Lack of concordance between genetic diversity estimates at the molecular and quantitative-trait levels

In many applications of population genetics, particularly in the field of conservation biology, estimates of molecular diversity are used as surrogate indicators of less easily acquired measures of genetic variation for quantitative traits. The general validity of this approach to inferring levels of quantitative genetic variation within populations is called into question by the demonstration that estimates of molecular and quantitative-genetic variation are essentially uncorrelated in natural populations of Daphnia, one of the few organisms for which multiple estimates of both quantities are available. On the other hand, molecular measures of population subdivision seem to give conservatively low estimates of the degree of genetic subdivision at the level of quantitative traits. This suggests that although molecular markers provide little information on the level of genetic variation for quantitative traits within populations, they may be valid indicators of population subdivision for such characters.     

PATTERNS OF GENETIC ARCHITECTURE FOR LIFE-HISTORY TRAITS AND MOLECULAR MARKERS IN A SUBDIVIDED SPECIES

Abstract Understanding the utility and limitations of molecular markers for predicting the evolutionary potential of natural populations is important for both evolutionary and conservation genetics. To address this issue, the distribution of genetic variation for quantitative traits and molecular markers is estimated within and among 14 permanent lake populations of Daphnia pulicaria representing two regional groups from Oregon. Estimates of population subdivision for molecular and quantitative traits are concordant, with Q _ST generally exceeding G _ST. There is no evidence that microsatellites loci are less informative about subdivision for quantitative traits than are allozyme loci. Character-specific comparison of Q _ST and G _ST support divergent selection pressures among populations for the majority of life-history traits in both coast and mountain regions. The level of within-population variation for molecular markers is uninformative as to the genetic variation maintained for quantitative traits. In D. pulicaria , regional differences in the frequency of sex may contribute to variation in the maintenance of expressed within-population quantitative-genetic variation without substantially impacting diversity at the genic level. These data are compared to an identical dataset for 17 populations of the temporary-pond species, D. pulex .     

Molecular and Quantitative Genetic Differentiation in Sitobion avenae Populations from Both Sides of the Qinling Mountains

Quantitative trait differences are often assumed to be correlated with molecular variation, but the relationship is not certain, and empirical evidence is still scarce. To address this issue, we sampled six populations of the cereal aphid Sitobion avenae from areas north and south of the Qinling Mountains, and characterized their molecular variation at seven microsatellite loci and quantitative variation at nine life-history traits. Our results demonstrated that southern populations had slightly longer developmental times of nymphs but much higher lifetime fecundity, compared to northern populations. Of the nine tested quantitative characters, eight differed significantly among populations within regions, as well as between northern and southern regions. Genetic differentiation in neutral markers was likely to have been caused by founder events and drift. Increased subdivision for quantitative characters was found in northern populations, but reduced in southern populations. This phenomenon was not found for molecular characters, suggesting the decoupling between molecular and quantitative variation. The pattern of relationships between F_ST and Q_ST indicated divergent selection and suggested that local adaptation play a role in the differentiation of life-history traits in tested S. avenae populations, particularly in those traits closely related to reproduction. The main role of natural selection over genetic drift was also supported by strong structural differences in G-matrices among S. avenae populations. However, cluster analyses did not result in two groups corresponding to northern and southern regions. Genetic differentiation between northern and southern populations in neutral markers was low, indicating considerable gene flow between them. The relationship between molecular and quantitative variation, as well as its implications for differentiation and evolution of S. avenae populations, was discussed.     

