Evolution of preproinsulin gene in birds.

The coding region of the preproinsulin gene has been cloned and partly sequenced in a variety of marine and terrestrial birds (28 species). All genes showed the "ancestral" structure with a large intron-2. The size of intron-2 changed considerably during the evolution of birds (2.4-4.2kb). The hydrophobicity of signal peptides was conserved. Bird C-peptides were predicted to be 28 amino acids long, but circulating C-peptides would be only 26 amino acids long, with Passer as a possible exception. Bird C-peptides were found to lack the sequences identified in mammals as responsible for peptide bioactivity and the structure of the central part. In contrast, predicted insulin sequences were highly conserved. Only two types of analog were identified: the hypoactive form (GluA8), present only in Anseriformes and the hyperactive form (His A8), present in all other species. Based on 3'-nucleotide sequence analysis (extending into intron-2), birds appeared to be monophyletic. Five groups were clearly identified: Paleognathae, Galliformes, Anseriformes, Passeriformes, and Charadriiformes. Paleognathae were suggested as the basal group, supporting the traditional view of avian evolution. Subsequent branching identified a gallo-anserae group and a group containing all other Neognathae. Surprisingly, Columba livia (Columbiforme order) clustered with Galliformes. With represented species, Procellariiformes and possibly Ciconiiformes, and Pelicaniformes were suggested as paraphyletic, in agreement with conclusions from some studies based on mitochondrial DNA sequences.