Evidence for a cost of sex in the freshwater snail Potamopyrgus antipodarum

Abstract

The maintenance of sexual reproduction by natural selection poses a paradox for evolutionary biology. Assuming 'all else equal,' a sexual lineage producing both males and females has a slower intrinsic growth rate than a clonal lineage producing only females. Hence, clones should rapidly outcompete their sexual counterparts, unless there are ecological or mutation-clearing advantages to sex, or unless the all-else-equal assumption is incorrect. Here we examined the crucial all-else-equal assumption using a lake population of freshwater snails (Potamopyrgus antipodarum) in which both diploid sexual and triploid parthenogenetic individuals coexist. We contrasted the variation in life-history traits between sexual and clonal forms to the variation in life histories among habitats and sampling locations. We also conducted a laboratory experiment in which sexual snails competed with a single clone for 1 yr. We found that size at maturity varied among habitats within the lake, but this difference was not attributable to reproductive mode. There were also no differences among clonal and sexual individuals with respect either to brood size or to the frequency of developmental errors in brooded embryos. Finally, in the laboratory competition experiment, the growth rate of sexual individuals was significantly lower than the growth rate of snails from a single clone. Taken together, these results suggest that the all else-equal assumption is correct, and that a cost of males exists in natural sexual populations of these snails.