The glucagon (GCG) peptide family consists of GCG, glucagon-like peptide 1 (GLP1), and GLP2, which are derived from a
common GCG precursor, and the glucose-dependent insulinotropic polypeptide (GIP). These peptides interact with cognate
receptors, GCGR, GLP1R, GLP2R, and GIPR, which belong to the secretin-like G protein-coupled receptor (GPCR) family. We
used bioinformatics to identify genes encoding a novel GCG-related peptide (GCRP) and its cognate receptor, GCRPR. The
GCRP and GCRPR genes were found in representative tetrapod taxa such as anole lizard, chicken, and Xenopus, and in
teleosts including medaka, fugu, tetraodon, and stickleback. However, they were not present in mammals and zebrafish.
Phylogenetic and genome synteny analyses showed that GCRP emerged through two rounds of whole genome duplication
(2R) during early vertebrate evolution. GCRPR appears to have arisen by local tandem gene duplications from a common
ancestor of GCRPR, GCGR, and GLP2R after 2R. Biochemical ligand-receptor interaction analyses revealed that GCRP had the
highest affinity for GCRPR in comparison to other GCGR family members. Stimulation of chicken, Xenopus, and medaka
GCRPRs activated Gas-mediated signaling. In contrast to chicken and Xenopus GCRPRs, medaka GCRPR also induced Gaq/11-
mediated signaling. Chimeric peptides and receptors showed that the K16M17K18 and G16Q17A18 motifs in GCRP and GLP1,
respectively, may at least in part contribute to specific recognition of their cognate receptors through interaction with the
receptor core domain. In conclusion, we present novel data demonstrating that GCRP and GCRPR evolved through gene/
genome duplications followed by specific modifications that conferred selective recognition to this ligand-receptor pair.