Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, Colorado, USA.
Department of Biology, University of Texas at Arlington, Arlington, Texas, USA.
Genome Biol Evol. 2022 Sep 6;14(9). doi: 10.1093/gbe/evac116.
Sex chromosomes diverge after the establishment of recombination suppression, resulting in differential sex-linkage of genes involved in genetic sex determination and dimorphic traits. This process produces systems of male or female heterogamety wherein the Y and W chromosomes are only present in one sex and are often highly degenerated. Sex-limited Y and W chromosomes contain valuable information about the evolutionary transition from autosomes to sex chromosomes, yet detailed characterizations of the structure, composition, and gene content of sex-limited chromosomes are lacking for many species. In this study, we characterize the female-specific W chromosome of the prairie rattlesnake (Crotalus viridis) and evaluate how recombination suppression and other processes have shaped sex chromosome evolution in ZW snakes. Our analyses indicate that the rattlesnake W chromosome is over 80% repetitive and that an abundance of GC-rich mdg4 elements has driven an overall high degree of GC-richness despite a lack of recombination. The W chromosome is also highly enriched for repeat sequences derived from endogenous retroviruses and likely acts as a "refugium" for these and other retroelements. We annotated 219 putatively functional W-linked genes across at least two evolutionary strata identified based on estimates of sequence divergence between Z and W gametologs. The youngest of these strata is relatively gene-rich, however gene expression across strata suggests retained gene function amidst a greater degree of degeneration following ancient recombination suppression. Functional annotation of W-linked genes indicates a specialization of the W chromosome for reproductive and developmental function since recombination suppression from the Z chromosome.
性染色体在重组抑制建立后发生分歧，导致参与遗传性别决定和二态特征的基因的性连锁差异。这个过程产生了雄性或雌性异配性系统，其中 Y 和 W 染色体仅存在于一种性别中，并且通常高度退化。性限 Y 和 W 染色体包含了关于从常染色体向性染色体进化的重要信息，但对于许多物种，性限染色体的结构、组成和基因内容的详细特征描述仍然缺乏。在这项研究中，我们对草原响尾蛇（Crotalus viridis）的雌性特异性 W 染色体进行了表征，并评估了重组抑制和其他过程如何塑造 ZW 蛇的性染色体进化。我们的分析表明，响尾蛇 W 染色体超过 80%是重复的，尽管缺乏重组，但富含 GC 的 mdg4 元件的大量存在导致了整体高度的 GC 丰富度。W 染色体还高度富含源自内源性逆转录病毒的重复序列，并且可能作为这些和其他逆转录元件的“避难所”。我们在至少两个进化层上注释了 219 个假定的功能性 W 连锁基因，这些基因至少跨越了根据 Z 和 W 配子体之间序列差异估计确定的两个进化层。这些层中最年轻的一层相对基因丰富，但是跨层的基因表达表明，在古老的重组抑制之后，退行性变化更大的情况下，保留了基因功能。W 连锁基因的功能注释表明，W 染色体在重组抑制后，专门用于生殖和发育功能。
