Amanda I. Tinoco, Lorna M. Y. Mitchison-Field, Jacob Bradford, +4 , and Phillip A. Cleve

PNAS June 5, 2023; 120 (24) e2216144120

https://doi.org/10.1073/pnas.2216144120

Significance

Coral reefs are biodiversity hotspots that are in decline due to stressors associated with climate change. Thus, a critical research goal is to achieve a deeper understanding of basic coral biology to inform conservation efforts. This goal has been difficult to achieve due to the lack of genetic tools for corals. Here, we used CRISPR/Cas9 mutagenesis to show that a predicted bicarbonate transporter that apparently evolved specifically in the common ancestor of the stony corals is indeed required for formation of the calcium-carbonate skeleton in young coral colonies. The ability to do such genetic analysis in adult corals should now allow critical tests of hypotheses about gene function and the generation of stable, genetically modified lines for research and conservation.

Abstract

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 SLC4γ, 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 SLC4γ 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 SLC4γ, but not the closely related and apparently ancestral SLC4β, 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 SLC4γ are defective in skeleton formation, with the severity of the defect in individual animals correlated with their frequencies of SLC4γ mutations. Taken together, the results suggest that the evolution of the stony corals involved the neofunctionalization of the newly arisen SLC4γ 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.

See https://www.pnas.org/doi/10.1073/pnas.2216144120