The change in quantitative genetic variation with inbreeding

Inbreeding is known to reduce heterozygosity of neutral genetic markers, but its impact on quantitative genetic variation is debated. Theory predicts a linear decline in additive genetic variance (V(A)) with increasing inbreeding coefficient (F) when loci underlying the trait act additively, but a nonlinear hump-shaped relationship when dominance and epistasis are important. Predictions for heritability (h2) are similar, although the exact shape depends on the value of h2 in the absence of inbreeding. We located 22 published studies in which the level of genetic variation in experimentally inbred populations (measured by V(A) or h2) was compared with that in outbred control populations. For life-history traits, the data strongly supported a nonlinear change in genetic variation with increasing F. V(A) and h2 were, respectively, 244% and 50% higher at F = 0.4 than in outbred populations, and dominance plus epistatic variance together exceeded additive variance by a factor of four. For nonfitness traits the decline was linear and estimates of nonadditive variance were small. These results confirm that population bottlenecks frequently increase V(A) in some traits, and imply that life-history traits are underlain by substantial dominance or epistasis. However, the importance of drift-induced genetic variation in conservation or evolutionary biology is questionable, in part because inbreeding depression usually accompanies inbreeding.     

Distinguishing adaptive from nonadaptive genetic differentiation: comparison of Q(ST) and F(ST) at two spatial scales

Genetic differentiation in 20 hierarchically sampled populations of wild barley was analyzed with quantitative traits, allozymes and Random Amplified Polymorphic DNAs (RAPDs), and compared for three marker types at two hierarchical levels. Regional subdivision for both molecular markers was much lower than for quantitative traits. For both allozymes and RAPDs, most loci exhibited minor or no regional differentiation, and the relatively high overall estimates of the latter were due to several loci with exceptionally high regional differentiation. The allozyme- and RAPD-specific patterns of differentiation were concordant in general with one another, but not with quantitative trait differentiation. Divergent selection on quantitative traits inferred from very high regional Q(ST) was in full agreement with our previous results obtained from a test of local adaptation and multilevel selection analysis. In contrast, most variation in allozyme and RAPD variation was neutral, although several allozyme loci and RAPD markers were exceptional in their levels of regional differentiation. However, it is not possible to answer the question whether these exceptional loci are directly involved in the response to selection pressure or merely linked to the selected loci. The fact that Q(ST) and F(ST) did not differ at the population scale, that is, within regions, but differed at the regional scale, for which local adaptation has been previously shown, implies that comparison of the level of subdivision in quantitative traits, as compared with molecular markers, is indicative of adaptive population differentiation only when sampling is carried out at the appropriate scale.     

Loss of genetic variability in social spiders: genetic and phylogenetic consequences of population subdivision and inbreeding

The consequences of population subdivision and inbreeding have been studied in many organisms, particularly in plants. However, most studies focus on the short‐term consequences, such as inbreeding depression. To investigate the consequences of both population fragmentation and inbreeding for genetic variability in the longer term, we here make use of a natural inbreeding experiment in spiders, where sociality and accompanying population subdivision and inbreeding have evolved repeatedly. We use mitochondrial and nuclear data to infer phylogenetic relationships among 170 individuals of Anelosimus spiders representing 23 species. We then compare relative mitochondrial and nuclear genetic variability of the inbred social species and their outbred relatives. We focus on four independently derived social species and four subsocial species, including two outbred–inbred sister species pairs. We find that social species have 50% reduced mitochondrial sequence divergence. As inbreeding is not expected to reduce genetic variability in the maternally inherited mitochondrial genome, this suggests the loss of variation due to strong population subdivision, founder effects, small effective population sizes (colonies as individuals) and lineage turnover. Social species have < 10% of the nuclear genetic variability of the outbred species, also suggesting the loss of genetic variability through founder effects and/or inbreeding. Inbred sociality hence may result in reduction in variability through various processes. Sociality in most Anelosimus species probably arose relatively recently (0.1–2 mya), with even the oldest social lineages having failed to diversify. This is consistent with the hypothesis that inbred spider sociality represents an evolutionary dead end. Heterosis underlies a species potential to respond to environmental change and/or disease. Inbreeding and loss of genetic variability may thus limit diversification in social Anelosimus lineages and similarly pose a threat to many wild populations subject to habitat fragmentation or reduced population sizes.     

