EVIDENCE FOR A COST OF SEX IN THE FRESH-WATER SNAIL POTAMOPYRGUS-ANTIPODARUM

The maintenance of sexual reproduction by natural selection poses a pa radox 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 shou ld rapidly outcompete their sexual counterparts, unless there are ecol ogical or mutation-clearing advantages to sex, or unless the all-else- equal assumption is incorrect. Here we examined the crucial all-else-e qual assumption using a lake population of freshwater snails (Potamopy rgus antipodarum) in which both diploid sexual and triploid parthenoge netic individuals coexist. We contrasted the variation in life-history traits between sexual and clonal forms to the variation in life histo ries among habitats and sampling locations. We also conducted a labora tory experiment in which sexual snails competed with a single clone fo r 1 yr. We found that size at maturity varied among habitats within th e lake, but this difference was not attributable to reproductive mode. There were also no differences among clonal and sexual individuals wi th respect either to brood size or to the frequency of developmental e rrors in brooded embryos. Finally, in the laboratory competition exper iment, the growth rate of sexual individuals was significantly lower t han the growth rate of snails from a single clone. Taken together, the se results suggest that the all-else-equal assumption is correct, and that a cost of males exists in natural sexual populations of these sna ils.