The origin of the mammalian form of GnRH in primitive fishes

The presence of neuroendocrine hormones in extant agnathan fishes suggests that a method of control involving these hormones was operating 500–600 million years ago in emerging vertebrates. Data on a limited number of species show that several members of the GnRH family of peptides may have arisen in non-teleost fishes. Lamprey (Petromyzon marinus) GnRH has a unique composition and has not been detected in other vertebrates. It is not yet clear whether the chicken II GnRH-like molecule arose in cartilaginous fishes, but a chromatographically and immunologically similar molecule is found in dogfish (Squalus acanthias) and ratfish (Hydrolagus colliei). Finally, a mammalian GnRH-like molecule is detected in three primitive bony fish: sturgeon (Acipenser transmontanus), reed fish (Calamoichthys calabaricus), and alligator gar (Lepidosteus spatula). Minor forms are also present, but are not yet characterized. Clearly, the basic structure of GnRH peptides was established in primitive fish. In contrast, at least three other identified forms of GnRH have been detected in teleosts or tetrapods: Salmon I, catfish I, and chicken I GnRH. Evidence for the presence of members of the GnRH family and the neurohypophysial hormone family in primitive fishes argues for the importance of neuroendocrine control throughout the history of vertebrates.