Consequences of Sex Differences in Selection and Sex-specific Genetic Architecture of Fitness

The sexes exhibit dramatic phenotypic differences. Ultimately, these differences are thought to be the result of sex differences in selection. My thesis takes a variety of approaches to investigate the consequences of these differences from an evolutionary genetics perspective. In Chapter 2 I surveyed the literature for estimates of sex-specific phenotypic selection in the wild and performed meta-analysis to understand the strength of sex-specific selection. Results of this study suggest that selection is often male-biased in magnitude and consistent with sexual selection theory, male-biased selection is driven by components of fitness related to mating success. In Chapter 3, I explored two potentially contradictory consequences of increased variance in fitness among males: stronger selection but also increased genetic drift. I used a mathematical model to explore how increased condition-dependency of male fitness relates to the efficacy of selection. Next, I estimated sex-specific fitness in a laboratory population of Drosophila melanogaster in environmental contexts intended to alter sex-specific variance. I show that increased variance in male fitness can facilitate stronger selection without a concomitant increase in the strength of genetic drift when a substantial amount of the variance in fitness is due to stochastic processes such as random juvenile mortality. In Chapter 4 I investigated the sex-specific genetic architecture of fitness using inbred lines of D. melanogaster. I used three approaches – quantitative genetics, genome wide association and a mutation load approach – each providing a unique perspective into the genetic underpinnings of male and female fitness. Finally, in Chapter 5 I used RNA-seq data to investigate the coincidence of two forms of sexually dimorphic gene regulation: sex-bias in gene expression (SBGE) and sexually dimorphic isoform usage (SDIU) in D. melanogaster. Results of this study suggest that both forms of gene regulation are widespread and tissue specific. Population genomic patterns of these genes are consistent with the idea that both SBGE and SDIU evolve to resolve sexual conflict but that they may represent alternative routes to resolution. Overall, my thesis reinforces a view that the sexes represent very different selective environments and provides novel insights into the causes and consequences of these sex differences.Ph.D