Abstract
Testosterone plays a central role in regulating the development of male traits across vertebrates. Although this role is well-established, how androgen regulation evolves to influence the development of traits differently across species remains less clear. In my dissertation, I use lizards in the genus Sceloporus to test how hormone-phenotype couplings and hormone-genome interactions evolve to facilitate the evolution of male traits and of sexual dimorphism. In Chapter 1, I perform a testosterone manipulation experiment in juveniles from two species: S. undulatus, which exhibits the ancestral state of sexually dimorphic ventral coloration that develops when testosterone induces melanin synthesis in males, and S. virgatus, which exhibits a derived sexually monomorphic coloration where testosterone does not induce color development and melanin is absent from the dermis. I find that ventral skin in the sexually dimorphic S. undulatus is significantly more responsive to testosterone than ventral skin in the sexually monomorphic S. virgatus, as quantified by the number of genes up- or downregulated by testosterone. In particular, genes related to melanin synthesis are strongly upregulated by testosterone in the sexually dimorphic species, but not in the sexually monomorphic species. My results suggest that tissue- and gene-level sensitivity to testosterone can evolve to facilitate the evolution of male traits. In Chapter 2, I use immunohistochemistry to test whether the abundance and distribution of androgen receptor protein differ in the skin of unmanipulated adults of both species, predicting that androgen receptor localizes to melanophores (the cell type that produces melanin) in S. undulatus but to a different cell type in S. virgatus. I find that skin from S. undulatus males has abundant androgen receptor that is localized to the most superficial part of the dermis. However, S. virgatus males exhibit very little androgen receptor, which is mostly observed in deeper parts of the dermis. Using antibodies for proteins characteristic of melanophores, I find that androgen receptor in S. undulatus males likely colocalizes within this cell type, which would facilitate androgen-induced melanin synthesis. However, I do not detect any melanophore markers in skin from S. virgatus males. This result suggests that the reduced androgen sensitivity in S. virgatus may be due to a significant reduction in androgen receptor expression, possibly driven by the loss of a single cell type crucial for androgen-induced melanin synthesis. In Chapter 3, I shift focus to test for sex- and species-specific effects of testosterone on the liver transcriptome. Using three species of Sceloporus (S. undulatus, S. virgatus, and S. merriami), I perform a testosterone manipulation experiment in juveniles and analyze gene expression in the liver. I find that males consistently have a stronger transcriptomic response to testosterone than females, but that the direction of testosterone-mediated gene expression is concordant between the sexes. However, I find that testosterone-mediated gene expression between species is evolutionarily labile, such that genes that are upregulated (or downregulated) by testosterone in one species are conversely downregulated (or upregulated) by testosterone in another. Further, the number of genes with significant treatment-by-species interactions increases with phylogenetic distance, suggesting that individual genes evolve to respond to testosterone in species-specific ways. Together, these results suggest that the transcriptome is characterized by evolutionary potential, facilitating the evolution of male traits and sexual dimorphism despite pleiotropic effects of testosterone.
