The evolutionary causes and consequences of sex and sexual dimorphism

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Sexual reproduction is widespread and, in most animals and some plants, involves two sexes. The long-term objective of the proposed research program is to understand the evolutionary causes and consequences of sexual reproduction, especially in systems with two sexes. One central aim is to quantify selection on sex and understand the various mechanisms by which this selection arises. The second aim is to understand the consequences of evolving with two separate sexes. One set of projects focuses on a plant that reproduces both sexually and asexually, the liverwort Marchantia polymorpha. We will study the population genomics of this species from different regions of North America to infer if populations vary in how frequently they reproduce sexually. If so, we will then infer whether higher rates of sex have positively influenced the efficacy of selection, as predicted. A second set of projects aims to understand selection on sex, one of the great unknowns of evolutionary biology. Well-established theory shows that selection on sex should work through how sex affects either the mean or the variance in fitness, yet there is almost no data evaluating these effects in nature. In a field study, we will measure the distribution of fitness of experimentally generated sexually- and asexually-derived offspring. Additional experiments will focus on testing key ecological sources of selection. A second set of projects focuses on differences between males and females. Selection often favours different phenotypes in the two sexes, yet these phenotypes are constructed from a largely shared set of genes. In recent years, studies in many animals have shown sexual dimorphism in gene expression is common, raising the possibility that many genes in the genome are selected for different expression patterns in the two sexes. However, the selection responsible for creating sex-biased gene expression (SBGE) is poorly understood. Using experimental fruit fly populations, we will examine changes in sex-biased gene expression (SBGE) in two different evolutionary contexts. In the first, we will examine how SBGE evolves in response to changes in mate competition, which is thought to be a major force favouring dimorphism. In the second, we examine how SBGE evolves in response to a manipulation that keeps a large portion of the genome evolving mostly in one sex, thereby greatly diminishing any conflicting selection from the other sex. One major goal of this work is to better understand whether genes that are already dimorphically expressed are under on-going selection for even stronger dimorphism and whether there are many genes that are not currently dimorphic but are under selection to become so. Collectively, the projects will provide a clearer picture of the evolution of SBGE and the consequences of evolving with a genome shared by two different sexes.