Characteristics of a Novel Tyrosinase Gene Involved in the Formation of Shell Color in Hard Clam Meretrix meretrix

Hard clam (Meretrix meretrix) has a rich shell color variation among individuals from light yellow to bluish gray, brown red, and black, which may associating with melanin. Tyrosinase (Tyr) is a key rate-limiting enzyme for the biosynthesis of melanin, which affects the dark color of animal skin, hair, fur, scales and feathers. Here, we isolated and characterized the full-length cDNA sequence of M. meretrix tyrosinase gene (MmTyr) which encodes a protein with 689 amino acids. Sequence characteristics and phylogenetic analysis showed that there are a variety of Tyr genes in mollusks, which can be divided into different categories. The abundance of MmTyr transcript was detected in six tissues including mantle, adductor muscle, digestive gland, foot, gill and siphon of adult M. meretrix with qRT-PCR. The results showed a higher expression specifically in the mantle, digestive gland and siphon, which was consistent with the phenotypic color difference. In the embryos and larvae at different developmental stages, high expression was found in the trochophore larvae and juvenile clams. Among three M. meretrix strains with different shell colors, the expression level of MmTyr and total melanin content in the mantle edge both showed significant differences, suggesting that MmTyr is involved in the formation of shell color. The results obtained in this study will improve our understanding of molluscan tyrosinase gene function and molecular mechanism of shell color pattern determination.