Evolution of glucagon genes

Abstract

Statistical analyses of DNA sequences of the preproglucagon genes from bovine, human, hamster, and anglerfish suggest that a gene duplication creating two anglerfish genes (AF I and II) occurred about 160 Myr ago, long after the separation of fish and mammals. The analyses further suggest that the internal duplication producing the glucagon and glucagon-like peptide II (GLP-II) regions occurred about 1.2 billion years ago, which would indicate that the GLP-II region was present in the ancestral anglerfish sequence but was silenced or deleted before the gene duplication separating AF I and II. The glucagon-like peptide I (GLP-I) was derived from a duplication of the ancestral glucagon region about 800 Myr ago. The rate of synonymous substitution in these genes is approximately 4.3 x 10-9 substitutions per year per synonymous site. The rate of nonsynonymous substitution in the signal peptide region is about 1.1 x 10-9 substitutions per year per nonsynonymous site, a high rate comparable to that in the C-peptide region of preproinsulin. The rate of nonsynonymous substitution in the glicentin-related pancreatic polypeptide (GRPP) region is 0.63 x 10-9 for the comparisons between mammalian species and 1.8 x 10-9 for the comparisons between fish and mammals; the moderate rate in mammals suggests a physiological role for GRPP. The glucagon region is extremely conservative; no nonsynonymous substitution is observed in the mammalian genes, and a nonsynonymous rate of 0.18 x 10-9 was obtained from the comparisons between fish and mammals. In the GLP-I region, the rate of nonsynonymous substitution was estimated to be 0.08 x 10-9 for the comparisons between mammalian species and 0.30 x 10-9 for the comparisons between fish and mammals. In the GLP-II region, the rate was estimated to be 0.25 x 10-9 for the comparisons between mammalian species. Thus, GLP-I and II are also very conservative, which suggests an important physiological role for these peptides.