The genetic and phenotypic expression of condition dependence and sexual dimorphism within and across the Drosophila melanogaster species group and the Rhopaloa clade

Abstract

In species where the sexes face different selective pressures, the evolution of sexual dimorphism may reflect the evolution towards divergent sex-specific trait optima. Sexual dimorphism, often considered in the context of exaggerated male secondary sexual traits, is thought to be driven primarily by sexual selection. Sexually selected traits tend to exhibit heightened nutritionally sensitive expression, called condition dependence. The condition dependent expression of sexually selected traits is proposed to act as a mechanism for the maintenance of genetic variation in sexually dimorphic traits. This genic capture model has become a common explanation for the maintenance of variation in exaggerated male weapons and ornaments. In this thesis, I explore the relationship between the evolution and expression of sexual dimorphism and condition dependence using Drosophila as a model system. In chapter two, we explore the intra- and inter-specific patterns of sexual dimorphism and condition dependence and their relationship in 29 species in the Drosophila melanogaster species group. We find that sexual and fecundity selection may be acting together to drive the expression of condition dependent sexual dimorphism within species. However, sexual selection alone may not be sufficient to drive the evolution of condition dependent sexual dimorphism across species. We show that in a strongly male-biased species, D. prolongata, the relationship between sexual dimorphism and condition-dependent trait expression is not consistent with a genic capture model. In chapter three, we use RNA-sequencing to identify the degree of similarity between patterns of gene expression which coordinate sexually dimorphic trait expression and condition dependent growth in the developing forelegs of male D. prolongata. We used 1) within-species comparisons: comparing patterns of gene expression during development between the sexually dimorphic forelegs and the nearly monomorphic midlegs and 2) across-species comparisons: comparing patterns of gene expression between the developing legs of D. prolongata and D. carrolli, and D. rhopaloa (exhibiting extreme male biased dimorphism, near monomorphism, and female-biased dimorphism, respectively). In D. prolongata, we find evidence that sex and developmental condition may have a shared role in shaping the genomic architecture of the developing legs. We do not find evidence to suggest that the relationship between sex-biased and condition-dependent gene expression is consistent with the predictions of the genic capture model. Together, these findings attempt to identify how condition dependent and sexually dimorphic variation shape the genetic architecture of sexually selected traits.