DO SELF-FERTILIZATION AND GENETIC DRIFT PROMOTE A VERY-LOW GENETIC-VARIABILITY IN THE ALLOTETRAPOLOID BULINUS-TRUNCATUS (GASTROPODA, PLANORBIDAE) POPULATIONS

Bulinus truncatus, one of the intermediate hosts of the genus Schistos oma is an hermaphrodite freshwater snail species occupying a variety o f environments over almost all Africa. These environments are subjecte d to large variations in water availability. B. truncatus is allotetra ploid and its populations exhibit various frequencies of aphallic indi viduals (unable to reproduce as male). Both traits probably favour a r eproduction by self-fertilization. Here we investigate the genetic str ucture of populations of B. truncatus of Niger and Ivory Coast using p rotein electrophoresis to analyse the influence of the environment and of both the last traits. To obtain an estimate of the true heterozygo sity in this allotetraploid species, we analyse independently the two diploid loci at each tetraploid locus. Our study indicates (i) an extr emely low intrapopulation polymorphism with most alleles fixed and the total absence of heterozygotes and (ii) low differentiation between p opulations. These results indicate high gene flow between populations. However, the existence of private alleles sometimes at high frequency , the low polymorphism and the lack of heterozygotes point to the role of both genetic drift and self-fertilization, the second amplifying t he genetic consequences of the first.