The loss of recombination triggers divergence between the sex chromosomes and promotes degeneration of the sex-limited chromosome. Several livebearers within the genus Poecilia share a male-heterogametic sex chromosome system that is roughly 20 million years old, with extreme variation in the degree of Y chromosome divergence. In P. picta, the Y is highly degenerate and associated with complete X chromosome dosage compensation. In contrast, although recombination is restricted across almost the entire length of the sex chromosomes in P. reticulata and P wingei, divergence between the X and the Y chromosome is very low. This clade therefore offers a unique opportunity to study the forces that accelerate or hinder sex chromosome divergence. We used RNA-seq data from multiple families of both P. reticulata and P. wingei, the species with low levels of sex chromosome divergence, to differentiate X and Y coding sequence based on sex-limited SNP inheritance. Phylogenetic tree analyses reveal that occasional recombination has persisted between the sex chromosomes for much of their length, as X- and Y-linked sequences cluster by species instead of by gametolog. This incomplete recombination suppression maintains the extensive homomorphy observed in these systems. In addition, we see differences between the previously identified strata in the phylogenetic clustering of X-Y orthologs, with those that cluster by chromosome located in the older stratum, the region previously associated with the sex-determining locus. However, recombination arrest appears to have expanded throughout the sex chromosomes more gradually instead of through a stepwise process associated with inversions.

中文翻译：

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通过不完全重组抑制维持孔雀鱼 Y 染色体完整性。


重组的丧失会引发性染色体之间的分歧，并促进性限制染色体的退化。 Poecilia 属内的几个活体生物共享大约 2000 万年前的雄性异配性染色体系统，Y 染色体分歧程度存在极大差异。在 P. picta 中，Y 是高度简并的，并且与完整的 X 染色体剂量补偿相关。相比之下，尽管重组在 P. reticulata 和 P. wingei 的性染色体的几乎整个长度上受到限制，但 X 和 Y 染色体之间的分歧非常低。因此，这个进化枝提供了一个独特的机会来研究加速或阻碍性染色体分歧的力量。我们使用来自性染色体分歧水平较低的物种 P. reticulata 和 P. wingei 的多个科的 RNA-seq 数据，根据性别限制的 SNP 遗传来区分 X 和 Y 编码序列。系统发育树分析表明，性染色体之间在其大部分长度上持续存在偶尔的重组，因为X和Y连锁序列按物种而不是配子体聚类。这种不完全重组抑制维持了在这些系统中观察到的广泛同态性。此外，我们还发现 XY 直向同源物的系统发育聚类中先前鉴定的各层之间存在差异，而那些按位于较旧层（先前与性别决定基因座相关的区域）的染色体进行聚类的层之间存在差异。然而，重组停滞似乎在整个性染色体中逐渐扩展，而不是通过与倒位相关的逐步过程。