Genetics, evolution, and adaptive significance of the selfing syndrome in the genus Capsella

The change from outbreeding to selfing is one of the most frequent evolutionary transitions in flowering plants. It is often accompanied by characteristic morphological and functional changes to the flowers (the selfing syndrome), including reduced flower size and opening. Little is known about the developmental and genetic basis of the selfing syndrome, as well as its adaptive significance. Here, we address these issues using the two closely related species Capsella grandiflora (the ancestral outbreeder) and red shepherd's purse (Capsella rubella, the derived selfer). In C. rubella, petal size has been decreased by shortening the period of proliferative growth. Using interspecific recombinant inbred lines, we show that differences in petal size and flower opening between the two species each have a complex genetic basis involving allelic differences at multiple loci. An intraspecific cross within C. rubella suggests that flower size and opening have been decreased in the C. rubella lineage before its extensive geographical spread. Lastly, by generating plants that likely resemble the earliest ancestors of the C. rubella lineage, we provide evidence that evolution of the selfing syndrome was at least partly driven by selection for efficient self-pollination. Thus, our studies pave the way for a molecular dissection of selfing-syndrome evolution.     

Genetic architecture and adaptive significance of the selfing syndrome in Capsella

The transition from outcrossing to predominant self-fertilization is one of the most common evolutionary transitions in flowering plants. This shift is often accompanied by a suite of changes in floral and reproductive characters termed the selfing syndrome. Here, we characterize the genetic architecture and evolutionary forces underlying evolution of the selfing syndrome in Capsella rubella following its recent divergence from the outcrossing ancestor C. grandiflora. We conduct genotyping by multiplexed shotgun sequencing and map floral and reproductive traits in a large (N= 550) F2 population. Our results suggest that in contrast to previous studies of the selfing syndrome, changes at a few loci, some with major effects, have shaped the evolution of the selfing syndrome in Capsella. The directionality of QTL effects, as well as population genetic patterns of polymorphism and divergence at 318 loci, is consistent with a history of directional selection on the selfing syndrome. Our study is an important step toward characterizing the genetic basis and evolutionary forces underlying the evolution of the selfing syndrome in a genetically accessible model system.     

Consequences of multiple mating‐system shifts for population and range‐wide genetic structure in a coastal dune plant

Evolutionary transitions from outcrossing to selfing can strongly affect the genetic diversity and structure of species at multiple spatial scales. We investigated the genetic consequences of mating‐system shifts in the North American, Pacific coast dune endemic plant Camissoniopsis cheiranthifolia (Onagraceae) by assaying variation at 13 nuclear (n) and six chloroplast (cp) microsatellite (SSR) loci for 38 populations across the species range. As predicted from the expected reduction in effective population size (N_e) caused by selfing, small‐flowered, predominantly selfing (SF) populations had much lower nSSR diversity (but not cpSSR) than large‐flowered, predominantly outcrossing (LF) populations. The reduction in nSSR diversity was greater than expected from the effects of selfing on N_e alone, but could not be accounted for by indirect effects of selfing on population density. Although selfing should reduce gene flow, SF populations were not more genetically differentiated than LF populations. We detected five clusters of nSSR genotypes and three groups of cpSSR haplotypes across the species range consisting of parapatric groups of populations that usually (but not always) differed in mating system, suggesting that selfing may often initiate ecogeographic isolation. However, lineage‐wide genetic variation was not lower for selfing clusters, failing to support the hypothesis that selection for reproductive assurance spurred the evolution of selfing in this species. Within three populations where LF and SF plants coexist, we detected genetic differentiation among diverged floral phenotypes suggesting that reproductive isolation (probably postzygotic) may help maintain the striking mating‐system differentiation observed across the range of this species.     

A Markov chain Monte Carlo approach for joint inference of population structure and inbreeding rates from multilocus genotype data

Nonrandom mating induces correlations in allelic states within and among loci that can be exploited to understand the genetic structure of natural populations (Wright 1965). For many species, it is of considerable interest to quantify the contribution of two forms of nonrandom mating to patterns of standing genetic variation: inbreeding (mating among relatives) and population substructure (limited dispersal of gametes). Here, we extend the popular Bayesian clustering approach STRUCTURE (Pritchard et al. 2000) for simultaneous inference of inbreeding or selfing rates and population-of-origin classification using multilocus genetic markers. This is accomplished by eliminating the assumption of Hardy-Weinberg equilibrium within clusters and, instead, calculating expected genotype frequencies on the basis of inbreeding or selfing rates. We demonstrate the need for such an extension by showing that selfing leads to spurious signals of population substructure using the standard STRUCTURE algorithm with a bias toward spurious signals of admixture. We gauge the performance of our method using extensive coalescent simulations and demonstrate that our approach can correct for this bias. We also apply our approach to understanding the population structure of the wild relative of domesticated rice, Oryza rufipogon, an important partially selfing grass species. Using a sample of n = 16 individuals sequenced at 111 random loci, we find strong evidence for existence of two subpopulations, which correlates well with geographic location of sampling, and estimate selfing rates for both groups that are consistent with estimates from experimental data (s approximately 0.48-0.70).     

