Genetic evidence for low outcrossing rates in polyploid freshwater snails (Ancylus fluviatilis)

Abstract

We did breeding experiments with the polyploid hermaphroditic freshwater snail Ancylus fluviatilis to determine outcrossing rates at the level of individuals. The study was in response to electrophoretic work on the genetic structure of natural populations which suggested only a limited extent of cross-fertilization. Snails were sampled from populations known to harbour electrophoretic markers suitable to detect all outcrossing events among the F$_{1}$ progeny of experimental breeding pairs. Of the 540 hatching juveniles scored for a three-locus genotype (Gpi-Lap-Aat), 237 were generated by isolated snails (n = 15), thus documenting the facility for uniparental reproduction. Among the 303 offspring sired by paired snails (n = 35), only seven proved to be cross-fertilized, giving an overall outcrossing rate of 2.3%. The two individuals using allosperm nevertheless produced most of their progeny uniparentally. Some degree of outcrossing was thus confirmed at the level of individual snails. Our results are consistent with theoretical predictions for polyploids, and provide genetic evidence for a breeding system reminiscent of highly selfing plants.