PROJECT SUMMARY Animals harbor bacterial symbionts that express specific traits that promote host fitness. How hosts select for bacteria with those traits remains poorly understood. A particularly useful model system to examine host- microbe symbioses is the symbiosis established between the Hawaiian bobtail squid Euprymna scolopes and the bioluminescent bacterium Vibrio fischeri. The host harbors population of V. fischeri within a specialized light organ. In exchange for bioluminescence, the host provides the bacterial symbionts with nutrients that promote their growth. The overall goal of this proposal is to increase understanding of how squid select for bioluminescent symbionts. Previous work has shown that mutants that lack the light-producing enzyme luciferase are able to colonize the light organ but become attenuated. Research efforts with mutants that lack key factors that enable resistance to oxidative stress will be examined in colonization experiments. A screen to identify systems of nutrient import that is linked to the generation of oxidative stress will also be performed. Together, these results will determine the physiological changes that affect nonluminous strains, which will increase understanding of how hosts select for functional symbionts. The candidate will receive training in microbiology research and opportunities for professional development, which will prepare the candidate for a career in microbiology research.