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 (similar to 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.