Involvement of insulin-like growth factor in intraspecific variation in growth of Pacific oyster Crassostrea gigas during winter

Insulin-like growth factor I (IGF-I) plays a critical role in regulating proliferation, differentiation, growth, and survival in fish as well as in humans. Few studies have investigated the involvement of IGFs in bivalves. In Korea, the Pacific oyster Crassostrea gigas is an important aquaculture species, and its growth is dependent on environmental conditions. Intrinsic factors that can be used as indicators to assess oyster growth based on individual size are limited. This study analyzed IGF-I, IGF-I receptor (IGF-IR), and IGF binding protein (IGFBP) in adductor muscle of C. gigas in winter using immunoblotting and reverse transcriptase-polymerase chain reaction. Levels of IGF-I and the IGF-IR beta subunit, and expression of C. gigas insulin receptor-related receptor and IGFBP messenger RNA were significantly higher in large individuals (LI) than small individuals (SI). The IGF-IR beta subunit was detected in the adductor muscle of both groups; however, phosphorylation of the IGF-IR beta subunit and extracellular signal-regulated kinase was greater in LI than SI. The condition index of C. gigas showed a similar pattern. The results of this study suggest that the IGF system is associated with growth rate differences in C. gigas during winter in Korea.