Evidence from a New Zealand snail for the maintenance of sex by parasitism

The widespread occurrence of sexual reproduction is an important problem in evolutionary theory, because of the cost of producing males in dioecious organisms and the allocation of limited resources in cosexes1–4. Here I compare the predictions of the major ecological hypotheses on the maintenance of sex by examining male frequency in Potamopyrgus antipodarum, a dioecious freshwater snail with both sexual and parthenogenetic populations5. The results do not-support the view that sex is maintained by a variable physical environment6–9, but they are consistent with the idea that sex is favoured by selection resulting from host–parasite interactions10–14.