EVOLUTION AND ECOLOGICAL CORRELATES OF UNIPARENTAL REPRODUCTION IN FRESH-WATER SNAILS

We review the spatial and temporal correlates of uniparental reproduction in freshwater snails as they pertain to the ecological hypotheses for the maintenance of biparental sex. The biogeographic evidence from two species (Potamopyrgus antipodarum and Bulinus truncatus) presently supports the Red Queen hypothesis that biparental reproduction is selected as a way to reduce the risk to progeny of parasite attack. Uniparental reproduction in these species is associated with low levels of infection by parasites (castrating digenetic trematodes), suggesting that parthenogenesis or self-fertilization can replace cross-fertilization when the risk of infection is low. In addition, in B. truncatus, the opportunity for cross-fertilization coincides with the season in which parasite attack is highest. In a third species (Campeloma decisum), parthenogenetic reproduction is correlated with latitude and the presence of a non-castrating trematode that may prevent cross-fertilization; these patterns suggest that parthenogenesis has been selected as a mechanism to assure reproduction. Finally, we discuss the spotty taxonomic distribution of parthenogenetic species.