Résumé
The distribution of neutral genetic variability within and among sets of populations results from the combined actions of genetic drift, migration, extinction and recolonization processes, mutation, and the mating system. We here analyzed these factors in 38 populations of the hermaphroditic snail Bulinus truncatus. The sampling area covered a large part of the species range. The variability was analyzed using four polymorphic microsatellite loci. A very large number of alleles (up to 55) was found at the level of the whole study. Observed heterozygote deficiencies within populations are consistent with very high selfing rates, generally above 0.80, in all populations. These should depress the variability within populations, because of low effective size, genetic hitchhiking, and background selection, whatever the model of mutation assumed. However, that some populations exhibit much more variability than others suggests that historical demographic processes (e.g., population size variation, bottlenecks, or founding events) may play a significant role. A hierarchical analysis of the distribution of the variability across populations indicates a strong pattern of isolation by distance, whatever the geographical scale considered. Our analysis also illustrates how the mutation rate may affect population differentiation, as different mutation rates result in different levels of homoplasy at microsatellite loci. The effects of both genetic drift and gene flow vary with the temporal and spatial scales considered in B. truncatus populations.