UC Davis

The evolution of biological sexes is a fascinating field of study with much that remains to be learned. Decades of studies in Drosophila and in other animal models have made it clear that reproductive tissues in both sexes are some of the most rapidly evolving organs: They diverge more rapidly in gene expression relative to non- reproductive tissues, the genes that are biased in expression to reproductive tissues often show rapid divergence relative to non reproductive biased genes, and the reproductive organs themselves sometimes even show their capacity for physiological and behavioral changes, such as the ability of certain female reproductive organs to store sperm.

To better understand the genetic and genomic mechanisms that underlie the rapid evolution of reproductive tissues in response to local adaptation, and to study the extent of parallelism in gene expression in these tissues across species, I study the gene expression changes that underlie the accessory glands and testes across two geographic regions in three species, as seen in chapter one. Since much less is known about the female contributions to reproduction, and to begin to understand the rapid evolution of some of their tissues, I have begun to characterize the similarities and differences in gene expression across the seminal receptacle, the spermathecae, and the parovaria in eight species. This second chapter will help us better understand how certain female reproductive organs can gain and lose sperm storage functions across evolutionary time. Lastly, and to better understand how stem cell niches are maintained across species, I begin to characterize some of the gene expression differences across such niches in various species of Drosophila, and ask the question of whether such niches use similar or different gene products for the maintenance of healthy stem cells in these niches.

Studying the rapid evolution of reproductive organs helps us characterize the shared gene expression patterns that are core to reproduction across distantly related species, while also elucidating the divergence in gene expression that allows for the rise of new molecules, physiologies, and behaviors. My three chapters of studies contribute to our understanding of male and female reproductive system function and elaborate on our limited knowledge of the evolution of female reproductive tissues. Studying the diversity of genetic and phenotypic features of closely and distantly related species will eventually allow us to better understand the co-evolution of male and female sexes across species, and will allow us to study the extent of gene expression plasticity across sexes: If different gene products related to successful fertilization are made by both males and females, how amenable are such reproductive genes to switching expression across sexes, and how common are such switches? And what effects do sex-specific selection and local adaptation have on shaping content and expression of a genome?