Heritable variation in bleaching responses and its functional genomic basis in reef-building corals (Orbicella faveolata)

Reef-building corals are highly sensitive to rising ocean temperatures, and substantial adaptation will be required for animals and the ecosystems they support to persist in changing ocean conditions. Genetic variation that might support adaptive responses has been measured in larval stages of some corals, but these estimates remain unavailable for adult corals and the functional basis of this variation remains unclear. In this study, we focused on the potential for adaptation in Orbicella faveolata, a dominant reef-builder in the Caribbean. We conducted thermal stress experiments using corals collected from natural populations in Bocas del Toro, Panama, and used multilocus SNP genotypes to estimate genetic relatedness among samples. This allowed us to estimate narrow-sense heritability of variation in bleaching responses, revealing that this variation was highly heritable (h2=0.91). This estimate suggests substantial potential for adaptive responses to warming by natural populations of O. faveolata in this region. To investigate the functional basis for this variation, we applied genomic and transcriptomic approaches enabled by expanding sequence resources for this species. We used a genetic linkage map we have recently developed to map associations between SNP genotypes and bleaching responses, and identified four genetic markers associated with bleaching. We also profiled gene expression in corals with contrasting bleaching phenotypes, uncovering substantial variation in responses to thermal stress between heat-tolerant and heat-susceptible corals. Integrating these genomic and transcriptomic data with quantitative genetic analysis provides a new perspective on the mechanistic basis for thermal tolerance phenotypes and the potential for adaptation to rising ocean temperatures.