Latitudinal divergence of common frog (Rana temporaria) life history traits by natural selection: evidence from a comparison of molecular and quantitative genetic data

The relative roles of natural selection and direct environmental induction, as well as of natural selection and genetic drift, in creating clinal latitudinal variation in quantitative traits have seldom been assessed in vertebrates. To address these issues, we compared molecular and quantitative genetic differentiation between six common frog (Rana temporaria) populations along an approximately 1600 km long latitudinal gradient across Scandinavia. The degree of population differentiation (QST approximately 0.81) in three heritable quantitative traits (age and size at metamorphosis, growth rate) exceeded that in eight (neutral) microsatellite loci (FST = 0.24). Isolation by distance was clear for both neutral markers and quantitative traits, but considerably stronger for one of the three quantitative traits than for neutral markers. QST estimates obtained using animals subjected to different rearing conditions (temperature and food treatments) revealed some environmental dependency in patterns of population divergence in quantitative traits, but in general, these effects were weak in comparison to overall patterns. Pairwise comparisons of FST and QST estimates across populations and treatments revealed that the degree of quantitative trait differentiation was not generally predictable from knowledge of that in molecular markers. In fact, both positive and negative correlations were observed depending on conditions where the quantitative genetic variability had been measured. All in all, the results suggest a very high degree of genetic subdivision both in neutral marker genes and genes coding quantitative traits across a relatively recently (< 9000 years) colonized environmental gradient. In particular, they give evidence for natural selection being the primary agent behind the observed latitudinal differentiation in quantitative traits.     

Experimental microevolution: transplantation of pink salmon into the European North

Human-mediated translocations of species beyond their native ranges can enhance evolutionary processes in populations introduced to novel environments. We studied such processes in several generations of pink salmon Oncorhynchus gorbuscha introduced to the European North of Russia using a set of morphological and life-history traits as well as molecular genetic markers with different selective values: protein-coding loci, mtDNA, microsatellites, and MHC. The introduction of reproductively isolated pink salmon broodlines of odd and even years yielded different results. The odd-year broodline established self-reproducing local populations in many rivers of new range, but sustainable changes in external morphology, reproduction, and life-history, as well as the impoverishment of the gene pool occurred. Their successful colonisation of the new range resulted in specialisation manifested in the rapid directional shifts in some highly heritable phenotypic traits accompanied by increased homozygosity at molecular markers as a consequence of genetic drift and selective processes. The returns of transplanted pink salmon of even-year broodline decreased sharply already in the second generation, but there was no marked reduction of genetic diversity. Our data, as well as the analysis of the history of all pink salmon transplantations beyond the species range, demonstrate comparatively greater success of introduced odd-year broodline and permit to assume different adaptive plasticity of the even- and odd-year broodlines in pink salmon, what is most likely determined by differences in their evolutionary histories. Population genetic data suggest that the even-year broodline probably diverged from the odd-year broodline relatively recently and, due to the founder effect, may have lost a part of its genetic variation with which adaptive plasticity potential is associated.     

Divergent selection along climatic gradients in a rare central European endemic species,Saxifraga sponhemica

Background and Aims The effects of habitat fragmentation on quantitative genetic variation in plant populations are still poorly known. Saxifraga sponhemica is a rare endemic of Central Europe with a disjunct distribution, and a stable and specialized habitat of treeless screes and cliffs. This study therefore used S. sponhemica as a model species to compare quantitative and molecular variation in order to explore (1) the relative importance of drift and selection in shaping the distribution of quantitative genetic variation along climatic gradients; (2) the relationship between plant fitness, quantitative genetic variation, molecular genetic variation and population size; and (3) the relationship between the differentiation of a trait among populations and its evolvability.Methods Genetic variation within and among 22 populations from the whole distribution area of S. sponhemica was studied using RAPD (random amplified polymorphic DNA) markers, and climatic variables were obtained for each site. Seeds were collected from each population and germinated, and seedlings were transplanted into a common garden for determination of variation in plant traits.Key Results In contrast to previous results from rare plant species, strong evidence was found for divergent selection. Most population trait means of S. sponhemica were significantly related to climate gradients, indicating adaptation. Quantitative genetic differentiation increased with geographical distance, even when neutral molecular divergence was controlled for, and Q_ST exceeded F_ST for some traits. The evolvability of traits was negatively correlated with the degree of differentiation among populations (Q_ST), i.e. traits under strong selection showed little genetic variation within populations. The evolutionary potential of a population was not related to its size, the performance of the population or its neutral genetic diversity. However, performance in the common garden was lower for plants from populations with reduced molecular genetic variation, suggesting inbreeding depression due to genetic erosion.Conclusions The findings suggest that studies of molecular and quantitative genetic variation may provide complementary insights important for the conservation of rare species. The strong differentiation of quantitative traits among populations shows that selection can be an important force for structuring variation in evolutionarily important traits even for rare endemic species restricted to very specific habitats.     

