Gonadotropin-releasing hormone (GnRH) in bony fish that are phylogenetically ancient: reedfish (Calamoichthys calabaricus), sturgeon (Acipenser transmontanus), and alligator gar (Lepisosteus spatula).

Three species of fish that are phylogenetically older than other members of the bony fish lineage were selected to determine if gonadotropin-releasing hormone (GnRH) is present in their brains. Brain extracts were prepared from each species and found to contain immunoreactive (ir) GnRH. To further characterize the molecular forms of GnRH in each species, the extracts were injected into a high pressure liquid chromatograph (HPLC). The elution time of each GnRH-like form was compared to those of the synthetic forms of the five known GnRHs. Several antisera were used to detect both the synthetic and unknown GnRHs in the HPLC fractions. All three species of fish had two forms of GnRH: a dominant form that is mammalian GnRH-like (mGnRH), and a minor form of irGnRH material that is similar to chicken GnRH-II (cGnRH-II). The other known forms of GnRH (salmon, lamprey, and chicken-I) were not detected. The appearance in these ancient bony fish of a mammalian-like form of GnRH, which has not been found in the jawless or cartilaginous fish studied to date, suggests that mGnRH arose in a common phylogenetic ancestor of the bony fish and tetrapods. This mGnRH-like molecule is known to have been conserved in the amphibian and mammalian lineage, but not in the reptilian or avian line. In addition, the presence of a cGnRH-II-like molecule in the bony fish examined here, and in the cartilaginous fish studied earlier, implies that this form of GnRH may have been present in an ancestor common to both of these classes of fish.