| Most microorganisms experience variable environments which at least occasionally involve periods of harsh and extreme conditions that impose stress and hinder optimal performance of microbial physiology, metabolism, survival, and fitness. Microorganisms have evolved sophisticated, elaborate, and complex stress responses to regulate and mitigate the negative effects of stress in an attempt to maintain homeostasis. The different stress responses (e.g., heat shock and starvation) involve crosstalk to each other and are cross-connected in manifold ways to determinants of symbiosis, defined here to include mutualisms, parasitisms, host-pathogen relationships, and commensalisms. How stress impacts microbial diversity, ecology, and evolution is not entirely clear and is an interesting question. The sepiolid squid-Vibrio mutualism is an ideal model system to explore this problem. The aim of this dissertation was to investigate how environmental variation, abiotic factors, exposure to novel hosts, and stressors influence the evolutionary ecology of the symbiont Vibrio fischeri and what the potential consequences are to its free-living and host-associated phases, along with the accompanying overall effect on the symbiosis itself. Growth curve studies showed temperature, salinity, and nutrient availability can synergistically interact and affect symbiont reproduction, abundance, competition, and allelopathy prior to their encounter with host squid. Furthermore, these results suggested the distribution patterns of the hosts could be a contributor in shaping how V. fischeri biomass production responds to abiotic factors and stress during the planktonic lifestyle. Subsequent co-infection studies with dissimilar strains in squid showed how proportions of different symbiont types can modify the outcome of host colonization, including the countervailing of competitive dominance. Microbial experimental evolution indicated that V. fischeri is capable of rapid adaptation to environmentally stressful and novel host environments. Both of these perspectives are related, since host immunity imparts its own particular set of advosarial stresses. Temporal population genetic studies of V. fischeri isolated from animals provided a macroscale evolutionary viewpoint into the coevolution of the symbionts with their host squid. Genomic resequencing of experimentally evolved V. fischeri in E. tasmanica identified genes potentially responsible for the adaptation to this host. Phenotypic characterization indicated evidence of convergent evolution between derived lines through E. tasmanica and "ET" wild isolates procured from field-caught specimens of this same squid host species. |
