Spatial effects and rare outcrossing events in Medicago truncatula (Fabaceae)

In order to elucidate the mechanisms underlying the large amount of RAPD po lymorphism found in 1990 in a population of the selfing; annual Medicago tr uncatula GAERTN. (Fabaceae), we have analysed most of the individuals (n = 363) from the same population 6 years later using microsatellite loci. We c onfirm the result of the earlier study, namely that this population is very polymorphic and highly subdivided, with approximately 37% of the variance distributed among subpopulations, only 50 m apart one from another. We use standard F-statistics analyses, linkage disequilibria, minimum spanning net work, multilocus assignment tests and spatial autocorrelation analyses to t est the hypotheses that spatial structure and outcrossing events are involv ed in maintaining the large amount of genetic diversity at the level of eac h subpopulation. Interestingly, fine-scale spatial structure could be obser ved in only one subpopulation suggesting that other mechanisms are acting e lsewhere. To the best of our knowledge, this is the first study of fine spa tial genetic structure in a predominantly selfing species.