Generation of Self-Fertilizing Hermaphroditic Fish from Gonochoristic Fish, Medaka (Oryzias latipes).

Introduction: Hermaphroditism is a mode of reproduction involving an individual animal that possesses both a testis and an ovary either sequentially or simultaneously. The mechanism creating hermaphrodites remains unknown. Previously, we identified foxl3 as the germline sex determination gene in a gonochoristic fish, medaka (Oryzias latipes). foxl3 loss-of-function (foxl3-/-) females produce functional sperm as well as eggs in the ovary. However, these two gametes are not self-fertilizing because of the histological separation of each gamete production. In this study, we attempted to generate self-fertilizing medaka from female medaka by modifying germline sex using foxl3-/- mutants and by using exogenous androgen to induce partial sex reversal of somatic cells.

Methods: foxl3-/- XX females were treated with 11-ketotestosterone (11-KT), a potent teleost fish androgen, at the sexually mature stage for 30 days (90-120 dph). Then, the fish were kept under normal conditions until they were either being dissected or crossed with infertile males.

Results and discussion: We showed that the foxl3-/- XX female medaka can be transformed into a self-fertilizing hermaphrodite by inducing the formation of a male-like structure with exogenous 11-KT. Self-fertilization occurs in either the ovarian cavity, the oviduct, or both where sperm is released from a tubule-like structure which is likely derived from germinal epithelium, suggesting that timely modification of 2 independent mechanisms, regulation of germline sex and partial sex reversal of somatic cells, are critical to change the reproduction mode. Our results will provide insights in developmental and evolutional occurrence of hermaphrodite vertebrates, facilitate an innovative technique to improve the efficient selection of fish with desirable traits, and contribute to the rescue of endangered species.