Molecular characterization of Pax7 and its role in melanin synthesis in Crassostrea gigas.

The paired-box 7 (Pax7) is a transcription factor crucial for skin color polymorphism. However, the mechanism underlying the pigmentation associated with Pax7 in mollusks have yet to be elucidated. In this study, the cDNA sequence of Pax7 in the Pacific oyster Crassostrea gigas (CgPax7) was characterized. Phylogenetically, the identity of deduced amino acid sequence was similar to that of other mollusks and contained 463 amino acids, with conserved features of paired domain (PRD), homeobox domain (HD) and octapeptide. Gene expression analysis revealed that CgPax7 was markedly increased at D-shaped larvae stage and ubiquitously expressed in six examined tissues in adult oyster. The result of whole-mount in situ hybridization (WMISH) showed a restricted pattern of CgPax7 expression on margins of shell valves at D-shaped and umbo larvae stages. Additionally, although CgPax7 silencing had no significant effect on CgMitf expression, it significantly inhibited the expressions of CgPax7, CgTyr, CgTyrp1, CgTyrp2 and CgCdk2, genes involved in Tyr-mediated melanin synthesis. Furthermore, CgPax7 knockdown obviously decreased the tyrosinase activity. Less brown-granules at mantle edge was detected by micrographic examination and melanosomes defect was observed by transmission electron microscopy. It was demonstrated that CgPax7 play a key role in melanin synthesis by regulating Tyr-pathway in C. gigas. These findings indicated the potential framework by which mollusks pigmentation.