Reproductive biology and population genetic structure of Kandelia candel (Rhizophoraceae), a viviparous mangrove species

The pollination biology, mating system, and population genetic structure of Kandelia candel were investigated. Field observations on its pollination and reproductive biology suggested that this species is pollinator dependent for fruit set, and bee activities can lead to substantial geitonogamous selfing. Quantitative analysis of the mating system parameters was performed using progeny arrays assayed for allozyme markers. Multilocus outcrossing rates (t(m)) were estimated to be 0.697 ± 0.091 and 0.797 ± 0.062 in two populations. In comparison to other plant species with mixed-mating system, the level of allozyme variation was very low in the 13 populations sampled along the coastlines of Hong Kong. At the species level, the proportion of polymorphic loci was 20%, number of alleles per locus was 1.2, and heterozygosity was 0.0362. The total gene diversity was primarily distributed within populations (H(S )= 0.0339), and the coefficient of genetic differentiation among populations was low (G(ST )= 0.064). This pattern of population genetic structure suggests that gene flow, primarily in the form of water-dispersed seedlings in viviparous mangrove species, is not as limited as previously thought. However, microgeographic pattern in allele frequency at the marker loci could still be detected between the western and eastern coastal populations.     

Palynology and intergeneric relationships in subtribe Hyoseridinae (CompositaerLactuceae)

Ceratopteris pteridoides is an inbreeding species. Intralocus homozygosity is deduced from the almost total absence of genetic load (recessive deleterious and lethal genes) in sporophytes from natural populations, and from gametangial sequences which indicate intragametophytic selfing. This mating system and the profound capacity of the species for vegetative reproduction, coupled with the small size and geographical isolation of many populations, strongly support the hypothesis of individual homozygosity. However, polymorphism is exhibited both within and between populations in specific, genetically mediated, gametophytic characteristics. The population genetic structure of C. pteridoides is similar to other inbreeding species.     

Genetic variation and the mating system in the rare Acacia sciophanes compared with its common sister species Acacia anfractuosa (Mimosaceae)

Acacia sciophanes is an extremely rare and Critically Endangered species known from two small populations separated by less than 7 km. Specifically we aimed to investigate whether rarity in A. sciophanes is associated with decreased levels of genetic variation and increased levels of selfing by comparing patterns of genetic variation and mating system parameters with its widespread and common sister species A. anfractuosa. Fourteen polymorphic allozyme loci were used to assess genetic diversity with four of these used in the estimation of mating system parameters. At the species level A. sciophanes has lower allelic richness, polymorphism, observed heterozygosity and gene diversity than A. anfractuosa and significantly lower levels of gene diversity at the population level. Both species have a mixed mating system but the largest population of A. sciophanes has lower levels of outcrossing, higher correlated paternity and increased bi-parental inbreeding compared with two A. anfractuosa populations. However, both correlated paternity and bi-parental inbreeding appear to be at least partly influenced by population size regardless of the species. We suggest that A. sciophanes is likely to be an intrinsically rare species and that in particular the lower levels of genetic diversity and increased selfing are a feature of a species that has the ability to persist in a few localised small populations. Despite recent extensive habitat destruction our comparative study provided no clear evidence that such events have contributed to the lower genetic diversity and increased selfing in A. sciophanes and we believe its ability to exist in small populations may not only be an important factor in its survival as a rare species but also indicates that it may be less susceptible to the impacts of habitat loss and fragmentation. The key to this species conservation will be the maintenance of suitable habitat, particularly through improved fire regimes and control of invasive weeds, that will allow the two small populations to continue to persist in extremely restricted areas of remnant vegetation.     

