The physiology, evolution, and genomics of thermal tolerance in reef-building corals [electronic resource]
Keywords: Biology.

Increasing temperatures associated with anthropogenic climate change are likely to surpass the thermal limits of many species in the coming decades. Predictions of species response to these changes are predicated on the assumption that heat tolerance is static through time. Countless studies, however, have demonstrated that individual organisms and populations can shift their upper thermal limits through either acclimation, in which individuals exposed to increased temperatures shift their physiology, or adaptation, in which whole populations shift their thermal limits through natural selection for the most heat tolerant genotypes. These processes are especially relevant to foundation species, such as reef-building corals, as these organisms are often sessile, and so cannot easily shift their range. In this dissertation, I examined patterns and processes involved in acclimation and adaptation in reef-building corals, including their impacts on upper thermal limits, the genes and cellular pathways involved, and the relative roles of the two mechanisms over small and large spatial scales. In Chapter 1, I conducted a time series analysis of acclimation. I found an increase in heat tolerance after one week at higher temperatures and a corresponding shift in gene expression, resulting in a dampened response to heat stress. In Chapter 2, I utilized a small-scale natural temperature gradient to search for genetic variants associated with corals adapted to different temperatures. I found 114 genetic variants that differed between warm and cool microclimates separated by less than 1km. In Chapter 3, I tested these variants using a reciprocal transplants between warm and cool microclimates. I found that corals from warm microclimates survived more often, but a subset of genetic variants associated with survival were also associated with lower growth, suggesting a tradeoff in overall fitness. In Chapters 4 and 5, I examined tradeoffs and mechanisms of adaptation and acclimation over a large spatial scale: across latitude. Using a combination of acclimation and heat stress experiments, I found evidence for a high degree of plasticity in upper thermal limit for corals at both low and high latitudes. Despite this plasticity, however, there were still significant fixed effects between locations, suggesting large roles for both acclimation and adaptation in determining heat tolerance across latitude. In Chapter 5, I examined the molecular basis of these shifts in thermal tolerance. I found divergence at both the sequence and gene expression levels for populations from low and high latitude locations, eventually leading to the discovery that the two populations were separate cryptic species: a warm adapted congener and a more broadly distributed congener. This allowed me to compare gene expression patterns between the two cryptic species with different evolutionary histories, finding transcriptional differences across a broad range of cellular functions that could be involved in adaptive thermal tolerance. I also found a set of genes whose expression was tightly linked with bleaching status; these genes are therefore good candidates 'biomarkers' that could be used for prediction of heat stress and tolerance. Together, these studies highlight important roles for both acclimation and adaptation in determining thermal tolerance in corals and provide insights useful for predicting the response of these ecologically and economically important taxa to future climate change.