Saito D, Tanaka M
Laboratory of Molecular Genetics for Reproduction, National Institute for Basic Biology, Higashiyama, Okazaki, Japan.
Sex Dev. 2009;3(2-3):99-107. doi: 10.1159/000223075. Epub 2009 Aug 10.
Medaka is a differentiated gonochoristic species with a male heterogametic sex determination. Here, we review recent studies on gonadal sex differentiation in medaka, as well as the experimental sex-reversal models available for this organism. The accumulated literature on teleost gonadal development facilitates comparative interspecies studies. Among these, comparison between medaka and zebrafish is of special interest, because zebrafish is an undifferentiated gonochorist that, nevertheless, shares many similarities with medaka and is also used as a small fish model for developmental biology. Accordingly, here we focus on the comparative aspects of gonadal development in medaka and zebrafish. In medaka, oogenesis begins in female gonads, whereas oogenesis is suppressed and germ cells remain in an undifferentiated state in male gonads. In zebrafish, oogenesis begins in all individuals, regardless of their future sex, while actual sex differentiation begins later in gonadal development, which means that degeneration of immature oocytes occurs in presumptive males, while oogenesis proceeds to completion in presumptive females. Despite these apparent differences between medaka and zebrafish, the process of gonadal development comprises similar stages: early oogenesis, early aromatase expression, later oocyte development or loss, sexually dimorphic expression of somatic genes, and spermatogenesis. We propose the concept of canalization as a key to gaining a comprehensive understanding of gonadal sex differentiation. In this respect, the possible role of the male-determining gene DMY/dmrt1bY is also discussed.
青鳉是一种具有雄性异配性别决定的两性分化物种。在此，我们综述了近期关于青鳉性腺性别分化的研究，以及适用于该生物体的实验性性别反转模型。有关硬骨鱼性腺发育的大量文献有助于进行种间比较研究。其中，青鳉和斑马鱼之间的比较尤为有趣，因为斑马鱼是一种未分化的两性异体物种，然而它与青鳉有许多相似之处，并且也被用作发育生物学的小鱼模型。因此，在这里我们重点关注青鳉和斑马鱼性腺发育的比较方面。在青鳉中，卵子发生始于雌性性腺，而在雄性性腺中卵子发生受到抑制，生殖细胞保持未分化状态。在斑马鱼中，卵子发生在所有个体中开始，无论其未来性别如何，而实际的性别分化在性腺发育后期开始，这意味着在推定的雄性中未成熟卵母细胞会退化，而在推定的雌性中卵子发生会持续到完成。尽管青鳉和斑马鱼之间存在这些明显差异，但性腺发育过程包括相似的阶段：早期卵子发生、早期芳香化酶表达、后期卵母细胞发育或消失、体细胞基因的性别二态性表达以及精子发生。我们提出了“ canalization”这一概念，作为全面理解性腺性别分化的关键。在这方面，还讨论了雄性决定基因DMY/dmrt1bY的可能作用。
