Coevolutionary diversification in Septobasidium, a fungal symbiont of scale insects

Symbioses are inextricably linked to coevolution because their nature of intimate ecological interaction. These interactions are at the same time the simplest because they involve only two organisms and extremely complex because no symbionts live truly isolated from the myriad of environmental factors impacting one or both organisms. In this thesis, I seek to describe and differentiate patterns of diversification in the symbiotic fungal genus Septobasidium, which lives symbiotically with scale insects. I used taxonomic, phylogenetic, and population genetic approaches to dissect several aspects of the ecology of this symbiosis genus wide, within species, and within individuals. I focus on the relationship between host specificity, life history traits such as dispersal or sexual reproduction and codiversification or coevolution.; With a world-wide sampling of Septobasidium, including three newly described species and two other genera of fungi associated with scale insects, I show that insect symbiosis has a single origin in the basidiomycete fungi. Then using a comparative phylogenetic approach with molecular data from both fungi and insects, I show that cospeciation is not a dominant mode of diversification in this symbiosis. Within this comparative phylogenetic framework I develop a novel approach for measuring specificity, and use it to detect correlation between specificity and a measure of the degree to which a fungal species acts mutualistically. I also reject correlation with phylogeny and a morphological character. Within species of Septobasidium from the southeastern United States, I use genetic data to show that host specificity does not indicate the level of codispersal or recombination among symbionts. I also utilize a generalized hypothesis testing approach to show the conditions under which it is possible to distinguish between geographic and symbiotic explanations of population structure in Septobasidium .; I conclude that evolutionary diversification of Septobasidium is impacted by diversification in its host insects, but that there is a disassociation of dispersal between the two symbionts enabling them to have distinctly unlinked evolutionary trajectories. I propose a model of evolution in Septobasidium in which specificity, geography, and host distribution act to occasionally link insect and fungal evolutionary trajectories.