Using Sex-Specific Robertson Covariances to Estimate Within- and Cross-Sex Responses to Selection on Reproductive...

AbstractIn organisms with separate sexes, the expected evolutionary change in a trait due to selection can be expressed using sex-specific Robertson covariances (RCs), that is, the additive genetic covariance between the trait and female relative fitness and the additive genetic covariance between the trait and male relative fitness. Sex-specific RCs capture the effects of (1) direct and indirect selection acting on the trait in the sex it is measured in ("within-sex selection") and (2) direct and indirect selection experienced by the underlying loci when expressed in the opposite sex ("cross-sex selection"). Using hemiclonal analysis in Drosophila melanogaster, we investigated the expected response to within-sex and cross-sex selection for a suite of traits involved in interlocus sexual conflict (IeSC) at male-biased, equal, and female-biased adult sex ratios. Our results are consistent with the idea that IeSC and sexual selection become stronger with the degree of male bias in adult sex ratio. The expected responses to cross-sex selection were small and typically concordant relative to the expected response to within-sex selection, with no evidence of intralocus sexual conflict for the traits we investigated. On the contrary, our findings imply that cross-sex selection may substantially boost the rate of adaptation in females.