Coral adaptations across ecological and evolutionary scales
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Abstract
Marine organisms occupy environments spanning wide distributions of conditions across large (e.g., latitudinal gradients) and small (e.g., different depths) scales. Populations that exist across these environments might be adapted to local conditions and exhibit genetic divergence between habitats, sometimes to the point of becoming different species or lineages within a species complex. Corals are excellent organisms in which to study dynamics across large geographic ranges spanning divergent environments. They not only contain many species rich genera and exhibit local adaptation and long-range dispersal potential, but they are also the structural engineers of coral reef ecosystems that are facing serious threat from warming oceans, acidification, and other anthropogenic disturbances. Thus, investigating mechanisms of adaptation to environmental conditions in corals is critical because these processes also secondarily impact many reef-dependent marine organisms. My dissertation investigates population genomic variation across different ecological (latitudinal, local and within colony gradients) and evolutionary (populations and species) scales. First, I obtained samples of the ubiquitous pacific reef-building coral Acropora hyacinthus from its subtropical habitat in the Ryukyus Islands and temperate habitat in mainland Japan and uncovered the presence of three cryptic lineages in the region. Of these three, only one exists in temperate environments and this lineage has also recently expanded its range even further north along the coast of mainland Japan with warming oceans. I found genetic structure separating the recently expanded site and the other northernmost edge sites from core temperate sites. This divergence existed despite a model of larval dispersal suggesting higher connectivity of marginal and core sites relative to pairs of marginal sites. These findings suggest that cryptic lineages evolved to occupy different niches along a latitudinal gradient and that range expansion has been facilitated by adaptions to higher latitudes. Second, I investigated the role of the coral’s algal symbiont and bacterial communities in adaptation across smaller spatial scales by characterizing these communities in the massive scleractinian coral Porites lobata across a sedimentation gradient and across individual colonies in Guam. I found that both algal and bacterial communities varied within a single colony, but only bacteria showed clear structuring by colony position and only rare bacterial taxa were structured by the sedimentation gradient. Lastly, I investigated the contributions of host and symbiont to thermal adaptation in the facultatively symbiotic corals Astrangia and Oculina. I uncovered the existence of four genetic lineages of coral, two within each genus, with inter-lineage differences in distributions, thermal performance, and symbiotic partnerships. I also found evidence for shared genetic variation between Oculina and Astrangia when they exist sympatrically, suggesting the potential for adaptive introgression between these lineages. The findings of this dissertation shed light on the complex and dynamic nature of coral populations, highlighting the importance of considering multiple spatial scales and levels of evolutionary divergence when studying adaptation in marine organisms. Overall, this dissertation advances our understanding of the population genomics of corals and the ecological and evolutionary processes that shape their adaptation to diverse environments.