Bulinus globosus, an intermediate host of schisto-somes, is a self-fertile hermaphrodite species whose regular mating system is cross-fertilization. Fitness of selfing individuals originating from two geographically remote populations (Elevi, Ivory Coast; Daikaina, Niger) was scored. Individuals were then paired either with individuals from their own population, or with individuals from the other population. Electrophoretic analysis showed that pairing ‘female’ Elevi and ‘male’ Daikaina individuals results in only a low level of outcrossing, whereas the reverse pairings produced almost only outcrossed offspring. This could reflect unilateral partial isolation between the two populations. However, analysis of fitness values within and among groups of self-fertilizing and cross-fertilizing individuals, as well as electrophoretic data, suggest that selfing (or partial selfing) is the regular mating system in Elevi. Such experiments are relevant to the biological control of schistosiomasis by introducing resistant snails from allopatric populations, because they allow an assessment of the fate of introduced resistant genes.

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