Award: OCE-1559105

Many studies have found significant variation in the benefits that different species of coral reef symbionts provide to their hosts and in how hosts respond to increasing ocean temperatures. In studying two common symbionts found within the octocoral, Antillogorgium bipinnata, we also found significant genetic variation in functional traits within species. Different genotypes of these two symbiont species, Breviolum antillogorgium and B. minutum, vary in population growth rate and photosynthetic and respiration rates. These same traits also vary in response to temperature and nutrients. This suggests that there may be potential for the host or the environment to select for symbiont genotypes with particular traits. Focusing more closely on B. antillogorgium, we conducted a long-term selection experiment in which we isolated individual genotypes that grew best at ambient (26 degrees C) and elevated (30 degrees C) temperatures. Isolates were allowed to continue to evolve at these temperatures for several hundreds of generations. We did not find any significant effect of historical growth temperature on symbiont traits measured in vitro or on host survival in hospite. However, we found significant variation in thermal tolerance of photophysiology among genotypes, but also broad thermal tolerance, as all genotypes showed positive growth at elevated temperature. However, we found no significant correlation between photophysiology and symbiont fitness, measured as population growth rate in culture. This suggests that selecting for symbiont traits likely to be most beneficial to hosts may prove difficult through in vitro selection experiments, as such experiments are likely to directly select for growth rate, rather than photophysiology traits. To more closely examine the effects of symbiont genetic variation on host fitness, we used the upside-down jellyfish, Cassiopea xamachana, and its associated symbiont, Symbiodinium microadriaticum, as a model system. We again found significant variation among genotypes in the temperature response of photophysiology traits and growth rate. We also found that the effect of temperature on asexual reproduction and the timing of development in the host was dependent on which genotype of symbiont infected the host. However, symbiont traits measured in vitro were not predictive of host fitness responses to temperature. All of this work suggests that, at least in the few species we examined here, there is sufficient genetic variation to allow for selection on symbiont traits in response to temperature. However, the nature of selection on traits will likely depend on the community context in which those interactions occur. It may be difficult to artificially select for symbiont traits that benefit the host without also including the host in the selection experiment. For fast-growing coral species in which symbiont genetic composition can be manipulated, such artificial selection experiments may be possible in the laboratory but may be more challenging for other species. More broadly, selection on species traits will depend on the community composition of the environment in which selection occurs, as well as context-dependent responses to changes in the environment. We distributed the results of this work to scientific audiences via publications and presentations at scientific conferences. Additionally, the results of this work were incorporated into introductory and graduate classes at California State University, Northridge. The grant supported outreach to Dearborn Elementary Charter Academy via their Science Lab and supported multiple years of teaching a graduate course on Transforming STEM Education. This research helped to train two post-doctoral scholars, ten graduate students, and 12 undergraduate students, most of whom are from an underrepresented group in ecology. Last Modified: 12/30/2022 Submitted by: Casey P Terhorst