High selfing and high inbreeding depression in peripheral populations of Juncus atratus

The mating system of a plant is the prime determinant of its population genetic structure. However, mating system effects may be modified by postzygotic mechanisms like inbreeding depression. Furthermore, historical as well as contemporary ecological factors and population characteristics, like the location within the species range can contribute to genetic variability. Using microsatellite markers we assessed the population genetic structure of the wind-pollinated Juncus atratus in 16 populations from peripheral and nearly central areas of the distribution range and studied the mating system of the species. In three peripheral populations, outcrossing rates at seeds stage were low (mean t(m) = 5.6%), suggesting a highly autogamous mating system. Despite this fact, on adult stage both individual heterozygosity (mean H(O) = 0.48) and gene diversity (mean H(E) = 0.58) were high even in small populations. Inbreeding coefficients were consistently low among all populations (mean F(IS) = 0.15). Within the three peripheral populations indirect estimates of lifetime inbreeding depression were surprisingly high (delta(eq) = 0.96) and inbreeding depression could be shown to act mostly on early seedling establishment. Similar conditions of autogamy combined with high inbreeding depression are typical for plants with a large lifetime genomic mutation rate that cannot avoid selfing by geitonogamy. However, the results presented here are unexpected for small-statured, herbaceous plants. Substantial genetic differentiation among all populations was found (mean F(ST) = 0.24). An isolation-by-distance pattern was apparent on large scale but not on local scale suggesting that the overall pattern was largely influenced by historical factors, e.g. colonization, whereas locally genetic drift was of greater importance than gene flow. Peripheral populations exhibited lower genetic diversity and higher inbreeding coefficients when compared with subcentral populations.     

Signatures of balancing selection are maintained at disease resistance loci following mating system evolution and a population bottleneck in the genus Capsella

ABSTRACT: BACKGROUND: Population bottlenecks can lead to a loss of variation at disease resistance loci, which could have important consequences for the ability of populations to adapt to pathogen pressure. Alternatively, current or past balancing selection could maintain high diversity, creating a strong heterogeneity in the retention of polymorphism across the genome of bottlenecked populations. We sequenced part of the LRR region of 9 NBS-LRR disease resistance genes in the outcrossing Capsella grandiflora and the recently derived, bottlenecked selfing species Capsella rubella, and compared levels and patterns of nucleotide diversity and divergence with genome-wide reference loci. RESULTS: In strong contrast with reference loci, average diversity at resistance loci was comparable between C. rubella and C. grandiflora, primarily due to two loci with highly elevated diversity indicative of past or present balancing selection. Average between-species differentiation was also reduced at the set of R-genes compared with reference loci, which is consistent with the maintenance of ancestral polymorphism. CONCLUSIONS: Historical or ongoing balancing selection on plant disease resistance genes is a likely contributor to the retention of ancestral polymorphism in some regions of the bottlenecked Capella rubella genome.     

The Arabidopsis genome sequence as a tool for genome analysis in Brassicaceae. A comparison of the Arabidopsis and Capsella rubella genomes

The annotated Arabidopsis genome sequence was exploited as a tool for carrying out comparative analyses of the Arabidopsis and Capsella rubella genomes. Comparison of a set of random, short C. rubella sequences with the corresponding sequences in Arabidopsis revealed that aligned protein-coding exon sequences differ from aligned intron or intergenic sequences in respect to the degree of sequence identity and the frequency of small insertions/deletions. Molecular-mapped markers and expressed sequence tags derived from Arabidopsis were used for genetic mapping in a population derived from an interspecific cross between Capsella grandiflora and C. rubella. The resulting eight Capsella linkage groups were compared to the sequence maps of the five Arabidopsis chromosomes. Fourteen colinear segments spanning approximately 85% of the Arabidopsis chromosome sequence maps and 92% of the Capsella genetic linkage map were detected. Several fusions and fissions of chromosomal segments as well as large inversions account for the observed arrangement of the 14 colinear blocks in the analyzed genomes. In addition, evidence for small-scale deviations from genome colinearity was found. Colinearity between the Arabidopsis and Capsella genomes is more pronounced than has been previously reported for comparisons between Arabidopsis and different Brassica species.     

