Significant population genetic structure of the Cameroonian fresh water snail, Bulinus globosus, (Gastropoda: Planorbidae) revealed by nuclear microsatellite loci analysis.

In order to characterize the demographic traits and spatial structure of Cameroonians Bulinus globosus, intermediate host of Schistosoma haematobium, genetic structure of seven different populations, collected from the tropical zone, was studied using six polymorphic microsatellites. Intrapopulation genetic diversity ranged from 0.37 to 0.55. Interpopulation genetic diversity variation clearly illustrated their significant isolation due to distance with gene flow substantially limited to neighbouring populations. The effective population sizes (Ne) were relatively low (from 3.0 to 18.6), which supposes a high rate from which populations would lose their genetic diversity by drift. Analysis of genetic temporal variability indicated fluctuations of allelic frequencies (35 of 42 locus-population combinations, P<0.05) characteristic of stochastic demography, and this is reinforced by events of bottlenecks detected in all populations. These findings demonstrated that Cameroonian B. globosus were mixed-maters with some populations showing clear preference for outcrossing. These data also suggest that genetic drift and gene flow are the main factors shaping the genetic structure of studied populations.

Strong genetic structure in Cameroonian populations of Bulinus truncatus (Gastropoda: Planorbidae), intermediate host of Schistosoma haematobium.

In order to evaluate the snail host contribution on the variability of mollusk/schistosome compatibility, the genetic structure of seven Cameroonian populations of the schistosome vector, Bulinus truncatus, was studied using four variable microsatellite loci. A substantial polymorphism mainly distributed among populations was observed. No heterozygous genotype was scored, confirming the high level of selfing rate occurring in B. truncatus populations. Contemporaneous samples were highly and significantly differentiated with a marginally significant correlation with geographical distances (P-value=0.069). The different sites sampled seemed to rarely exchange migrants with very small Nm (∼0.22 or below). The data also suggest that B. truncatus subpopulations might be composed of very small and isolated units at much smaller surfaces than what was investigated. Even if more data (in particular more loci) will be needed to confirm these issues, they suggest that restricted gene flow plays an important role in maintaining differentiation among snail populations in the transmission foci, potentially leading to specific adaptation between each B. truncatus population and its local Schistosoma haematobium population.