Invasion success and genetic diversity of introduced populations of guppies Poecilia reticulata in Australia

High genetic diversity is thought to characterize successful invasive species, as the potential to adapt to new environments is enhanced and inbreeding is reduced. In the last century, guppies, Poecilia reticulata, repeatedly invaded streams in Australia and elsewhere. Quantitative genetic studies of one Australian guppy population have demonstrated high additive genetic variation for autosomal and Y-linked morphological traits. The combination of colonization success, high heritability of morphological traits, and the possibility of multiple introductions to Australia raised the prediction that neutral genetic diversity is high in introduced populations of guppies. In this study we examine genetic diversity at nine microsatellite and one mitochondrial locus for seven Australian populations. We used mtDNA haplotypes from the natural range of guppies and from domesticated varieties to identify source populations. There were a minimum of two introductions, but there was no haplotype diversity within Australian populations, suggesting a founder effect. This was supported by microsatellite markers, as allelic diversity and heterozygosity were severely reduced compared to one wild source population, and evidence of recent bottlenecks was found. Between Australian populations little differentiation of microsatellite allele frequencies was detected, suggesting that population admixture has occurred historically, perhaps due to male-biased gene flow followed by bottlenecks. Thus success of invasion of Australia and high additive genetic variance in Australian guppies are not associated with high levels of diversity at molecular loci. This finding is consistent with the release of additive genetic variation by dominance and epistasis following inbreeding, and with disruptive and negative frequency-dependent selection on fitness traits.     

Lack of genetic structure among ecologically adapted populations of an Australian rainforest Drosophila species as indicated by microsatellite markers and mitochondrial DNA sequences

Although fragmented rainforest environments represent hotspots for invertebrate biodiversity, few genetic studies have been conducted on rainforest invertebrates. Thus, it is not known if invertebrate species in rainforests are highly genetically fragmented, with the potential for populations to show divergent selection responses, or if there are low levels of gene flow sufficient to maintain genetic homogeneity among fragmented populations. Here we use microsatellite markers and DNA sequences from the mitochondrial ND5 locus to investigate genetic differences among Drosophila birchii populations from tropical rainforests in Queensland, Australia. As found in a previous study, mitochondrial DNA diversity was low with no evidence for population differentiation among rainforest fragments. The pattern of mitochondrial haplotype variation was consistent with D. birchii having undergone substantial past population growth. Levels of nuclear genetic variation were high in all populations while F(ST) values were very low, even for flies from geographically isolated areas of rainforest. No significant differentiation was observed between populations on either side of the Burdekin Gap (a long-term dry corridor), although there was evidence for higher gene diversity in low-latitude populations. Spatial autocorrelation coefficients were low and did not differ significantly from random, except for one locus which revealed a clinal-like pattern. Comparisons of microsatellite differentiation contrasted with previously established clinal patterns in quantitative traits in D. birchii, and indicate that the patterns in quantitative traits are likely to be due to selection. These results suggest moderate gene flow in D. birchii over large distances. Limited population structure in this species appears to be due to recent range expansions or cycles of local extinctions followed by recolonizations/expansions. Nevertheless, patterns of local adaptation have developed in D. birchii that may result in populations showing different selection responses when faced with environmental change.     

