Transcriptomic and Physiological Analysis Reveal Melanin Synthesis-Related Genes and Pathways in Pacific Oysters (Crassostrea gigas)

Shell color is one of the shell traits of molluscs, which has been regarded as an economic trait in some bivalves. Pacific oysters (Crassostrea gigas) are important aquaculture shellfish worldwide. In the past decade, several shell color strains of C. gigas were developed through selective breeding, which provides valuable materials for research on the inheritance pattern and regulation mechanisms of shell color. The inheritance patterns of different shell colors in C. gigas have been identified in certain research; however, the regulation mechanism of oyster pigmentation and shell color formation remains unclear. In this study, we performed transcriptomic and physiological analyses using black and white shell oysters to investigate the molecular mechanism of melanin synthesis in C. gigas. Several pigmentation-related pathways, such as cytochrome P450, melanogenesis, tyrosine metabolism, and the cAMP signaling pathway were found. The majority of differentially expressed genes and some signaling molecules from these pathways exhibited a higher level in the black shell oysters than in the white, especially after L-tyrosine feeding, suggesting that those differences may cause a variation of tyrosine metabolism and melanin synthesis. In addition, the in vitro assay using primary cells from mantle tissue showed that L-tyrosine incubation increased cAMP level, gene and protein expression, and melanin content. This study reveals the difference in tyrosine metabolism and melanin synthesis in black and white shell oysters and provides evidence for the potential regulatory mechanism of shell color in oysters.