POPULATION-DYNAMICS INFERRED FROM TEMPORAL VARIATION AT MICROSATELLITE LOCI IN THE SELFING SNAIL BULINUS-TRUNCATUS

We analyzed short-term forces acting on the genetics of subdivided pop ulations based on a temporal survey of the microsatellite variability in the hermaphrodite freshwater snail Bulinus truncatus. This species inhabits temporary habitats, has a short generation time and exhibits variable rates of selfing. We studied the variability over three sampl ing dates in 12 Sahelian populations (1161 individuals). Classical gen etic parameters (estimators of H-o, H-e, f, selfing rate and Fst) show ed limited change over time whereas important temporal changes of alle lic frequencies were detected for 10 of the ponds studied. These varia tions are not easily explained by selection, sampling drift and geneti c drift alone and may be due to periodic migration. Indeed the habitat s occupied by the populations studied are subject to large temporal fl uctuations owing to annual cycles of drought and flood. In such ponds our results support a demographic model of population expansions and c ontractions under which available habitats, after the rainy season, ar e colonized by individuals originating from a smaller number of refuge s (areas that never dry out in the deepest parts of the ponds). In con trast, selfing appeared to be an important force affecting the genetic structure in permanent ponds.