Molecular and quantitative genetic differentiation in European populations of Silene latifolia (Caryophyllaceae)

BACKGROUND AND AIMS: Among-population differentiation in phenotypic traits and allelic variation is expected as a consequence of isolation, drift, founder effects and local selection. Therefore, investigating molecular and quantitative genetic divergence is a pre-requisite for studies of local adaptation in response to selection under variable environmental conditions. METHODS: Among- and within-population variation were investigated in six geographically separated European populations of the white campion, Silene latifolia, both for molecular variation at six newly developed microsatellite loci and for quantitative variation in morphological and life-history traits. To avoid confounding effects of the maternal environment, phenotypic traits were measured on greenhouse-reared F(1) offspring. Tests were made for clinal variation, and the correlations among molecular, geographic and phenotypic distances were compared with Mantel tests. KEY RESULTS: The six populations of Silene latifolia investigated showed significant molecular and quantitative genetic differentiation. Geographic and phenotypic distances were significantly associated. Age at first flowering increased significantly with latitude and exhibited a Q(st) value of 0.17 in females and 0.10 in males, consistent with adaptation to local environmental conditions. By contrast, no evidence of isolation-by-distance and no significant association between molecular and phenotypic distances were found. CONCLUSIONS: Significant molecular genetic divergence among populations of Silene latifolia, from the European native range is consistent with known limited seed and pollen flow distances, while significant quantitative genetic divergence among populations and clinal variation for age at first flowering suggest local adaptation.     

Evidence for Fine-Scale Natal Homing Among Island Beach Spawning Sockeye Salmon, Oncorhynchus nerka

Salmonid fishes aggregate for breeding at spatially defined, suitable habitats. These aggregations may evolve into discrete populations when precise natal homing leads to reproductive isolation, and local regimes of selection lead to adaptation. Population structure is often defined by persistent differences in selectively neutral genetic markers and in mean values of morphological and life-history traits between locations. This approach is limited by the spatial scale at which traits diverge; low levels of reproductively successful straying, combined with similar selective pressures on life-history traits resulting from similar habitat features and environmental conditions, can significantly reduce the power of these discriminatory methods. We compared data on three life-history traits and polymorphism of DNA microsatellites for evidence of population subdivision among sockeye salmon spawning on spatially discrete but physically similar beaches on islands in Iliamna Lake, Alaska. We found small but significant differences in average body length, body depth and age composition between sites as well as significant interactions between site and year. These interactions, reflecting random variation in growth or recruitment among sites, are a powerful tool for discriminating populations with similar mean trait values. These results suggest fine-scale homing to natal sites, but the microsatellite data revealed no evidence of restricted gene flow among sites. There seems to be enough straying among the populations to prevent differentiation at neutral traits but enough homing for them to be functionally distinct.     

Random amplified polymorphic DNA (RAPD) and quantitative trait analyses across a major phylogeographical break in the Mediterranean ragwort Senecio gallicus Vill. (Asteraceae)

Random amplified polymorphic DNA (RAPD) and quantitative trait variation of the widespread and ephemeral Senecio gallicus were surveyed in 11 populations sampled from the Iberian Peninsula and southern France. The aim of the study was to compare population relationships and levels of geographical differentiation with chloroplast (cp) DNA and allozyme variation assessed previously in the same populations. Employing multivariate statistics, a moderate level of intraspecific differentiation was observed among populations from Iberian coastal and inland regions for both RAPDs and quantitative traits. However, RAPDs provided greater resolution in identifying additional population structure within the hypothesized, Pleistocene refugial source area of the species in coastal Iberia. A major part of the geographical subdivision in RAPD and quantitative traits was concordant with the coastal vs. inland divergence as previously inferred from cpDNA haplotype frequencies, but strongly contrasted with the geographical uniformity of the species for allozymes. This concordance across various nuclear and cytoplasmic markers (RAPDs/quantitative traits, cpDNA) suggests that geographical uniformity for allozymes is more attributable to low rates of evolution and/or small genome sampling rather than high rates of pollen dispersal, slow rates of nuclear lineage sorting, or indirect balancing selection. The present study underscores the value of using additional classes of nuclear markers for narrowing the numbers of competing causal hypotheses about intraspecific cpDNA-allozyme discrepancies and their underlying evolutionary processes.     

