Figure 3

Proposed evolutionary model of chordate PACAP-like (A) and GCG-like (B) members. Percentage of amino acid sequence identity/similarity of the different peptide groups is indicated and gene organisation of the coding region (excluding occasional species-specific gene organisation) is represented. Secretin family members are proposed to have evolved via exon and gene/chromosome duplication events from a common ancestor exon in the chordate radiation. Similarity between the deduced consensus sequences of the peptide groups in the same subfamily is higher than 62% within the vertebrate GCG-like members and 66% for the PACAP-like subfamily with the exception of SCT in which only the mammalian and chicken members have been identified. Boxes represent exons and lines introns and coding exons are indicated by the peptide abbreviation. Dashed lines indicate undefined evolutionary pathways. (A) Chordate PACAP and PRP and vertebrate VIP and PH share the same gene precursor and GHRH and SCT are encoded by a single exon. PACAP and VIP share the highest amino acid conservation and SCT is the most divergent and to date has only been identified in tetrapods. (B) Vertebrate GCG, GLP1 and GLP2 are encoded in the same gene precursor which arose by exon duplication events. GIP is encoded by a single exon in a different precursor which has a similar gene organisation with GCG/GLP precursor.