The genetic architecture of sexual dimorphism

Abstract: Phenotypic differences between the sexes evolve largely because selection favours a different complement of traits in either sex. Theory suggests that, despite its frequency, sexual dimorphism should be generally constrained from evolving because the sexes share much of their genome. While selection can lead to adaptation in one sex, correlated responses to selection can be maladaptive in the other. In this thesis I use Drosophila to examine the extent to which the shared genome constrains the evolution of sexual dimorphism and whether the sex chromosomes might play a special role in resolving intralocus sexual conflict.Gene expression data shows that intersexual genetic correlations are generally high, suggesting that genes often affect both sexes. The intersexual genetic correlation is negatively associated with sex-bias in expression in D. melanogaster, and the rate of change in sex-bias between D. melanogaster and six closely related species, showing that a sex-specific genetic architecture is a prerequisite for the evolution of sex difference. In further studies I find that genetic variance affecting lifespan is found in the male-limited Y chromosome within a population, which could offer a route to the evolution of further sexual dimorphism in lifespan, though the amount of variance was small suggesting adaptive potential from standing genetic variance is limited. Genetic variance on the X chromosome is also expected to be depleted once the sex chromosomes evolve, but here I find no evidence of depletion in either sex. Dosage compensation does not appear to double the male X-linked genetic variance, but this effect may be complex to detect. Finally, the X chromosome appears to be enriched for sex-specific genetic variance, and the consequences of this are explored using a variety of analytical methods to test biologically meaningful aspects of G-matrix structure.In summary, this thesis suggests that the evolution of sexual dimorphism is generally constrained by the shared genome, but intralocus sexual conflict could be resolved by novel mutations on the Y chromosomes, and by standing sex-specific genetic variance on the X chromosome. It highlights a special role for the X chromosome in the evolution of sexual dimorphism.