Sex differences in the strength of selection under facultative sex

Facultative sex, the ability to reproduce both sexually and asexually, is widespread across the tree of life. In anisogamous species, the frequency of sex modulates selection on traits with sex-specific expression. Current theory on conditional gene expression posits that the strength of selection on loci only expressed by a subset of individuals, and/or in a subset of environments, is proportional to the frequency of expressers in the population. We show here that this assumption does not hold when the subsets in question are males or females (because of the Fisher condition) and is most important in facultatively sexual populations. In this case, the proportion of sexually produced offspring is not determined by male frequency (sex ratio), but on relative female investment in sexual versus asexual reproduction. This breaks the link between the frequency of expressers and selection for loci with sex-specific expression. However, certain conditions can re-establish this link, for example male traits that predict mating success better under strong male-male competition, and sex ratio affecting the relative fecundity of sexual versus asexual females. Our work highlights the importance of the Fisher condition for understanding the efficiency of selection and has implications for differences in the genetic load among sexes.