccharum (sugar maple). We compared genetic variation and structure for populations of sugar maple found within old-growth forested area and in area that has re-colonized since logging. In this study the percent polymorphic loci and allelic richness estimates were reduced in the logged populations compared to old-growth populations. Jackknifed estimates of population genetic differentiation showed significantly higher differentiation among logged populations, with this result being consistently seen when individuals within populations were grouped according to diameter at breast height. The result of decreased genetic variation and higher levels of genetic structure among logged populations suggests that even one extensive bout of logging can alter the level and distribution of genetic variation in this forest tree species.     

Genome and population dynamics under selection and neutrality: an example of S-allele diversity in wild cherry (Prunus avium L.)

Self-incompatibility, a common attribute of plant development, forms a classical paradigm of balancing selection in natural populations, in particular negative frequency-dependent selection. Under negative frequency-dependent selection population genetics theory predicts that the S-locus, being in command of self-incompatibility, keeps numerous alleles in equal frequencies demonstrating a wide allelic range. Moreover, while natural populations exhibit a higher within population genetic diversity, a reduction of population differentiation and increase of effective migration rate is expected in comparison to neutral loci. Allelic frequencies were investigated in terms of distribution and genetic structure at the gametophytic self-incompatibility locus in five wild cherry (Prunus avium L.) populations. Comparisons were also made between the differentiation at the S-locus and at the SSR loci. Theoretical expectations under balancing selection were congruent to the results observed. The S-locus showed broad multiplicity (16 S-alleles), high genetic diversity, and allelic isoplethy. Genetic structure at the self-incompatibility locus was almost four times lower than at 11 nSSR loci. Analysis of molecular variance revealed that only 5?% of the total genetic variation concerns differentiation among populations. In conclusion, the wealth of S-allelic diversity found in natural wild cherry populations in Greece is useful not only in advancing basic population genetics research of self-incompatibility systems in wild cherry but also in the development of breeding programs.     

Using microsatellite diversity in wild Anegada iguanas (<Emphasis Type="Italic">Cyclura pinguis</Emphasis>) to establish relatedness in a captive breeding group of this critically endangered species

Awareness of the genealogical relationships between founder animals in captive breeding programs is essential for the selection of mating pairs that maintain genetic diversity. If captive founder relationships are unknown they can be inferred using genetic data from wild populations. Here, we report the results of such an analysis for six Cyclura pinguis (Sauria: Iguanidae) acquired as adults in 1999 by the San Diego Zoo Institute for Conservation Research to begin a captive breeding program for this critically endangered species. The six founder animals were reportedly hatched in captivity from eggs collected on Anegada in 1985. No records exist, however, as to where on Anegada the eggs were collected or from how many nests they originated. To assist determination of genealogical relationships, we genotyped the six captive founders, their first six offspring, and 33 wild adult iguanas from Anegada at 23 informative microsatellite loci. With these data, we estimated allele frequencies among the wild samples and then estimated the relatedness of the captive population. Using likelihood inference, we determined that three closely related pairs exist among the six captive founders and that each pair is not closely related to the other two. In addition, we were able to assign parentage for all six of the founders’ offspring tested, one of which had been previously misdiagnosed. Using the assigned parentage and inferred relatedness of the six founders, we calculated mean kinship for each of the six founders and their five living offspring. Finally, based on the allelic diversity of the wild iguanas sampled, we conclude that the C. pinguis population on Anegada is not excessively inbred; however, further investigation is warranted.     

Efficient mitigation of founder effects during the establishment of a leading-edge oak population

Numerous plant species are shifting their range polewards in response to ongoing climate change. Range shifts typically involve the repeated establishment and growth of leading-edge populations well ahead of the main species range. How these populations recover from founder events and associated diversity loss remains poorly understood. To help fill this gap, we exhaustively investigated a newly established population of holm oak (Quercus ilex) growing more than 30 km ahead of the nearest larger stands. Pedigree reconstructions showed that plants belong to two non-overlapping generations and that the whole population originates from only two founder trees. The four first-generation trees that have reached maturity showed disparate mating patterns despite being full-sibs. Long-distance pollen immigration was notable despite the strong isolation of the stand: 6 per cent gene flow events in acorns collected on the trees (n = 255), and as much as 27 per cent among their established offspring (n = 33). Our results show that isolated leading-edge populations of wind-pollinated forest trees can rapidly restore their genetic diversity through the interacting effects of efficient long-distance pollen flow and purging of inbred individuals during recruitment. They imply that range expansions of these species are primarily constrained by initial propagule arrival rather than by subsequent gene flow.     

