Functional conservation and divergence of duplicated insulin-like growth factor 2 genes in grass carp (Ctenopharyngodon idellus)

Insulin-like growth factor 2 (IGF2) is a potent mitogenic and survival factor involved in the regulation of growth, development and reproduction in animals. Only one IGF2 gene exists in mammals. Recently, two igf2 genes have been identified in zebrafish, which presumably resulted from gene duplication. However, sequence information of duplicated igf2s and their functional regulation in other teleost fish is still unknown. Here, we report the identification of two igf2 cDNAs in grass carp, Ctenopharyngodon idellus. Like their human ortholog, grass carp igf2a and igf2b mRNAs encoded two structurally distinct mature IGF peptides. Both of them were detected by RT-PCR throughout embryogenesis. Ubiquitous expression of igf2b mRNAs was observed in embryos, whereas igf2a mRNAs were expressed mainly in the notochord and brain with in situ hybridization. In adult fish, igf2b mRNAs were transcribed in multiple tissues, whereas igf2a mRNAs were detected mainly in the liver. Hepatic levels of igf2a and igf2b transcripts were both up-regulated by growth hormone injection. Furthermore, the levels of hepatic igf2a and igf2b mRNAs decreased significantly during starvation and were rebounded rapidly after re-feeding. Our results suggest that duplicated igf2 genes have evolved divergent yet played an overlapping biological role in regulating grass carp growth and development.