Response of mate harm to sex-separated gene pools

Competition among males to fertilize females can generate sex differences in selection. At the genetic level, alternative alleles can be favoured in each sex (intralocus sexual conflict, IASC). At the phenotypic level, males can evolve traits that are harmful to the females with whom they interact. Here we used experimental evolution in Drosophila melanogaster to examine how relaxing IASC affected the evolution of mate harm. Genetically variable Chromosome 2s evolved in two separate pools within each population. One pool experienced patrilinear inheritance (segregated like a Y-chromosome) and male-limited selection, while the alternative pool segregated like an X-chromosome and experienced female-biased selection. We measured female fitness when exposed to males carrying either chromosome type at either normal (continuous) or reduced (periodic) male exposure, over three time periods. Males carrying either type of chromosome were harmful to females, but males carrying a male-limited chromosome displayed increased harmfulness, suggesting ongoing selection for mate harm that is constrained under normal inheritance. The magnitude and direction of the effect of male interaction on female fitness was highly sensitive to time of measurement, and we observed a brief period where increased interaction with males was beneficial to females.