Evolution and functional analysis of the Pif97 gene of the Pacific oyster Crassostrea gigas

Mollusc shell matrix proteins (SMPs) are important functional components embedded in the shell and play a role in shell formation. A SMP (Pif177) was identified previously from the nacreous layer of the Japanese pearl oyster Pinctada fucata, and its cleavage products (named pfPif97 and pfPif80 proteins) were found to bind to the chitin framework and induce aragonite crystal formation and orient the c axis. In this study, a homologue of pfPif177 was cloned from the mantle of the Pacific oyster Crassostrea gigas, containing the homologue of pfPif97 only and not pfPif80. This finding hints at the large divergence in gene structure between the two species. This homologue (cgPif97) shares characteristics with pfPif97, and suggests that the biological functions of these two proteins may be similar. The expression pattern of cgPif97 in different tissues and development stages indicates that it may play an important role in shell formation of the adult oyster. The morphology of the inner shell surface was affected by injected siRNA of cgPif97 and the calcite laths of the shell became thinner and narrower when the siRNA dose increased, suggesting that the cgPif97 gene plays an important role in calcite shell formation in C. gigas. In conclusion, we found evidence that the Pif177 gene evolved very fast but still retains a similar function among species [Current Zoology 59 (1): 109–115, 2013].