Strong isolation by distance among local populations of an endangered butterfly species (Euphydryas aurinia)

The marsh fritillary (Euphydryas aurinia) is a critically endangered butterfly species in Denmark known to be particularly vulnerable to habitat fragmentation due to its poor dispersal capacity. We identified and genotyped 318 novel SNP loci across 273 individuals obtained from 10 small and fragmented populations in Denmark using a genotyping‐by‐sequencing (GBS) approach to investigate its population genetic structure. Our results showed clear genetic substructuring and highly significant population differentiation based on genetic divergence (F _ST) among the 10 populations. The populations clustered in three overall clusters, and due to further substructuring among these, it was possible to clearly distinguish six clusters in total. We found highly significant deviations from Hardy–Weinberg equilibrium due to heterozygote deficiency within every population investigated, which indicates substructuring and/or inbreeding (due to mating among closely related individuals). The stringent filtering procedure that we have applied to our genotype quality could have overestimated the heterozygote deficiency and the degree of substructuring of our clusters but is allowing relative comparisons of the genetic parameters among clusters. Genetic divergence increased significantly with geographic distance, suggesting limited gene flow at spatial scales comparable to the dispersal distance of individual butterflies and strong isolation by distance. Altogether, our results clearly indicate that the marsh fritillary populations are genetically isolated. Further, our results highlight that the relevant spatial scale for conservation of rare, low mobile species may be smaller than previously anticipated.     

THE DISTRIBUTION OF SELFING RATES IN HOMOSPOROUS FERNS

The models of Lande and Schemske predict that among species in which the selfing rate is largely under genetic control and not subject to tremendous environmental variation, the distribution of selfing rates should be bimodal. When this prediction was tested empirically using data from the literature for species of angiosperms and gymnosperms, the distribution of outcrossing rates for all species was clearly bimodal. To provide another empirical test of the prediction, we analyzed mating-system data for 20 species of Pteridophyta (ferns). Homosporous ferns and their allies are unique among vascular plants because three types of mating are possible: intragametophytic selfing (selfing of an individual gametophyte); intergametophytic selfing (analogous to selfing in seed plants); and intergametophytic crossing (analogous to outcrossing in seed plants). The distribution of intragametophytic selfing rates among species of homosporous ferns is clearly uneven. Most species of homosporous ferns would be classified as extreme outcrossers. In contrast, a few species are nearly exclusively inbreeding. In only a few populations of Dryopteris expansa and Hemionitis palmata and a single population of Blechnum spicant do we see convincing evidence of a mixed mating system. The uneven distribution of selfing rates we observed for homosporous ferns, coupled with a corresponding bimodality of the magnitude of genetic load, strongly supports the model.     

Hidden genetic variance contributes to increase the short‐term adaptive potential of selfing populations

Standing genetic variation is considered a major contributor to the adaptive potential of species. The low heritable genetic variation observed in self‐fertilizing populations has led to the hypothesis that species with this mating system would be less likely to adapt. However, a non‐negligible amount of cryptic genetic variation for polygenic traits, accumulated through negative linkage disequilibrium, could prove to be an important source of standing variation in self‐fertilizing species. To test this hypothesis, we simulated populations under stabilizing selection subjected to an environmental change. We demonstrate that, when the mutation rate is high (but realistic), selfing populations are better able to store genetic variance than outcrossing populations through genetic associations, notably due to the reduced effective recombination rate associated with predominant selfing. Following an environmental shift, this diversity can be partially remobilized, which increases the additive variance and adaptive potential of predominantly (but not completely) selfing populations. In such conditions, despite initially lower observed genetic variance, selfing populations adapt as readily as outcrossing ones within a few generations. For low mutation rates, purifying selection impedes the storage of diversity through genetic associations, in which case, as previously predicted, the lower genetic variance of selfing populations results in lower adaptability compared to their outcrossing counterparts. The population size and the mutation rate are the main parameters to consider, as they are the best predictors of the amount of stored diversity in selfing populations. Our results and their impact on our knowledge of adaptation under high selfing rates are discussed.     

Genetic Diversity and Population Structure in Polygonum cespitosum: Insights to an Ongoing Plant Invasion

Molecular markers can help elucidate how neutral evolutionary forces and introduction history contribute to genetic variation in invaders. We examined genetic diversity, population structure and colonization patterns in the invasive Polygonum cespitosum, a highly selfing, tetraploid Asian annual introduced to North America. We used nine diploidized polymorphic microsatellite markers to study 16 populations in the introduced range (northeastern North America), via the analyses of 516 individuals, and asked the following questions: 1) Do populations have differing levels of within-population genetic diversity? 2) Do populations form distinct genetic clusters? 3) Does population structure reflect either geographic distances or habitat similarities? We found low heterozygosity in all populations, consistent with the selfing mating system of P. cespitosum. Despite the high selfing levels, we found substantial genetic variation within and among P. cespitosum populations, based on the percentage of polymorphic loci, allelic richness, and expected heterozygosity. Inferences from individual assignment tests (Bayesian clustering) and pairwise F _ST values indicated high among-population differentiation, which indicates that the effects of gene flow are limited relative to those of genetic drift, probably due to the high selfing rates and the limited seed dispersal ability of P. cespitosum. Population structure did not reflect a pattern of isolation by distance nor was it related to habitat similarities. Rather, population structure appears to be the result of the random movement of propagules across the introduced range, possibly associated with human dispersal. Furthermore, the high population differentiation, genetic diversity, and fine-scale genetic structure (populations founded by individuals from different genetic sources) in the introduced range suggest that multiple introductions to this region may have occurred. High genetic diversity may further contribute to the invasive success of P. cespitosum in its introduced range.     

