Evolution of the Hypothalamic-Pituitary-Gonadal Axis Regulation in Vertebrates Revealed by Knockout Medaka.

Reproduction is essential for life, but its regulatory mechanism is diverse. The analysis of this diversity should lead us to understand the evolutionary process of the regulation of reproduction. In mammals, the hypothalamic-pituitary-gonadal axis plays an essential role in such regulation, and each component, hypothalamic GnRH, and pituitary gonadotropins, LH, and FSH, is indispensable. However, the common principle of the hypothalamic-pituitary-gonadal axis regulation among vertebrates remains unclear. Here, we used a teleost medaka, which is phylogenetically distant from mammals, and analyzed phenotypes of gene knockouts (KOs) for GnRH, LH, and FSH. We showed that LH release, which we previously showed to be directly triggered by GnRH, is essential for ovulation in females, because KO medaka of GnRH and LH were anovulatory in spite of the full follicular growth and normal gonadosomatic index, and spawning could be induced by a medaka LH receptor agonist. On the other hand, we showed that FSH is necessary for the folliculogenesis, because the follicular growth of FSH KO medaka was halted at the previtellogenic stage, but FSH release does not necessarily require GnRH. By comparing these results with the previous studies in mammals that both GnRH and LH are necessary for folliculogenesis, we propose a hypothesis as follows. During evolution, LH was originally specialized for ovulation, and regulation of folliculogenesis by GnRH-LH (pulsatile release) was newly acquired in mammals, which enabled fine tuning of reproduction through hypothalamus.