Genetic structure of island populations of <Emphasis Type="Italic">Prunus lannesiana</Emphasis> var. <Emphasis Type="Italic">speciosa</Emphasis> revealed by chloroplast DNA,AFLP and nuclear SSR loci analyses

The wild flowering cherry Prunus lannesiana var. speciosa is highly geographically restricted, being confined to the Izu Islands and neighboring peninsulas in Japan. In an attempt to elucidate how populations of this species have established we investigated the genetic diversity and differentiation in seven populations (sampling 408 individuals in total), using three kinds of genetic markers: chloroplast DNA (cpDNA), amplified fragment length polymorphisms (AFLPs), and 11 nuclear SSR polymorphic loci. Eight haplotypes were identified based on the cpDNA sequence variations, 64 polymorphic fragments were scored for the AFLP markers, and a total of 154 alleles were detected at the 11 nuclear SSR loci. Analysis of molecular variance showed that among-population variation accounted for 16.55, 15.04 and 7.45% of the total detected variation at the cpDNA, AFLPs, and SSR loci, respectively. Thus, variation within populations accounted for most of the genetic variance for all types of markers, although the genetic differentiation among populations was also highly significant. For cpDNA variation, no clear structure was found among the populations, except that of the most distant island, although an “isolation by distance” pattern was found for each marker. Both neighbor-joining trees and structure analysis indicate that the genetic relationships between populations reflect geological variations between the peninsula and the islands and among the islands. Furthermore, hybridization with related species may have affected the genetic structure, and some genetic introgression is likely to have occurred.     

Fast evolutionary genetic differentiation during experimental colonizations

Founder effects during colonization of a novel environment are expected to change the genetic composition of populations, leading to differentiation between the colonizer population and its source population. Another expected outcome is differentiation among populations derived from repeated independent colonizations starting from the same source. We have previously detected significant founder effects affecting rate of laboratory adaptation among Drosophila subobscura laboratory populations derived from the wild. We also showed that during the first generations in the laboratory, considerable genetic differentiation occurs between foundations. The present study deepens that analysis, taking into account the natural sampling hierarchy of six foundations, derived from different locations, different years and from two samples in one of the years. We show that striking stochastic effects occur in the first two generations of laboratory culture, effects that produce immediate differentiation between foundations, independent of the source of origin and despite similarity among all founders. This divergence is probably due to powerful genetic sampling effects during the first few generations of culture in the novel laboratory environment, as a result of a significant drop in N _e. Changes in demography as well as high variance in reproductive success in the novel environment may contribute to the low values of N _e. This study shows that estimates of genetic differentiation between natural populations may be accurate when based on the initial samples collected in the wild, though considerable genetic differentiation may occur in the very first generations of evolution in a new, confined environment. Rapid and significant evolutionary changes can thus occur during the early generations of a founding event, both in the wild and under domestication, effects of interest for both scientific and conservation purposes.     

The impact of founder events and introductions on genetic variation in the muskox <Emphasis Type="Italic">Ovibos moschatus</Emphasis> in Sweden

The muskox Ovibos moschatus (Zimmerman 1780) is a specialised arctic mammal with a highly fragmented circumpolar distribution, with native populations in Canada and east Greenland and introduced populations in west Greenland, Alaska, Siberia and Eurasia. In 1971, five O. moschatus individuals from an introduced population in Norway migrated to Sweden. After a peak population of 36 individuals in the mid-1980s, the Swedish population now numbers seven individuals, making it vulnerable to both demographic and genetic stochasticity (i.e. inbreeding). Here, we analyse genetic variation among native and introduced populations of O. moschatus to evaluate the genetic effect of sequential founder events in this species. Our results show that genetic variation among native and introduced O. moschatus populations do not conform entirely to the expectations from sequential founder events, most likely because of random processes associated with introduction. In the Swedish population, a calf resulting from the mating of a wild cow and a captive Greenlandic bull contributes significantly to the current genetic variation. Thus, even a single outbreeding event may, at least momentarily, increase the genetic variation and potentially prevent inbreeding depression. Our results should aid the long-term preservation of O. moschatus in Sweden and Europe.     

