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Abstract
The persistence of host-symbiont mutualisms depends on the alignment of host and symbiont fitness interests. However, many mutualisms persist through processes, such as horizontal transmission, that can readily decouple host and symbiont fitness. How these mutualisms persist despite the potential costs remains unknown. My dissertation examines the eco-evolutionary dynamics that underlie the persistence of horizontally transmitted mutualisms. I evaluated these dynamics using a naturally occurring interaction between the insect host Anasa tristis and its horizontally transmitted bacterial symbiont Caballeronia spp. I began by testing the hypothesis that coevolution underlies the persistence of horizontally transmitted mutualisms. I tested for evidence of pairwise and diffuse coevolution by measuring patterns of host-symbiont specificity. I observed patterns of specialization consistent with diffuse coevolution, suggesting a potential pathway by which cooperation is maintained within this horizontally transmitted interaction. Specifically, selection from a range of host species may maintain fixed cooperative traits across populations of their shared generalist symbionts. I then directly tested whether symbiont transmission environment, like those experienced under vertical and horizontal transmission, alters the direction of selection for cooperative symbiont traits using experimental evolution. I experimentally passaged a Paraburkholderia symbiont of A. tristis hosts through four transmission environments, including between A. tristis hosts, between A. tristis hosts and soil, through soil, and through standard culture media. I found that symbionts passaged through the host environment rapidly evolved deleterious traits affecting host survival. In contrast, when symbiont evolution is decoupled from the host, deleterious symbiont traits evolve a slower rate. This demonstrates that transmission environment can alter the direction of selection for cooperative symbiont traits. Contrary to expectation, this work suggests, in some cases, vertical transmission can facilitate misalignment of host and symbiont fitness, even more rapidly than horizontal transmission. Overall, by combining analysis of natural populations with experimental evolution, this dissertation illuminates a pathway by which horizontally transmitted mutualisms may persist and provides new insights into the role of transmission mode within host-symbiont mutualisms.