HIGH-LEVEL OF POLYMORPHISM AND SPATIAL STRUCTURE IN A SELFING PLANT-SPECIES, MEDICAGO-TRUNCATULA (LEGUMINOSAE), SHOWN USING RAPD MARKERS

Using RAPD markers and one morphological marker, we studied the among- and within-population structure in a selfing annual plant species, Me dicago truncatula GAERTN. About 200 individuals, sampled from four pop ulations subdivided into three subpopulations each, were scored for 22 markers. It was found that the within-population variance component a ccounted for 55% of the total variance, while the among-population var iance component accounted for 45%. Eighteen percent of the total varia nce was due to within-population structure (i.e., among subpopulations ). Thus, 37% of the total variance was within subpopulations. Using a multilocus approach, it was found that no multilocus genotype was comm on to two populations. Two of the four studied populations were compos ed of few (less than or equal to 6) multilocus genotypes, whereas the other two had many (greater than or equal to 15) multilocus genotypes. In the most polymorphic population (37 genotypes), only one genotype was found to be common to two subpopulations. Resampling experiments s how that, depending on the population, three to 16 polymorphic loci we re necessary and sufficient to score all multilocus genotypes in the p opulation. When these data are compared to published results, it appea rs that on some occasions, the number of genotypes per population of s elfing species might be larger than would be expected from the sole co nsideration of effective population size. The large within-subpopulati on genetic variance observed in some populations could be explained by either small neighborhood sizes within subpopulations, or by outcross ing following migration through seed and/or pollen.