普通野生稻小种群的交配系统与遗传多样性

小种群的遗传动态是保育遗传学关注的核心问题之一,而种群遗传动态又与交配系统密切相关.普通野生稻(Oryza rufipogon Griff.)是具有重要经济价值的濒危物种,目前其种群规模都较小,研究其小种群交配系统与遗传变异性对普通野生稻的保护具有重要意义.运用7对SSR引物,对采自江西东乡普通野生稻小种群的36份种茎和其中20个家系共计601份子代进行了分析.结果显示:该种群的表观异交率为0.318,多位点法估计(MLTR)的多位点异交率为0.481;50%以上的子代共享亲本,非随机交配明显;东乡普通野生稻种群交配系统属于混合交配类型.比较亲本和子代种群的遗传变异性显示:子代种群比亲本种群遗传变异性更丰富;子代种群的杂合子不足与种群变小自交比例上升有关;而亲本种群杂合子过剩可能与杂合基因型的选择优势有关.这些结果说明创造条件扩大种群规模对普通野生稻的原生境保护显得尤为重要.     

Increased selfing and decreased effective pollen donor number in peripheral relative to central populations in Picea sitchensis (Pinaceae)

Because mating system can be influenced by effective neighborhood size, density, and isolation, populations at range peripheries may differ from those in the center. The importance of peripheral populations to conservation and evolution is controversial, and additional information about their genetic structure and evolutionary dynamics will inform conservation strategies. In wind-pollinated species, selfing rate is generally negatively correlated with population size and density, and inbreeding may therefore increase toward range peripheries. Picea sitchensis has a long and narrow range along the Pacific Coast of North America that tapers toward the northern and southern peripheries. We investigated whether central and peripheral populations differ in mating system parameters. The results suggest that population position within the range has a strong effect on mating system, and geographic isolation appears to be associated with higher selfing. The estimated effective number of pollen donors was much higher in the center of the range (mean = 18.5) than at the periphery (mean = 3.6), while selfing rate increased from 7.3% in central populations to as high as 35.2% in the northern, isolated population. These strong geographical patterns suggest mating system is influenced by both population size and isolation at range peripheries.     

The evolution and maintenance of monoecy and dioecy in Sagittaria latifolia (Alismataceae)

Plant species rarely exhibit both monoecious and dioecious sexual systems. This limits opportunities to investigate the consequences of combined versus separate sex function on mating patterns and genetic variation and the analysis of factors responsible for the evolution and maintenance of the two sexual systems. Populations of the North American clonal aquatic Sagittaria latifolia are usually either monoecious or dioecious and often grow in close geographic proximity. We investigated mating patterns, genetic structure, and relationships between the two sexual systems using allozyme variation in populations from southern Ontario, Canada. As predicted, selfing rates in monoecious populations (n = 6, mean = 0.41) were significantly higher than in dioecious populations (n = 6, mean = 0.11). Moreover, marker-based estimates of inbreeding depression (delta) indicated strong selection against inbred offspring in both monoecious (mean delta = 0.83) and dioecious (mean delta = 0.84) populations. However, the difference in selfing rate between the sexual systems was not reflected in contrasting levels of genetic variation. Our surveys of 12 loci in 15 monoecious and 11 dioecious populations revealed no significant differences in the proportion of polymorphic loci (P), number of alleles per locus (A), or observed and expected heterozygosity (H(o) and H(e), respectively). Strong inbreeding depression favoring survival of outcrossed offspring may act to maintain similar levels of diversity between monoecious and dioecious populations. Despite geographical overlap between the two sexual systems in southern Ontario, a dendrogram of genetic relationships indicated two distinct clusters of populations largely corresponding to monoecious and dioecious populations. Reproductive isolation between monoecious and dioecious populations appears to be governed, in part, by observed differences in habitat and life history. We suggest that selfing and inbreeding depression in monoecious populations are important in the transition from monoecy to dioecy and that the maintenance of distinct sexual systems in S. latifolia is governed by interactions between ecology, life history, and mating.     

