Abstracts (first author)
Evidence for cost of sex and parasite-mediated selection in a natural population of co-existing sexual and asexual snails
One of the main hypotheses for why sexual reproduction is so common despite the cost of producing males proposes that negative frequency-dependence selection by co-evolving parasites provides an advantage for rare genotypes. However, this mechanism fails to predict advantage to sex when asexual assemblages are diverse.
The challenge to sex posed by a diverse set of asexual lineages depends on the extent to which they realize their theorized two-fold advantage. Here, we used field-based comparisons of the rate of population growth between obligate sexual vs. multiple lineages of obligate asexual Potamopyrgus antipodarum, a New Zealand freshwater snail. We also used a population genetics approach to evaluate whether the temporal changes of population genetic structure follow patterns expected under parasite-mediated selection: faster clonal turnover in locations where parasite pressure is higher.
The reproductive output of asexual lineages measured using experimental enclosures (cages anchored to the bottom of the lake) over a course of one year was as high as that of the best sexual families, which implies the cost of sex. We also found that the reproductive output of asexual lineages depends on the habitat, which implies that environmental heterogeneity may select for habitat-specific clonal assemblages.
The genetic structure of the asexual population changed significantly over a 4-year period (4-8 generations) in shallow and mid-water habitats (high parasite pressure), but not in the deep habitat (low parasite pressure).
Our results show that the fitness of many asexual lineages is high enough to impose the cost of sex and that clonal turnover is faster in high infection sites. While the latter result is consistent with the parasite hypothesis for the maintenance of sex, the high relative fitness of many asexual lineages suggests that other mechanisms are needed to explain the persistence of sex in the face of a diverse array of asexual competitors.