Molecular cloning and differential expression in tissues of a tyrosinase gene in the Pacific oyster Crassostrea gigas

Tyrosinases are a group of type-3 copper proteins that catalyze the first two reactions in the melanin biosynthesis in organisms ranging from bacteria, fungi, plants to animals. Tyrosinases are not only involved in pigmentation, but also play an important role in the innate immunity in invertebrates. Additionally, tyrosinases are also known to be involved in the biogenesis and pigmentation of shells. The recently published Crassostrea gigas genome sequences revealed that the Pacific oyster possesses at least 26 tyrosinase isoforms. However, their molecular features are largely understudied. In fact, the full-length mRNA sequence was determined for one of the tyrosinase genes (i.e., CgTry1; aka cgi-tyr1). Here we report the full-length transcript of a second C. gigas tyrosinase (CgTyr2) sequence and the determination of its sequence features characteristic to the tyrosinase family proteins. We also showed that CgTyr2 gene was differentially expressed with the highest level of expression in mantle edges, suggesting its potential role in the formation of periostracum/pigmentation. Our comprehensive phylogenetic reconstructions supported that hemocyanins possibly evolved from a tyrosinase by an ancient gene duplication followed by functional differentiation, and the current large number of tyrosinase isoforms in C. gigas and other mollusks were originated from multiple gene duplication events that took places before and after mollusk species were established.