Sws 2 Gene Positively Regulates Melanin Production in Plectropomus leopardus Skin via Direct Regulation of the Synthesis of Retinoic Acid.

Opsins are a class of transmembrane proteins encoded by opsin genes, and they play a variety of functional roles. Short wavelength-sensitive opsin 2 ( sws 2), one of the five classes of visual opsin genes, mainly senses blue light. Previous research has indicated that sws 2 is essential for melanocyte formation in fish; however, its specific role in skin color differentiation remains to be elucidated. Here, we identified the sws 2 gene in a prized reef-dwelling fish, Plectropomus leopardus . The full-length P. leopardus sws 2 gene encodes a protein consisting of 351 amino acids, and exhibits substantial homology with other fish species. The expression of the sws 2 gene was widespread across P. leopardus tissues, with high expression in eye and skin tissues. Through immunohistochemistry and in situ hybridization analyses, we discovered that the sws 2 gene was primarily localized in the rod and cone cells of the retina, and epidermal cells of the skin. Furthermore, dsRNA interference was used for sws 2 gene knockdown in living P. leopardus to elucidate its function in skin color differentiation. Black-color-related genes, melanin contents, and tyrosinase activity in the skin significantly decreased after sws 2 knockdown ( p < 0.05), but red-color-related genes and carotenoid and lutein contents significantly increased ( p < 0.05). Retinoic acid injection produced the opposite results. Our results suggested that the sws 2 gene influences P. leopardus skin color regulation by affecting vitamin synthesis and melanin-related gene expression levels. This study establishes a foundation for elucidating the molecular mechanisms by which sws 2 regulates melanocyte formation in fish skin.