CHANGE OF GENETIC ARCHITECTURE IN RESPONSE TO SEX

A traditional view is that sexual reproduction increases the potential for phenotypic evolution by expanding the range of genetic variation upon which natural selection can act. However, when nonadditive geneti c effects and genetic disequilibria underlie a genetic system, genetic slippage (a change in the mean genotypic value contrary to that promo ted by selection) in response to sex may occur. Additionally, dependin g on whether natural selection is predominantly stabilizing or disrupt ive, recombination may either enhance or reduce the level of expressed genetic variance. Thus, the role of sexual reproduction in the dynami cs of phenotypic evolution depends heavily upon the nature of natural selection and the genetic system of the study population. In the prese nt study, on a permanent lake Daphnia pulicaria population, sexual rep roduction resulted in significant genetic slippage and a significant i ncrease in expressed genetic variance for several traits. These observ ations provide evidence for substantial genetic disequilibria and nona dditive genetic effects underlying the genetic system of the study pop ulation. From these results, the fitness function of the previous clon al selection phase is inferred to be directional and/or stabilizing. T he data are also used to infer the effects of natural selection on the mean and the genetic variance of the population.