Combined effects of bottlenecks and selfing in populations of Corella eumyota, a recently introduced sea squirt in the English Channel

The success of an exotic species depends notably on its capacity to initiate a new population from a few individuals, to survive genetic bottlenecks and to adapt locally. Species with multiple reproductive strategies (e.g. mixed-mating system with both self- and cross-fertilization) can be efficient colonizers. Herein we focus on Corella eumyota , an exotic ascidian that has rapidly invaded English Channel coasts in recent years. Interestingly, this brooding hermaphroditic ascidian is capable of self-fertilization in the laboratory. We developed 12 microsatellite markers from an enriched library of genomic DNA to investigate the level of inbreeding and selfing in two putatively native populations (South Africa, N  = 34, and New Zealand, N  = 28) and to examine if founder effects were possibly associated with its recent introduction in two French populations (Perros-Guirec, N  = 22 and Brest; N  = 25). Genetic polymorphism was very low in both native populations (i.e. less than 60% of the loci were polymorphic) and even lower in the introduced populations, one of which was monomorphic at all loci, suggesting a recent bottleneck. F _is and a new method based on multi-locus heterozygosity were used to provide estimates of inbreeding. A high selfing rate was inferred in the South Africa population with both methods ( s  = 0.90), whereas in the other native population (New Zealand) a lower but significant estimate of selfing rate ( s  = 0.29) was obtained with the multi-locus method. This variability of population selfing rate might be explained by a mixed-mating system, allowing C. eumyota to reproduce through inbreeding and outbreeding according to mating possibilities; this trait may have favoured the rapid establishment of new populations in Europe.     

Genetic entities and mating system in hermaphroditic Fucus spiralis and its close dioecious relative F. vesiculosus (Fucaceae, Phaeophyceae)

To date, molecular markers have not settled the question of the specific status of the closely related, but phylogenetically unresolved, brown seaweeds, hermaphroditic Fucus spiralis and dioecious Fucus vesiculosus, nor their propensity for natural hybridization. To test the degree of species integrity and to assess effect of the mating system on the population genetic structure, 288 individuals coming from parapatric (discontinuous) and sympatric (contiguous) spatial configurations at two sites were genotyped with five microsatellite loci. Using a Bayesian admixture analysis, our results show that F. spiralis and F. vesiculosus comprise clearly distinct genetic entities (clusters) generally characterized by cosexual and unisexual individuals, respectively. Genetic diversity within each entity suggests that F. spiralis reproduces primarily through selfing while F. vesiculosus is characterized by an endogamous breeding regime. Nevertheless, aberrant sexual phenotypes were observed in each cluster, no diagnostic alleles were revealed and 10% of study individuals were intermediate between the two genetic entities. This pattern can be explained by recent divergence of two taxa with retention of ancestral polymorphism or asymmetrical, introgressive hybridization. However, given (i) coincident monomorphism at three loci in spiralis clusters and (ii) that significantly more intermediates were observed in sympatric stations than in parapatric stations, we argue that interspecific gene flow has occurred after divergence of the two taxa. Finally, we show that whether recently separated or recently introgressive, the divergent breeding systems probably contribute to species integrity in these two taxa.     

How multilocus genotypic pattern helps to understand the history of selfing populations: a case study in Medicago truncatula

The occurrence of populations exhibiting high genetic diversity in predominantly selfing species remains a puzzling question, since under regular selfing genetic diversity is expected to be depleted at a faster rate than under outcrossing. Fine-scale population genetics approaches may help to answer this question. Here we study a natural population of the legume Medicago truncatula in which both the fine-scale spatial structure and the selfing rate are characterized using three different methods. Selfing rate estimates were very high ( approximately 99%) irrespective of the method used. A clear pattern of isolation by distance reflecting small seed dispersal distances was detected. Combining genotypic data over loci, we could define 34 multilocus genotypes. Among those, six highly inbred genotypes (lines) represented more than 75% of the individuals studied and harboured all the allelic variation present in the population. We also detected a large set of multilocus genotypes resembling recombinant inbred lines between the most frequent lines occurring in the population. This finding illustrates the importance of rare recombination in redistributing available allelic diversity into new genotypic combinations. This study shows how multilocus and fine-scale spatial analyses may help to understand the population history of self-fertilizing species, especially to make inferences about the relative role of foundation/migration and recombination events in such populations.