| Symbiosis, the living together of unlike organisms, is critically important to the evolution of life on Earth. Virtually all animals host beneficial microbial symbionts, some of which are critical for normal host development and survival. Leaf-cutter ants farm a fungal cultivar as their primary nutrient source. The ants also carry a specialized Actinobacterial exosymbiont, in the genus Pseudonocardia, which produces antifungal compounds that inhibit Escovopsis, a parasite specialized on the ants' garden. In this dissertation, I use the leaf-cutter ant-Pseudonocardia mutualism as a model for host-beneficial microbe symbiosis, exploring across three scales: from genus to individual to genetic interactions. In chapter two, I employ 11 culturing techniques to explore whether Atta leaf-cutter ants, which lack visible exosymbionts, associate with Pseudonocardia. I isolated Pseudonocardia from all colonies attempted using at least one method, with bead-beating of workers being the most efficient. An Elongation Factor-Tu phylogeny placed three strains into ant-associated clades, with the majority falling into two novel clades. Additional work will be needed to localize and further characterize these strains. In chapter three, I used subcolony experiments to isolate the components and timing needed for proper exosymbiont acquisition in Acromyrmex ants. Major workers carrying visible Pseudonocardia need to interact with newly eclosed ants in a narrow window of a few hours after eclosion in order for successful transmission to occur. These constraints on colonization are apparently mechanisms to ensure specificity. Specificity-producing mechanisms seem likely to occur in many symbioses to prevent host exploitation. In chapter four, I examined gene expression in symbiotic and aposymbiotic ants ten days after eclosion. I found differences in oxidation-reduction processes, reflecting established changes in aposymbiotic ant respiration rates. I also found upregulation in chitin metabolism, likely linked to host-associated molecular patterns. Finally, I found upregulation in aposymbiotic ants' NF-Kappa B pathway, perhaps corresponding to lower numbers of all non-ant reads including the ants' fungal cultivar food source, associated garden bacteria, and known bacteria symbionts across the cost-benefit spectrum, with Pseudonocardia the most prevalent. This work highlights the importance of specificity in ant-Pseudonocardia interactions and the likely importance of specificity as a first principle in symbiosis. |
