Quantitative genetic variation in Daphnia: Temporal changes in genetic architecture

Nonadditive genetic variation and genetic disequilibrium are two important factors that influence the evolutionary trajectory of natural populations. We assayed quantitative genetic variation in a temporary-pond-dwelling popu lation of Daphnia pulex over a full season to examine the role of nonadditi ve genetic variation and genetic disequilibrium in determining the short-te rm evolutionary trajectory of a cyclic parthenogen. Quantitative traits wer e influenced by three factors: (1) clonal selection significantly changed t he population mean phenotype during the course of the growing season; (2) s exual reproduction and recombination led to significant changes in life-his tory trait means and the levels of expressed genetic variation, implying th e presence of substantial nonadditive genetic variation and genetic disequi librium; and (3) Egg-bank effects were found to be an important component o f the realized year-to-year change. Additionally, we examined the impact of genetic disequilibria induced by clonal selection on the genetic (co)varia nce structure with a common principal components model. Clonal selection ca used significant changes in the (co)variance structure that were eliminated by a single bout of random mating, suggesting that a build-up of disequili bria was the primary source of changes in the (co)variance structure. The r esults of this study highlight the complexity of natural selection operatin g on populations that undergo alternating phases of sexual and asexual repr oduction.