Glycoprotein hormone in the pituitary of hagfish and its evolutionary implications

The pituitary gland is present in all vertebrates, from agnathans (jawless vertebrates) to mammals, but not in invertebrates. Reproduction in gnathostomes (jawed vertebrates) is controlled by two pituitary gonadotropins (GTHs), luteinizing hormone and follicle-stimulating hormone, which are part of the pituitary glycoprotein hormone (GPH) family. Hagfishes, which lack both jaws and vertebrae, are considered the most primitive vertebrate known, living or extinct. Accordingly, they are of particular importance in understanding the evolution of the pituitary GPHs and their functions related to vertebrate reproduction. Nevertheless, key elements of the reproductive endocrine system in hagfish have yet to be elucidated. Our current report has revealed the first identification of a functional GPH composed of two subunits that possess gonadotropic action at the pituitary of brown hagfish. It seems most likely that an ancestral GPH gave rise to only one GTH in hagfish pituitary and that multiplicity of GPHs arose later during the early evolution of gnathostomes. This paper briefly summarizes the latest findings on the hagfish GPH from an evolutionary point of view.