Identification of a laccase gene involved in shell periostracal tanning of the clam Meretrix petechialis

Tanning is a complex extracellular process that is a mechanism for stabilizing proteinaceous extracellular structures. Phenoloxidases play important roles in cross-linking during tanning, and laccase (EC 1.10.3.2) is a member of the phenoloxidase enzyme class. In this study, we identified a laccase gene (MpLac) from the clam Meretrix petechialis and found that MpLac might be involved in shell periostracal tanning of clams. Using whole-mount in situ hybridization, we found that MpLac mRNA in the larval clam was mainly expressed in the mantle edge. In the adult clam, our quantitative real-time PCR results showed that the mantle was also a tissue with a high MpLac expression level; in addition, by combining the results of fluorescence in situ hybridization, H&E staining and transmission electron microscopy, we found that the inner epithelium of the outer fold of the mantle edge, which is involved in periostracum formation, was the exact region in which MpLac mRNA was expressed. Furthermore, knocking down the expression of MpLac by RNA interference (RNAi) bleached the new shell periostracum. All of our results suggest the involvement of MpLac in shell periostracal tanning of M. petechialis.