Expression of tyrosinase-like protein genes and their functional analysis in melanin synthesis of Pacific oyster (Crassostrea gigas)

Color polymorphism in Mollusca is of great interest for consumer preference. Although the heritability of shell color variation has been conducted by experimental crossing, little is known about molecular basis involved in these patterns. Tyrosinase-like proteins are important enzymes which are members of the type-3 copper protein superfamily. In this research, two tyrosinase-like protein genes including CgTyp-1 and CgTyp-3 were identified in the Pacific oyster Crassostrea gigas. Tissue expression analysis showed that CgTyp-1 and CgTyp-3 were dominantly expressed in the mantle. Particularly, they were expressed significantly higher in the edge mantle than that in the central mantle whether on the left or right mantles. Additionally, expressions of CgTyp-1 and CgTyp-3 were mainly found in the black shell color oysters, with relative lower levels in the white shell color oysters. In situ hybridization showed that positive signals for CgTyp-1 and CgTyp-3 were both detected within the outer epithelium of the outer fold either in the black or white shell color oysters. After interference, the expression levels of CgTyp-1 and CgTyp-3 mRNA were significantly attenuated, and the efficiency of RNAi reached 84.72% and 71.58%, respectively. Besides, knockdown CgTyp-1 or CgTyp-3, obviously decreased the tyrosinase activity of mantles. Furthermore, the number of the melanosomes within epithelium of the outer fold was sharply reduced by silencing of each Typ. These findings argue that CgTyp-1 and CgTyp-3 may be involved in the melanin synthesis, which lends insight into regulation mechanism of shell pigmentation in C. gigas.