Nakamura Masahisa
Department of Biology, Faculty of Education and Integrated Arts and Sciences, Waseda University, 2-2 Wakamatsu, Shinjuku-ku, Tokyo 162-8480, Japan.
Semin Cell Dev Biol. 2009 May;20(3):271-82. doi: 10.1016/j.semcdb.2008.10.003. Epub 2008 Oct 17.
The heterogametic sex is male in all mammals, whereas it is female in almost all birds. By contrast, there are two heterogametic types (XX/XY and ZZ/ZW) for genetic sex determination in amphibians. Though the original heterogametic sex was female in amphibians, the two heterogametic types were probably interchangeable, suggesting that sex chromosomes evolved several times in this lineage. Indeed, the frog Rana rugosa has the XX/XY and ZZ/ZW sex-determining systems within a single species, depending on the local population in Japan. The XY and ZW geographic forms with differentiated sex chromosomes probably have a common origin as undifferentiated sex chromosomes resulted from the hybridization between the primary populations of West Japan and Kanto forms. It is clear that the sex chromosomes are still undergoing evolution in this species group. Regardless of the presence of a sex-determining gene in amphibians, the gonadal sex of some species can be changed by sex steroids. Namely, sex steroids can induce the sex reversal, with estrogens inducing the male-to-female sex reversal, whereas androgens have the opposite effect. In R. rugosa, gonadal activity of CYP19 (P450 aromatase) is correlated with the feminization of gonads. Of particular interest is that high levels of CYP19 expression are observed in indifferent gonads at time before sex determination. Increases in the expression of CYP19 in female gonads and CYP17 (P450 17alpha-hydroxylase/C17-20 lyase) in male gonads suggest that the former plays an important role in phenotypic female determination, whereas the latter is needed for male determination. Thus, steroids could be the key factor for sex determination in R. rugosa. In addition to the role of sex steroids in gonadal sex determination in this species, Foxl2 and Sox3 are capable of promoting CYP19 expression. Since both the genes are autosomal, another factor up-regulating CYP19 expression must be recruited. The factor, which may be located on the X or W chromosome, intervenes directly or indirectly, in the transcriptional regulation of the CYP19 gene for feminization in amphibians. A factor up-regulating CYP17 expression remains to be identified.
在所有哺乳动物中，异配性别为雄性，而在几乎所有鸟类中，异配性别为雌性。相比之下，两栖动物的遗传性别决定存在两种异配类型（XX/XY和ZZ/ZW）。虽然两栖动物最初的异配性别为雌性，但这两种异配类型可能是可互换的，这表明性染色体在该谱系中进化了好几次。事实上，日本皱蛙在一个物种内存在XX/XY和ZZ/ZW两种性别决定系统，这取决于日本当地的种群。具有分化性染色体的XY和ZW地理形态可能有着共同的起源，因为未分化的性染色体是日本西部主要种群与关东形态杂交产生的。很明显，该物种群中的性染色体仍在进化。尽管两栖动物中存在性别决定基因，但某些物种的性腺性别可被性类固醇改变。也就是说，性类固醇可诱导性别逆转，雌激素诱导雄性向雌性的性别逆转，而雄激素则有相反的作用。在日本皱蛙中，CYP19（P450芳香化酶）的性腺活性与性腺的雌性化相关。特别有趣的是，在性别决定之前的未分化性腺中观察到高水平的CYP19表达。雌性性腺中CYP19表达的增加以及雄性性腺中CYP17（P450 17α-羟化酶/C17-20裂解酶）表达的增加表明，前者在表型雌性决定中起重要作用，而后者是雄性决定所必需的。因此，类固醇可能是日本皱蛙性别决定的关键因素。除了性类固醇在该物种性腺性别决定中的作用外，Foxl2和Sox3能够促进CYP19表达。由于这两个基因都是常染色体基因，所以必须招募另一个上调CYP19表达的因子。该因子可能位于X或W染色体上，直接或间接地干预两栖动物中CYP19基因的转录调控以实现雌性化。上调CYP17表达的因子仍有待确定。
