Is sex-biased expression adaptive?

We observe differences between females and males across the tree of life. These differences can be observed on a range of levels, from body size and shape to animal behaviour, down to differences within the cell. One important cellular mechanism involves how genes produce messenger molecules to regulate other downstream cellular processes. We refer to this as gene expression. There are considerable gaps in scientists understanding of which genes are responsible for sex differences in numerous organismal features. Therefore, investigating gene expression is key to identifying the role of sex differences in adaptation through natural selection. Gene expression also differs between the sexes. However, we are unsure of whether said differences contribute to organisms being better adapted to their environment, or whether differences accumulate without carrying evolutionary advantages. If differences between females and males are not adaptive, we can make some predictions based on how many individuals exist in each species (or population). Evolutionary biologists refer to this parameter as population size. When population size is low, i.e. there are only a few individuals of a given species, we predict that non-adaptive sex differences will readily accumulate. Alternatively, if most differences between females and males are adaptive, large populations are predicted to accumulate more of these differences. My project will improve our understanding of how sex differences in gene expression contribute to sex differences in bodily features and functions. It will also address how important these differences are for adaptation. Using mathematical models, I will investigate scenarios where populations range from small to large and where sex differences can evolve. Using online databases, I will collect gene expression data to find differences between females and males across a wide range of species, from humans to insects. With this data, I will describe the relationship between sex differences and population size. Finally, I will compare sex differences between two closely related species of fruit fly a cosmopolitan species that is found worldwide and an island species that lives in an archipelago in the Indian ocean as I expose them to a cold and a hot temperature environment in the lab. The two species have vastly different population sizes. Comparing the two will shed light on how sex differences in gene expression are affected by both temperature and population size.