Emerging Perspectives on Gonadotropin Regulation in Vertebrates Revealed by the Discovery of FSH-RH in Teleosts

Vertebrate gonadal function is regulated by pituitary gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These hormones are considered to be regulated by hypothalamic factor(s). Since the discovery of gonadotropin-releasing hormone (GnRH) in mammals, which stimulates the secretion of both FSH and LH, GnRH had been believed to be the sole gonadotropin-releasing hormone in vertebrates for more than 5 decades. However, recent studies have identified an alternative primary regulator of FSH in teleosts, leading to the hypothesis that FSH and LH are regulated by different factors in teleosts (dual GnRH model). This contrasts with the situation in mammals, where a single GnRH regulates both hormones (solo GnRH model). Importantly, although underlying mechanisms likely differ, both teleosts and mammals reproduce efficiently and have convergently evolved similar phenomena, including steroid feedback regulation. In this review, by comparing these taxa, we summarize mechanistic differences and propose an evolutionary scenario based on current experimental evidence.