Dissertation (Ph.D.) - Indiana University, Biology, 2021The effect of host-microbe interactions on diverse aspects of host biology are increasingly appreciated across biological disciplines, yet the roles played by these interactions in shaping host evolution remain poorly understood. My dissertation research seeks to address these and related issues using the dung beetle genus Onthophagus. Previous work in this genus has demonstrated that mothers reliably pass to their offspring a conserved group of gut microbes, and that these vertically inherited microbes enhance offspring growth, development, and survival, especially under stress. In the first three chapters of my dissertation, I employed a manipulative method which allowed for the exchange of gut microbiota between Onthophagus species. Using this technique, I was able to first show that different species have diverged to make use of non-interchangeable gut microbiota, and that disruption of these specific host-microbiota relationships has potentially long-term evolutionary consequences. Secondly, I then showed that this host-microbiota species specificity can arise over evolutionarily short timespans, including recently divergent, broadly sympatric and often syntopic sister species sharing virtually identical ecologies. In my third chapter, I was able to show that Onthophagusmicrobiota may influence population adaptation to local thermal conditions. However, contrary to my original hypotheses, results suggested that local host microbiome interactions may limit, rather than enhance, host fitness. Finally, in my fourth chapter, I employed a microbial sequencing approach to provide an in-depth assessment of the taxonomic composition of the gut microbiota of several dung beetle species, and to determine to what extent microbiome composition changes when hosts are introduced to novel geographic ranges. As a whole, my dissertation employs a diversity of methodologies to better understand the evolutionary and ecological ramifications of dung beetle microbiome symbioses.