Investigating patterns of parallel genetic change in repeated adaptation

The phenomenon of repeated evolution runs counter to expectations about the role of contingency in adaptation. However, many examples of independently acquired similar traits show that evolution sometimes does follow the same path. Factors influencing the probability of such an event include selection, trait complexity and relatedness. Previous investigations of repeated adaptation have primarily focused on low-complexity traits subject to strong selection. Studies of systems with varying levels of trait complexity, selection, and relatedness are needed to evaluate the relative contributions of these factors. The series of studies reported here 1) establishes a system for inquiry into the role of parallel adaptation among hosts and parasites and 2) provides an assessment of the role of parallel genetic change in the evolution of a complex trait. In Chapter 2, I show that all-female broods in a line of Drosophila borealis are caused by infection with a male-killing strain of Wolbachia that is very closely-related to another male-killing strain infecting a geographically and evolutionarily distant species of Drosophila. This host-parasite system, together with two other known male-killing Wolbachia strains infecting Drosophila provides a framework for investigating the role of parallel evolution in the independent acquisition of the male-killing trait among Wolbachia, as well as in the adaptation of divergent hosts to similar male-killing parasites. In Chapters 3-5, I investigate the role of parallel genetic change in a complex trait in two species of Drosophila by searching for evidence of adaptation in the Drosophila americana homologs of genes thought to underlie adaptation to climate in Drosophila melanogaster. In Chapter 3, I investigate the D. americana homolog of Alcohol dehydrogenase (Adh). In contrast with D. melanogaster, which segregates functionally distinct variants in Adh that represent local adaptation to climate, D. americana segregates little variation. This is surprising, especially because Adh of D. americana is