In many species, sexual differentiation is a vital prelude to reproduction, and disruption of this process can have severe fitness effects, including sterility. It is thus interesting that genetic systems governing sexual differentiation vary among—and even within—species. To understand these systems more, we investigated a rare example of a frog with three sex chromosomes: the Western clawed frog, Xenopus tropicalis. We demonstrate that natural populations from the western and eastern edges of Ghana have a young Y chromosome, and that a male-determining factor on this Y chromosome is in a very similar genomic location as a previously known female-determining factor on the W chromosome. Nucleotide polymorphism of expressed transcripts suggests genetic degeneration on the W chromosome, emergence of a new Y chromosome from an ancestral Z chromosome, and natural co-mingling of the W, Z, and Y chromosomes in the same population. Compared to the rest of the genome, a small sex-associated portion of the sex chromosomes has a 50-fold enrichment of transcripts with male-biased expression during early gonadal differentiation. Additionally, X. tropicalis has sex-differences in the rates and genomic locations of recombination events during gametogenesis that are similar to at least two other Xenopus species, which suggests that sex differences in recombination are genus-wide. These findings are consistent with theoretical expectations associated with recombination suppression on sex chromosomes, demonstrate that several characteristics of old and established sex chromosomes (e.g., nucleotide divergence, sex biased expression) can arise well before sex chromosomes become cytogenetically distinguished, and show how these characteristics can have lingering consequences that are carried forward through sex chromosome turnovers.

在许多物种中，性别分化是繁殖的重要前奏，破坏这一过程可能会带来严重的适应性影响，包括不育。因此，有趣的是，控制性别分化的遗传系统在物种之间甚至物种内部都不同。为了更深入地了解这些系统，我们研究了具有三个性染色体的青蛙的稀有例子：西方爪状青蛙，热带非洲爪蟾。。我们证明，来自加纳西部和东部边缘的自然种群具有年轻的Y染色体，并且该Y染色体上的男性决定因素与W染色体上以前已知的女性决定因素处于非常相似的基因组位置。表达的转录物的核苷酸多态性表明，W染色体上的遗传变性，祖先Z染色体上出现了新的Y染色体，以及同一种群中W，Z和Y染色体的自然混合。与基因组的其余部分相比，性染色体的一小部分与性相关的部分在早期性腺分化过程中具有男性偏向表达的转录物富集了50倍。此外，热带假单胞菌在配子发生过程中重组事件的发生率和基因组位置具有性别差异，与至少两种其他非洲爪蟾属物种相似，这表明重组性别差异在全属范围内。这些发现与有关性染色体重组抑制相关的理论预期是一致的，证明了性染色体在细胞遗传学上得以区分之前，早已建立的性染色体的一些特征（例如核苷酸差异，性别偏向的表达）就可以出现，并表明了这些特征是如何产生的。会产生挥之不去的后果，这些后果会通过性染色体更新而延续下来。