Sexual reproduction is generally believed to yield beneficial effects via the expansion of expressed genetic variation, which increases the efficiency of selection and the adaptive potential of a population. However, when nonadditive gene action is involved, sex can actually impede the adaptive progress of a population. If selection promotes coupling disequilibria between genes of similar effect, recombination and segregation can result in a decrease in expressed genetic variance in the offspring population. In addition, when nonadditive gene action underlies a quantitative trait, sex can produce a change in trait means in a direction opposite to that favored by selection. In this study we measured the change in genotypic trait means and genetic variances across a sexual generation in four populations of the cyclical parthenogen Daphnia pulicaria, which vary predictably in their incidence of sexual reproduction. We show that both the costs and benefits of sex, as measured by changes in means and variances in life-history traits, increase substantially with decreasing frequency of sex.
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