ALLOZYME VARIATION IN SOME POPULATIONS OF THE FRESHWATER SNAILS LYMNAEA PEREGRA, L. AURICULARIA AND L. STAGNALIS (GASTROPODA: PULMONATA)

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Abstract
Three species of the genus Lymnaea (Gastropoda, Pulmonata, Basommatophora) were examined for genetic variation at 11 enzymatic loci in samples collected in Brittany (North-Western France). Variability was greatest in L. percgra, but the limited area of sampling did not allow specific inferences. The UPGMA clustering of unbiased genetic distances (Nei, 1978) isolated first L. auricularia samples, and secondly the single L. stagnalis sample from the L. peregra set. This scheme of relationships was opposed to the identification of alleles shared by the three species at the studied loci Analysis of L. peregra population structure by F-statistics (Weir & Cockerham, 1984) suggested high inbreeding and raised the possible role of self-fertilization. The contradiction between high variability and high homozygosity is discussed. Furthermore, population differentiation was rather weak (although statistically significant), despite some remarkable differences among loci. Nm estimates using Wright's (1951) and Slatkin's (1985) methods revealed a gene flow unable to prevent the effect of genetic drift among populations. The relative homogeneity of populations with weak genetic exchange is discussed

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