Amplification of Immune Genes in Ancient Stony Corals for Adapting to Unstable Marine Environments

The Late Devonian period was known for disturbances such as lower temperatures and abnormal ocean carbon and nitrogen levels, leading to the extinction of approximately 21% of genus-level and 16% of family-level marine organisms. However, evolutionary responses of marine organisms to hardships have not yet been fully explored, even though these organisms may soon face another extinction event. Stony corals, one of the few marine organisms that survived the Late Devonian Period, may provide some insight into the adaptive evolution mechanism underlying survival in unstable marine environments. The current study revealed that the gene families related to signal transduction and immunity, such as G protein-coupled receptors and Toll-like receptors, expanded in stony coral ancestors (SCAs), possibly improving the efficiency of stress and immune responses and maintaining internal environmental homeostasis. Interestingly, the first horizontal gene transfer event of MSHA from actinomycetes to corals and the subsequent expansion in SCAs were discovered. MSHA encodes D-inositol 3-phosphate glycosyltransferase, which is naturally found in actinomycetes and is responsible for the synthesis of mycothiol with antibacterial properties. The MSHA family members diverged throughout the development of stony corals, but their essential function in glycosyl transfer remained unchanged. Therefore, the evolutionary history of ancient coral shows that efficient signal transduction and increased immunity may have driven the survival of SCAs throughout the Late Devonian period, which may provide new insights into how current corals avoid extinction.