Symbiosis can be a major source of evolutionary innovation and a driver of ecological diversification. Insects, in particular, partner with a diversity of heritable bacterial endosymbionts that affect them in myriad ways, ranging from mutualistic to parasitic. Species of the genus Drosophila, however, harbor only Wolbachia and Spiroplasma. While Wolbachia infections in Drosophila are well characterized, much less is known about the consequences of Spiroplasma infections. Spiroplasma is a male-killer in some species of Drosophila, and recent work has demonstrated that this bacterium can also confer protection against a nematode parasite in one Drosophila species. Many other species of Drosophila, however, are infected with Spiroplasma that do not cause male-killing, and their fitness effects are unclear. To discern the impacts of Spiroplasma infections in Drosophila, my dissertation work seeks to characterize the distribution of Spiroplasma among Drosophila by investigating the genetic diversity of Spiroplasma infecting Drosophila, examining the bacterial density dynamics of diverse Spiroplasma strains infecting different Drosophila species and exploring the fitness consequences of Spiroplasma infection in a cactophilic Drosophila species. Using a multilocus phylogenetic analysis, I show that there have been at least five introductions of four very different types of Spiroplasma into Drosophila, with a single origin of the male-killing Spiroplasma in Drosophila. Horizontal transmission has played an important role in driving the distribution of Spiroplasma among Drosophila, and I provide some evidence that it may be occurring within populations of certain Drosophila species. I detected no recombination, however, among Spiroplasma from different phylogenetic clades, suggesting that Spiroplasma may be similar to beneficial bacterial endosymbionts trapped in their host with no opportunity for recombination. Using quantitative PCR, I assessed the bacterial density dynamics of Spiroplasma, which vary among bacterial strain and Drosophila species. Although a number of facultative endosymbionts play a role in host plant specialization, this does not seem to be the case for the Spiroplasma strain infecting one population of Drosophila with high infection prevalence. This characterization of the Drosophila/Spiroplasma symbiosis lays much of the groundwork necessary to effectively explore the consequences of Spiroplasma infections in Drosophil
