A genomic perspective on the evolution of sexual dimorphism: The role of hormones and gene expression

Sexual dimorphism is common throughout the animal kingdom, and many dimorphisms appear to arise due to differences in the expression of genes. Signaling molecules, such as the steroid hormone testosterone, are one possible way to mediate sexually dimorphic phenotypes and gene expression. Testosterone affects many sexually dimorphic phenotypes, including immune function, aggression, and metabolic activity. Both sexes respond to experimental manipulation of testosterone, and levels of testosterone are correlated between males and females across species. Thus, there may be sexual conflict over the optimal level of circulating hormone. This conflict can be reduced, however, it the sexes differ in the mechanisms of their response to testosterone. To investigate sex differences in how gene expression responds to testosterone, we focused on the dark-eyed junco (Junco hyemalis)- an ecological model species that has been heavily studied for sex differences and for the effects of testosterone, but has, until recently, lacked genomic tools. This dissertation developed new genomic tools for the species, including transcriptomes (sequence of all expressed genes) and microarrays to measure gene expression. I demonstrated that males and females share a core transcriptional response to testosterone that is supplemented by many sex-specific changes in gene expression. The functions of these genes are related to many of the known phenotypic effects of testosterone. This gene expression response is observed in multiple tissues and in response to both experimental and natural variation in testosterone, and may suggest that juncos are reducing sexual conflict over circulating hormone levels by modifying the transcriptional response to elevated hormone. In addition, comparing males and females directly suggests that many sex differences in behavior and physiology may be mediated by gene expression in the junco. By adding the investigation of gene expression to a system with well-studied natural history, this dissertation presents novel insights into the genomic mechanisms underlying sexual dimorphism and response to hormones in an ecologically relevant context.