Visual adaptation of opsin gene expression to the aquatic environment in sea turtles

Several vertebrate taxa, cetaceans, sirenia, pinnipeds, and sea snakes have adapted to the marine aquatic environment. In the species of these taxa, marine adaptation has resulted in shifts in the absorption spectra of opsin pigments and/or the degeneration or duplication of opsin genes. Thus, marine adaptation has strongly affected the evolution of opsins. In sea turtles, however, the effect of adaptation from freshwater to marine environments on opsin evolution has not been studied. In this study, we determined the high-throughput RNA sequences extracted from eyes of two sea turtles (green turtle: Chelonia mydas, loggerhead: Caretta caretta) and two freshwater turtle species (three-keeled pond turtle: Mauremys reevesii, softshell turtle: Pelodiscus sinensis) and investigated the amino acid evolution and expression of the opsin gene. We found that most of the sea turtle lineage-specific amino acid substitutions did not alter amino acid properties and did not include previously known substitutions for turning absorption spectra of opsin pigments, suggesting no adaptive amino acid substitutions in the opsins during marine adaptation in sea turtles. Instead, the blue-sensitive opsin (SWS2) gene expression was higher in sea turtles than in freshwater turtles. These results suggest that sea turtles may have adapted their vision to the blue light-rich marine environment by increasing SWS2 expression.