Protoporphyrin IX metabolism mediated via translocator protein (CgTspO) involved in orange shell coloration of pacific oyster (Crassostrea gigas)

Mollusc shell color polymorphism is influenced by various factors. Pigments secreted in vivo by animals play a critical role in shell coloration. Among the different shell-color hues, orange pigmentation has been partially attributed to porphyrins. However, the detailed causal relationship between porphyrins and orange-shell phenotype in molluscs remains largely unexplored. The various strains of Pacific oyster (Crassostrea gigas) with different shell color provide useful models to study the molecular regulation of mollusc coloration. Accordingly, oysters with orange and gold-shells, exhibiting distinct porphyrin distributions, were selected for analysis of total metabolites and gene expression profile through mantle metabolomic and transcriptomic studies. Translocator protein (TspO) and protoporphyrin IX (PPIX) were identified as potential factors influencing oyster shell-color. The concentration of PPIX was measured using HPLC, while expression profiling of CgTspO was analyzed by qPCR, in situ hybridization, Western blotting, and immunofluorescence techniques. Moreover, the roles of CgTspO in regulating PPIX metabolism and affecting the orange-shell-coloration were investigated in vitro and in vivo. These studies indicate that PPIX and its associated metabolic protein, CgTspO may serve as new regulators of orange-shell-coloration in C. gigas. Data of this study offer new insights into oyster shell coloration and enhancing understandings of mollusc shell color polymorphism.