Abstract

After the discovery in 1996 of the growth hormone secretagogue-receptor type-1a (GHS-R1a) as an orphan G-protein coupled receptor, many research groups attempted to identify the endogenous ligand. Finally, Kojima and colleagues successfully isolated the peptide ligand from rat stomach extracts, determined its structure, and named it ghrelin (Kojima et al. 1999). The GHS-R1a is now accepted to be the ghrelin receptor. The existence of the ghrelin system has been demonstrated in many animal classes through biochemical and molecular biological strategies as well as through genome projects. Our work, which has concentrated on identifying the ghrelin receptor and its ligand ghrelin in laboratory animals, particularly non-mammalian vertebrates, has provided new insights into the molecular evolution of the ghrelin receptor. In mammals, it is assumed that the ghrelin receptor evolution is in line with the plate tectonics theory. In contrast, the evolution of the ghrelin receptor in non-mammalian vertebrates differs from that of mammals: multiplicity of the ghrelin receptor isoforms is observed in non-mammalian vertebrates only. This multiplicity is due to genome duplication and polyploidization events that particularly occurred in Teleostei. Furthermore, it is likely that the evolution of the ghrelin receptor is distinct from that of its ligand, ghrelin, since only one ghrelin isoform has been detected in all species examined so far. In this review, we summarize current knowledge related to the molecular evolution of the ghrelin receptor in mammalian and non-mammalian vertebrates.