Preservation and Research Center, City of Yokohama, 155-1 Asahi Ward, Yokohama 241-0804, Japan.
Hikiiwa Park Center, 1629 Inari-cho, Tanabe 646-0051, Japan.
Philos Trans R Soc Lond B Biol Sci. 2021 Sep 13;376(1833):20200105. doi: 10.1098/rstb.2020.0105. Epub 2021 Jul 26.
Sex chromosomes generally evolve from a homomorphic to heteromorphic state. Once a heteromorphic system is established, the sex chromosome system may remain stable for an extended period. Here, we show the opposite case of sex chromosome evolution from a heteromorphic to a homomorphic system in the Japanese frog One geographic group, Neo-ZW, has ZZ-ZW type heteromorphic sex chromosomes. We found that its western edge populations, which are geographically close to another West-Japan group with homomorphic sex chromosomes of XX-XY type, showed homozygous genotypes of sex-linked genes in both sexes. Karyologically, no heteromorphic sex chromosomes were identified. Sex-reversal experiments revealed that the males were heterogametic in sex determination. In addition, we identified another similar population around at the southwestern edge of the Neo-ZW group in the Kii Peninsula: the frogs had homomorphic sex chromosomes under male heterogamety, while shared mitochondrial haplotypes with the XY group, which is located in the east and bears heteromorphic sex chromosomes. In conclusion, our study revealed that the heteromorphic sex chromosome systems independently reversed back to or turned over to a homomorphic system around each of the western and southwestern edges of the Neo-ZW group through hybridization with the West-Japan group bearing homomorphic sex chromosomes. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part II)'.
性染色体通常从同形状态进化为异形状态。一旦建立了异形系统，性染色体系统可能会在很长一段时间内保持稳定。在这里，我们展示了日本青蛙性染色体进化从异形到同形系统的相反情况。一个地理群体 Neo-ZW 具有 ZZ-ZW 型异形性染色体。我们发现，其西部边缘种群与具有同形性染色体 XX-XY 型的另一个西部日本群体在地理上接近，表现出两性中性别连锁基因的纯合基因型。在核型上，没有发现异形性染色体。性反转实验表明，雄性在性别决定中是异配性别。此外，我们在纪伊半岛 Neo-ZW 群体的西南边缘附近发现了另一个类似的种群：青蛙在雄性异配性别下具有同形性染色体，而与位于东部并具有异形性染色体的 XY 群体共享线粒体单倍型。总之，我们的研究表明，异形性染色体系统通过与具有同形性染色体的西部日本群体杂交，在 Neo-ZW 群体的西部和西南部边缘各自独立地反转回或翻转成同形系统。本文是主题为“挑战性染色体进化的范式：以脊椎动物为重点的实证和理论见解（第二部分）”的一部分。
