DIVERGENCE OF INSULIN-LIKE-GROWTH-FACTORS-I AND INSULIN-LIKE-GROWTH-FACTORS-II IN THE ELASMOBRANCH, SQUALUS-ACANTHIAS

Recent studies have shown that vertebrates, including teleostean fishe s, amphibians, birds and mammals, contain two distinct insulin-like gr owth factor (IGF) genes, In contrast agnathans, represented by hagfish , apparently have only one IGF that has features characteristic of bot h IGF-I and IGF-II. Between these groups the elasmobranchs occupy a cr itical position in terms of the phylogeny of IGFs. We sought to determ ine if gene duplication and divergence of IGF-I and IGF-II occurred be fore or after divergence of elasmobranchs from other vertebrates by cl oning IGF-like molecules from Squalus acanthias. Our analysis shows th at Squalus liver produces two distinct IGF-like molecules, One has gre ater sequence identity to, and conserved features characteristic of, k nown IGF-I molecules, while the other is more IGF-II like. These resul ts suggest that the prototypical IGF molecule duplicated and diverged in an ancestor of the extant gnathostomes.