Divergence of insulin‐like growth factors I and II in the elasmobranch, Squalus acanthias

Abstract

Recent studies have shown that vertebrates, including teleostean fishes, amphibians, birds and mammals, contain two distinct insulin‐like growth factor (IGF) genes. In contrast agnathans, represented by hagfish, apparently have only one IGF that has features characteristic of both IGF‐I and IGF‐II. Between these groups the elasmobranchs occupy a critical position in terms of the phylogeny of IGFs. We sought to determine if gene duplication and divergence of IGF‐I and IGF‐II occurred before or after divergence of elasmobranchs from other vertebrates by cloning IGF‐like molecules from Squalus acanthias. Our analysis shows that Squalus liver produces two distinct IGF‐like molecules. One has greater sequence identity to, and conserved features characteristic of, known IGF‐I molecules, while the other is more IGF‐II like. These results suggest that the prototypical IGF molecule duplicated and diverged in an ancestor of the extant gnathostomes.