| Intragenomic conflict arises when transmission of one part of the genome interferes with transmission of other regions. Such conflict can occur at multiple scales: between alleles, homologous chromosomes, or entire genomes. Because heritable elements can vary in their transmission through males versus females, intragenomic conflict is often manifested in sex ratio distortion. This thesis investigates the population genetic consequences of two sex-ratio distorting genetic elements. The first set of studies focus on male-killing Wolbachia, a bacterial endosymbiont that infects the fly Drosophila innubila. First, the density of Wolbachia varies among individuals of D. innubila, which affects offspring sex ratio. Hence, the dynamics of infection within fly populations may result from processes affecting the growth and regulation of bacterial populations within individuals, as well as from selection among cytoplasmic lineages that vary in bacterial density. Second, the population-level effects of this infection are significant, and molecular data indicate that this male-killing infection is evolutionarily old. Despite a large effective population size of the host species and strong selection to evolve resistance, the D. innubila-Wolbachia association appears to be at a stable equilibrium that is maintained by imperfect maternal transmission of the bacteria rather than partial resistance in the host species. Third, the infection occurs in all of the main populations of D. innubila, with the proportion of females in the population proportional to the prevalence of Wolbachia infection. Molecular data and assays of Wolbachia's phenotypic effects indicate that evolutionary interactions between D. innubila and Wolbachia occur at the species rather than population level, but that the host-parasite association is ecologically dynamic within local populations. The second part of this thesis focuses on X-chromosome meiotic drive in Drosophila recens. Across the species' range, non-driving chromosomes exhibit high levels of polymorphism, whereas X-drive is associated with a single whole-chromosome haplotype that harbors little variation. This apparent suppression of recombination is corroborated experimentally. Molecular data indicate X-drive is monophyletic in D. recens and ∼200,000 years old. The results of this work demonstrate the sex-ratio distorting effects of intra-genomic conflict affect population-level sex ratios, current patterns of polymorphism, and long-term patterns of genome evolution. |
