Genetic differentiation,dispersal and mating system in the schistosome-transmitting freshwater snail Biomphalaria glabrata

Biomphalaria glabrata is the main intermediate host of Schistosoma mansoni in America and one of the most intensely studied species of freshwater snail, yet very little is known about its population biology. Here, we used seven highly polymorphic microsatellite loci to analyse genetic diversity in populations from three regions (Lesser Antilles, Venezuela and southern Brazil). Considerable genetic variation was detected, with an average (s.d.) H(0) = 0.32 (0.24). More diversity per population was found in the Valencia lake basin in Central Venezuela, which suggests an influence of dispersal (via inter-population connectivity) on the restoring of genetic diversity after the demographic bottlenecks recurrently experienced by populations. A marked population structure was detected and there seems to be a relationship between mean differentiation and genetic diversity within regions. There is also a significant isolation-by-distance pattern. The Lesser Antilles populations appear clearly differentiated from the rest, which suggests a single colonisation event followed by local radiation within these islands or multiple colonisation events from the same source area. Our results indicate that B. glabrata essentially cross-fertilises, with little variation in selfing rates among populations. However, significant deficits in heterozygotes and linkage disequilibria were detected in two Venezuelan populations suggesting a mixture of at least two different genetic entities, probably with differences in their respective mating systems.