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 Fx progeny of experimental breeding pairs. Of the 540 hatching juveniles scored for a three-locus genotype ( G p i -L a p -A a t) ,237 were generated by isolated snails = 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 selling plants.

Molecular markers and evolutionary processes in hermaphrodite freshwater snails

“…Most of the diploid fern species have high rates of intergametophytic crossing and there is evidence for substantial levels of intragametophytic seI fing in allopolyploid species, whereas their diploid progenitors often carry high genetic loads and are primarily outcrossers (Soltis and Soltis, 1987;Watano and Masuyama, 1991). Comparable da ta for hermaphrodite snail systems are just beginning to appear, again from studies on Ancylus (Städler et al , 1993;Städler, in preparation) and Bulinus truncatus, where both population genetic structure and the occurrence of aphally suggest low levels of outcrossing (Jarne et al , 1992b;Njiokou et al, 1993a,b). Despite these encouraging leads, the lack of protein polymorphism in many freshwater snails has precluded a direct estimate of selfing rates, so that the current data base remains very scanty (Jarne and Charlesworth, 1993;Städler and Jarne, in preparation).…”

“…Streit, 1976Streit, , 1985. However, they provide adequate model systems to contribute to the expanding research agenda of modern population biology, as exemplified by the implications of modes of reproduction, sex allocation, and life-history evolution (e.g., Schrag and Read, 1992;Jarne et al, 1991Jarne et al, , 1992bNjiokou et al, 1993b;Städler et al, 1993;Wethington and Dillon, 1993).…”

“…Recently, studies on freshwater pulmonate snails have begun to contribute to our understanding of these processes (e.g., Städler et al, 1993;Njiokou et al, 1993a). Gene silencing may prove to be a key factor in post-polyploidization evolution by promoting genetic and very likely phenotypic divergence.…”

“…In general, they allow estimation of allelie frequeneies and heterozygosities, but also insights into the mating system (Njiokou et al, 1993b;Städler et al , 1993). Allozymes bear the additional advantage that inferences about hybridity, introgression, and polyploidy can be drawn from gel banding patterns even where diploid ancestral taxa are unavailable for study.…”