Role of the bicarbonate transporter SLC4 gamma in stony-coral skeleton formation and evolution

Coral reefs are highly diverse ecosystems of immense ecological, economic, and aesthetic importance built on the calcium-carbonate based skeletons of stony corals. The formation of these skeletons is threatened by increasing ocean temperatures and acidification, and a deeper understanding of the molecular mechanisms involved may assist efforts to mitigate the effects of such anthropogenic stressors. In this study, we focused on the role of the predicted bicarbonate transporter SLC4 gamma, which was suggested in previous studies to be a product of gene duplication and to have a role in coral-skeleton formation. Our comparative-genomics study using 30 coral species and 15 outgroups indicates that SLC4 gamma is present throughout the stony corals, but not in their non-skeleton-forming relatives, and apparently arose by gene duplication at the onset of stony coral evolution. Our expression studies show that SLC4 gamma, but not the closely related and apparently ancestral SLC4 gamma, is highly upregulated during coral development coincident with the onset of skeleton deposition. Moreover, we show that juvenile coral polyps carrying CRISPR/Cas9-induced mutations in SLC4 gamma are defective in skeleton formation, with the severity of the defect in individual animals correlated with their frequencies of SLC4 gamma mutations. Taken together, the results suggest that the evolution of the stony corals involved the neofunctionalization of the newly arisen SLC4 gamma for a unique role in the provision of concentrated bicarbonate for calcium-carbonate deposition. The results also demonstrate the feasibility of reverse-genetic studies of ecologically important traits in adult corals.