Inbreeding depression and male survivorship in Drosophila: implications for senescence theory

The extent to which inbreeding depression affects longevity and patterns of survivorship is an important issue from several research perspectives, including evolutionary biology, conservation biology, and the genetic analysis of quantitative traits. However, few previous inbreeding depression studies have considered longevity as a focal life-history trait. We maintained laboratory populations of Drosophila melanogaster at census population sizes of 2 and 10 male-female pairs for up to 66 generations and performed repeated assays of male survivorship throughout this time period. On average, significant levels of inbreeding depression were observed for median life span and age-specific mortality. For age-specific mortality, the severity of inbreeding depression increased over the life span. We found that a baseline inbreeding load of 0.307 lethal equivalents per gamete affected age-specific mortality, and that this value increased at a rate of 0.046 per day of the life span. With respect to some survivorship parameters, the differentiation of lineages was nonlinear with respect to the inbreeding coefficient, which suggested that nonadditive genetic variation contributed to variation among lineages. These findings provide insights into the genetic basis of longevity as a quantitative trait and have implications regarding the mutation-accumulation evolutionary explanation of senescence.     

Pleiotropic quantitative trait loci contribute to population divergence in traits associated with life-history variation in Mimulus guttatus

Evolutionary biologists seek to understand the genetic basis for multivariate phenotypic divergence. We constructed an F2 mapping population (N = 539) between two distinct populations of Mimulus guttatus. We measured 20 floral, vegetative, and life-history characters on parents and F1 and F2 hybrids in a common garden experiment. We employed multitrait composite interval mapping to determine the number, effect, and degree of pleiotropy in quantitative trait loci (QTL) affecting divergence in floral, vegetative, and life-history characters. We detected 16 QTL affecting floral traits; 7 affecting vegetative traits; and 5 affecting selected floral, vegetative, and life-history traits. Floral and vegetative traits are clearly polygenic. We detected a few major QTL, with all remaining QTL of small effect. Most detected QTL are pleiotropic, implying that the evolutionary shift between these annual and perennial populations is constrained. We also compared the genetic architecture controlling floral trait divergence both within (our intraspecific study) and between species, on the basis of a previously published analysis of M. guttatus and M. nasutus. Eleven of our 16 floral QTL map to approximately the same location in the interspecific map based on shared, collinear markers, implying that there may be a shared genetic basis for floral divergence within and among species of Mimulus.     

Can population genetic structure be predicted from life-history traits?

Population genetic structure is a key parameter in evolutionary biology. Earlier comparative studies have shown that genetic structure depends on species ecological attributes and life-history traits, but species phylogenetic relatedness had not been accounted for. Here we reevaluate the relationships between genetic structure and species traits in seed plants. Each species is characterized by a set of life-history and ecological features as well as by its geographic range size, its heterozygote deficit, and its genetic structure at nuclear and organelle markers to distinguish between pollen- and seed-mediated gene flow. We use both a conventional regression approach and a method that controls for phylogenetic relationships. Once phylogenetic conservatism and covariation among traits are taken into account, genetic structure is shown to be related with only a few synthetic traits, such as mating system for nuclear markers and seed dispersal mode or geographic range size for organelle markers. Along with other studies on invasiveness or rarity, our work illustrates the fact that predicting the fate of species across a broad taxonomic assemblage on the basis of simple traits is rarely possible, a testimony of the highly contingent nature of evolution.     