Conservation of genetic diversity in slippery elm (<Emphasis Type="Italic">Ulmus rubra</Emphasis>) in Wisconsin despite the devastating impact of Dutch elm disease

Forest pest epidemics are responsible for many population declines reported in forest trees. While forest tree populations tend to be genetically diverse, in principle mortality resulting from disease could diminish that genetic diversity and alter the genetic structure of the remnant populations with consequences for the ability of a species to adapt to changing environments. Slippery elm (Ulmus rubra Muhl.) is a long-lived, wind-pollinated forest tree with a native range covering essentially all of eastern North America. Dutch elm disease (DED) caused by an introduced fungal pathogen (Ophiostoma ulmi) devastated North American elm populations, including slippery elm, beginning in the 1930s. Estimates of the numbers of elms lost to DED are unknown but range into the hundreds of millions of trees given their former abundance. In this study, the genotypes of 77 herbarium specimens collected between 1890 and 2004 in Wisconsin, and of 100 slippery elm trees from five wild Wisconsin populations, were characterized using 13 microsatellite loci. Levels of genetic diversity were compared between the herbarium specimens collected pre- and post-DED spread in Wisconsin. In addition, the levels of genetic diversity and degree of genetic differentiation were quantified in the five wild populations. The allelic diversity and expected levels of heterozygosity were similar between the pre- and post-DED herbarium specimens. The five wild populations were only slightly differentiated and no genetic bottleneck was detected for any population. At least in Wisconsin, slippery elm apparently has maintained levels of genetic diversity that could facilitate adaptation to future climatic and environmental changes.     

Hidden founder effects: small‐scale spatial genetic structure in recently established populations of the grassland specialist plant Anthyllis vulneraria

The long‐term establishment success of founder plant populations has been commonly assessed based on the measures of population genetic diversity and among population genetic differentiation, with founder populations expected to carry sufficient genetic diversity when population establishment is the result of many colonists from multiple source populations (the ‘migrant pool’ colonization model). Theory, however, predicts that, after initial colonization, rapid population expansion may result in a fast increase in the extent of spatial genetic structure (SGS), independent of extant genetic diversity. This SGS can reduce long‐term population viability by increasing inbreeding. Using 12 microsatellite markers, we inferred colonization patterns in four recent populations of the grassland specialist plant Anthyllis vulneraria and compared the extent of SGS between recently established and old populations. Assignment analyses of the individuals of recent population based on the genetic composition of nine adjacent putative source populations suggested the occurrence of the ‘migrant pool’ colonization model, further confirmed by high genetic diversity within and low genetic differentiation among recent populations. Population establishment, however, resulted in the build‐up of strong SGS, most likely as a result of spatially restricted recruitment of the progeny of initial colonists. Although reduced, significant SGS was nonetheless observed to persist in old populations. The presence of SGS was in all populations associated with elevated inbreeding coefficients, potentially affecting the long‐term viability of these populations. In conclusion, this study illustrates the importance of studying SGS next to population genetic diversity and differentiation to adequately infer colonization patterns and long‐term establishment success of plant species.     

陕西省林麝mtDNA D-loop区序列结构和种群遗传多样性

林麝(Moschus berezovskii)曾广泛分布于中国,由于盗猎和栖息地缩小,秦岭地区野生种群数量迅速下降,圈养繁殖种群已成立了几十年,但大多数圈养种群的遗传背景不清,种群规模增长非常缓慢。为了给这一物种的保护和管理提供有用的信息,调查了陕西省林麝1个圈养种群3个野生种群线粒体DNA(mt DNA)D-Loop 632 bp片段的遗传多样性和种群结构。在69个个体中其碱基组成为A+T的平均含量63.2%高于G+C含量36.8%,共检测到变异位点171个(约占总位点数的27.05%)。核苷酸多样性(Pi)为0.04424,平均核苷酸差异数(K)为19.908。69个个体分属32个单倍型,单倍型间的平均遗传距离(P)为0.070。32个单倍型构建的NJ系统树聚为3个分支,4个林麝群体中的单倍型是随机分布的。4个群体的平均遗传距离为0.043(标准误SE为0.005),凤县养殖场群体与留坝和陇县群体的亲缘关系较远。单倍型间的平均遗传距离为0.043,可见其遗传分化尚未达到种群分化的水平。结果表明,陕西省林麝群体mt DNA D-loop区序列存在着较丰富的变异和遗传多样性,凤县野生群体和凤县养殖场群体的核苷酸多样性和单倍型多样较高,养殖场种群没有出现近亲繁殖及遗传多样性下降的情况。凤县野生群体和凤县养殖场群体两者遗传分化较小,存在着较高的基因流水平。     

