The differentiation of sex chromosomes is thought to be interrupted by relatively frequent sex chromosome turnover and/or occasional recombination between sex chromosomes (fountain-of-youth model) in some vertebrate groups as fishes, amphibians, and lizards. As a result, we observe the prevalence of homomorphic sex chromosomes in these groups. Here, we provide evidence for the loss of sex chromosome heteromorphism in the Amazonian frogs of the genus Engystomops, which harbors an intriguing history of sex chromosome evolution. In this species complex composed of two named species, two confirmed unnamed species, and up to three unconfirmed species, highly divergent karyotypes are present, and heteromorphic X and Y chromosomes were previously found in two species. We describe the karyotype of a lineage estimated to be the sister of all remaining Amazonian Engystomops (named Engystomops sp.) and perform chromosome painting techniques using one probe for the Y chromosome and one probe for the non-centromeric heterochromatic bands of the X chromosome of E. freibergi to compare three Engystomops karyotypes. The Y probe detected the Y chromosomes of E. freibergi and E. petersi and one homolog of chromosome pair 11 of Engystomops sp., suggesting their common evolutionary origin. The X probe showed no interspecific hybridization, revealing that X chromosome heterochromatin is strongly divergent among the studied species. In the light of the phylogenetic relationships, our data suggest that sex chromosome heteromorphism may have occurred early in the evolution of the Amazonian Engystomops and have been lost in two unnamed but confirmed candidate species.

中文翻译：

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青蛙的性染色体进化——来自 Engystomops 属染色体绘制的有益见解

在鱼类、两栖动物和蜥蜴等一些脊椎动物群中，性染色体的分化被认为被相对频繁的性染色体更新和/或性染色体之间偶尔的重组（青春之泉模型）所打断。因此，我们观察到这些组中同态性染色体的普遍性。在这里，我们为 Engystomops 属的亚马逊蛙中性染色体异型性的丧失提供了证据，该属具有有趣的性染色体进化历史。在这个由两个命名物种、两个已确认的未命名物种和多达三个未确认物种组成的物种复合体中，存在高度不同的核型，并且先前在两个物种中发现了异形 X 和 Y 染色体。我们描述了一个谱系的核型，该谱系估计是所有剩余的亚马逊 Engystomops（命名为 Engystomops sp.）的姐妹，并使用一个探针用于 Y 染色体和一个探针用于 X 染色体的非着丝粒异染色带进行染色体绘制技术。 E. freibergi 比较三种 Engystomops 核型。Y 探针检测到 E. freibergi 和 E. petersi 的 Y 染色体以及 Engystomops sp. 的第 11 对染色体的一个同源物，表明它们有共同的进化起源。X 探针未显示出种间杂交，表明 X 染色体异染色质在所研究的物种之间存在很大差异。根据系统发育关系，