Microbial Genome Evolution Due to Multifaceted Symbiosis within the Tsetse Fly (Diptera: Glossinidae)

Microbes are capable of rapid genetic modification, enabling the habitation of a wide field of niches, including forming interdependent associations with macroscopic hosts. While ancient multipartite mutualisms have been shown to involve metabolic complementation, little is known concerning the early genomic adaptations leading towards co-residence within a novel host. The overall objective of this research is to gain insight on genome evolution resulting from symbiosis, particularly by examining bacteria with varying levels of host dependency and times of establishment. The tsetse fly (Diptera: Glossinidae) serves as a relatively simple model system to investigate evolutionary aspects of symbiosis, while also maintaining medical and agricultural significance as vectors of African trypanosomes. In addition to potentially harboring trypanosomes, the tsetse enteric microbiota consists of two gamma-Proteobacteria: the anciently associated obligate mutualist Wigglesworthia spp. and the recently established commensal Sodalis glossinidius. The genomes of Wigglesworthia spp. (isolated from Glossina morsitans (Wgm) and G. brevipalpis (Wgb)), Sodalis and Trypanosoma brucei subspp. have been sequenced and annotated, facilitating empirical studies exploring potential partner interactions and adaptations. My work first examines the importance of nutrient provisioning, specifically thiamine (Vitamin B1), for the maintenance of a stable symbiotic environment within the tsetse host. These studies demonstrated that Sodalis required exogenous thiamine for proliferation due to the erosion of biosynthetic capabilities, while Wigglesworthia thiamine biosynthetic loci expression was influenced by the functional demand for this nutrient. My research also explored how distinct symbiont metabolic capabilities, retained by Wgm, but lacking in the Wgb genome, contribute to host biology and phenotypic variation. Wgm chorismate and folate (Vitamin B9) biosynthesis increased during times of nutrient stress, such as pregnancy and trypanosome infection, and was found to be critical for host biology. Lastly, genetic adaptations leading towards symbiont diversification and establishment in novel hosts were investigated. To accomplish this, molecular phylogenetic analyses were performed on Sodalis and closely related bacteria using genome regions traditionally associated with accelerated evolution, such as surface encoding loci and internal transcribed spacer regions, further increasing the resolution of this clade. This enhanced knowledge of tsetse symbionts increases our understanding of tsetse biology, potentially contributing to disease control strategies, and offers additional insights regarding fundamental evolutionary aspects involved in microbial symbiosis