Gene flow between populations of two invertebrates in springs

1. Using allozymes, we analysed genetic structure of the freshwater gastropod Bythinella dunkeri and the freshwater flatworm Crenobia alpina. The two species are habitat specialists, living almost exclusively in springs. The sampled area in Hesse (Germany) covers a spatial scale of 20 km and includes two river drainages. From the biology of the two species we expected little dispersal along rivers. However, the possibility exists that groundwater provide suitable pathways for dispersal. 2. In B. dunkeri heterozygosity decreased from west to east. For some alleles we found clines in this geographic direction. These clines generated a positive correlation between geographic distance and genetic differentiation. Furthermore patterns of genetic variation within populations suggested that populations may have been faced with bottlenecks and founder effects. If populations are not in population genetic equilibrium, such founder effects would also explain the rather high amount of genetic differentiation between populations (10%). 3. For C. alpina the mean number of alleles decreased with increasing isolation of populations. Genetic differentiation between populations contributed 19% to the total genetic variation. Genetic differentiation was not correlated to geographic distance, but compared with B. dunkeri variability of pairwise differentiation between pairs of populations was higher in C. alpina. 4. Overall B. dunkeri appears to be a fairly good disperser, which may use groundwater as dispersal pathway. Furthermore populations seem to be not in equilibrium. In contrast C. alpina forms rather isolated populations with little dispersal between springs and groundwater seems to play no important role for dispersal.     

From nature to the laboratory: the impact of founder effects on adaptation

Most founding events entail a reduction in population size, which in turn leads to genetic drift effects that can deplete alleles. Besides reducing neutral genetic variability, founder effects can in principle shift additive genetic variance for phenotypes that underlie fitness. This could then lead to different rates of adaptation among populations that have undergone a population size bottleneck as well as an environmental change, even when these populations have a common evolutionary history. Thus, theory suggests that there should be an association between observable genetic variability for both neutral markers and phenotypes related to fitness. Here, we test this scenario by monitoring the early evolutionary dynamics of six laboratory foundations derived from founders taken from the same source natural population of Drosophila subobscura. Each foundation was in turn three‐fold replicated. During their first few generations, these six foundations showed an abrupt increase in their genetic differentiation, within and between foundations. The eighteen populations that were monitored also differed in their patterns of phenotypic adaptation according to their immediately ancestral founding sample. Differences in early genetic variability and in effective population size were found to predict differences in the rate of adaptation during the first 21 generations of laboratory evolution. We show that evolution in a novel environment is strongly contingent not only on the initial composition of a newly founded population but also on the stochastic changes that occur during the first generations of colonization. Such effects make laboratory populations poor guides to the evolutionary genetic properties of their ancestral wild populations.     

Impact of founder population, drift and selection on the genetic diversity of a recently translocated tree population

Recently established, temperate tree populations combine a high level of differentiation for adaptive traits, suggesting rapid genetic evolution, with a high level of genetic diversity within population, suggesting a limited impact of genetic drift and purifying selection. To study experimentally the evolutionary forces in a recently established population, we assessed the spatial and temporal patterns of genetic diversity within a disjunct population of Cedrus atlantica established 140 years ago in south-eastern France from a North African source. The population is expanding through natural regeneration. Three generations were sampled, including founder trees. We analysed 12 isozyme loci, three of which were previously found in tight association with selected genes, and quantitative traits. No bottleneck effect was detected in the founder generation, but a simple test of allelic association revealed an initial disequilibrium which disappeared in the following generations. The impact of genetic drift during secondary evolution was limited, as suggested by the weak temporal differentiation. The genetic load was not reduced after 3 generations, and the quantitative variation for adaptive traits did not change either. Thus, initial genetic changes first proceed from a rapid re-organisation of the diversity through mating and recombination, whereas genetic erosion through drift and selection is delayed due to temporal and spatial stochasticity. Two life-history traits of trees contribute to slowing down the processes of genetic erosion: perenniality and large spatial scale. Thus, one would expect recently established tree populations to have a higher diversity than older ones, which seems in accordance with experimental surveys.