A quantitative review of heterozygosity–fitness correlations in animal populations

The ease of obtaining genotypic data from wild populations has renewed interest in the relationship between individual genetic diversity and fitness-related traits (heterozygosity–fitness correlations, or HFC). Here we present a comprehensive meta-analysis of HFC studies using powerful multivariate techniques which account for nonindependence of data. We compare these findings with those from univariate techniques, and test the influence of a range of factors hypothesized to influence the strength of HFCs. We found small but significantly positive effect sizes for life-history, morphological, and physiological traits; while theory predicts higher mean effect sizes for life-history traits, effect size did not differ consistently with trait type. Newly proposed measures of variation were no more powerful at detecting relationships than multilocus heterozygosity, and populations predicted to have elevated inbreeding variance did not exhibit higher mean effect sizes. Finally, we found evidence for publication bias, with studies reporting weak, nonsignificant effects being under-represented in the literature. In general, our review shows that HFC studies do not generally reveal patterns predicted by population genetic theory, and are of small effect (less than 1% of the variance in phenotypic characters explained). Future studies should use more genetic marker data and utilize sampling designs that shed more light on the biological mechanisms that may modulate the strength of association, for example by contrasting the strength of HFCs in mainland and island populations of the same species, investigating the role of environmental stress, or by considering how selection has shaped the traits under investigation.     

Estimating inbreeding depression in natural plant populations using quantitative and molecular data

Inbreeding and inbreeding depression are important issues in the biology and conservation of natural plant and animal populations, primarily when subpopulation sizes are reduced due to habitat fragmentation. In this study, we propose a method for estimating inbreeding depression in progenies of natural plant populations, combining the estimation of the fixation index by codominant markers with the experimental evaluation of quantitative traits. Our technique estimates apparent inbreeding depression in structured natural populations using the linear regression of phenotypic means on the inbreeding coefficients estimated with codominant markers. This method was applied to data from 112 maternal progenies of 10 natural subpopulations of Eugenia dysenterica DC, a fruiting tree species from the Brazilian savanna (Cerrado). The results show that the proposed method was efficient at detecting the presence of inbreeding depression for seedling emergence and initial growth traits in the species. This corroborates the importance of maintaining high levels of heterozygosity for in situ conservation or genetic restoration of natural populations.     

Quantitative and molecular genetic variation in sympatric populations of Medicago laciniata and M. truncatula (Fabaceae): relationships with eco-geographical factors

Medicago laciniata is restricted to south of the Mediterranean basin and it extends in Tunisia from the inferior semi-arid to Saharan stages, whereas M. truncatula is a widespread species in such areas. The genetic variability in four Tunisian sympatric populations of M. laciniata and M. truncatula was analysed using 19 quantitative traits and 20 microsatellites. We investigated the amplification transferability of 52 microsatellites developed in M. truncatula to M. laciniata. Results indicate that about 78.85% of used markers are valuable genetic markers for M. laciniata. M. laciniata displayed significantly lower quantitative differentiation among populations (QST=0.12) than did M. truncatula (QST=0.45). However, high molecular differentiations, with no significant difference, were observed in M. laciniata (FST=0.48) and M. truncatula (FST=0.47). Several quantitative traits exhibited significantly smaller QST than FST for M. laciniata, consistent with constraining selection. For M. truncatula, the majority of traits displayed no statistical difference in the level of QST and FST. Furthermore, these traits are significantly associated with eco-geographical factors, consistent with selection for local adaptation rather than genetic drift. In both species, there was no significant correlation between genetic variation at quantitative traits and molecular markers. The site-of-origin explains about 5.85% and 11.27% of total quantitative genetic variability among populations of M. laciniata and M. truncatula, respectively. Established correlations between quantitative traits and eco-geographical factors were generally more moderate for M. laciniata than for M. truncatula, suggesting that the two species exhibit different genetic bases of local adaptation to varying environmental conditions. Nevertheless, no consistent patterns of associations were found between gene diversity (He) and environmental factors in either species.