The sex chromosomes of most anuran amphibians are characterized by homomorphy in both sexes, and evolution to heteromorphy rarely occurs at the species or geographic population level. Here, we report sex chromosome heteromorphy in geographic populations of the Japanese Tago’s brown frog complex (2n = 26), comprising Rana sakuraii and R. tagoi. The sex chromosomes of R. sakuraii from the populations in western Japan were homomorphic in both sexes, whereas chromosome 7 from the populations in eastern Japan were heteromorphic in males. Chromosome 7 of R. tagoi, which is distributed close to R. sakuraii in eastern Japan, was highly similar in morphology to the Y chromosome of R. sakuraii. Based on this and on mitochondrial gene sequence analysis, we hypothesize that in the R. sakuraii populations from eastern Japan the XY heteromorphic sex chromosome system was established by the introduction of chromosome 7 from R. tagoi via interspecies hybridization. In contrast, chromosome 13 of R. tagoi from the 2 large islands in western Japan, Shikoku and Kyushu, showed a heteromorphic pattern of constitutive heterochromatin distribution in males, while this pattern was homomorphic in females. Our study reveals that sex chromosome heteromorphy evolved independently at the geographic lineage level in this species complex.
Cytogenet Genome Res

中文翻译：

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日本塔哥棕蛙群地理种群性别染色体异质性的演变

大多数无环两栖动物的性染色体都具有两性同态的特征，并且在物种或地理种群水平上很少发生向异型的进化。在这里，我们报告了日本塔哥的棕色青蛙群（2 n = 26）的地理种群中的性染色体异质性，其中包括樱树蛙和R. tagoi。来自日本西部人群的樱红细菌的性染色体在两个性别上都是同态的，而来自日本东部人群的樱木的7号染色体在男性中是同态的。分布在日本东部樱花附近的R. tagoi的7号染色体在形态上与日本的Y染色体高度相似R. sakuraii。基于此以及线粒体基因序列分析，我们假设在日本东部的樱木种群中，通过种间杂交引入了来自塔霍菌的第7号染色体，从而建立了XY异型性染色体系统。相比之下，来自日本西部两个大岛（四国和九州）的塔戈河杆菌的13号染色体在男性中显示出组成型异染色质分布的异型模式，而在女性中则表现出同态性。我们的研究表明，性染色体异质性在该物种复合体的地理谱系水平上独立进化。
细胞遗传学基因组研究