Genomic analysis of many nonmodel species has uncovered an incredible diversity of sex chromosome systems, making it possible to empirically test the rich body of evolutionary theory that describes each stage of sex chromosome evolution. Classic theory predicts that sex chromosomes originate from a pair of homologous autosomes and recombination between them is suppressed via inversions to resolve sexual conflict. The resulting degradation of the Y chromosome gene content creates the need for dosage compensation in the heterogametic sex. Sex chromosome theory also implies a linear process, starting from sex chromosome origin and progressing to heteromorphism. Despite many convergent genomic patterns exhibited by independently evolved sex chromosome systems, and many case studies supporting these theoretical predictions, emerging data provide numerous interesting exceptions to these long-standing theories, and suggest that the remarkable diversity of sex chromosomes is matched by a similar diversity in their evolution. For example, it is clear that sex chromosome pairs are not always derived from homologous autosomes. In addition, both the cause and the mechanism of recombination suppression between sex chromosome pairs remain unclear, and it may be that the spread of recombination suppression is a more gradual process than previously thought. It is also clear that dosage compensation can be achieved in many ways, and displays a range of efficacy in different systems. Finally, the remarkable turnover of sex chromosomes in many systems, as well as variation in the rate of sex chromosome divergence, suggest that assumptions about the inevitable linearity of sex chromosome evolution are not always empirically supported, and the drivers of the birth–death cycle of sex chromosome evolution remain to be elucidated. Here, we concentrate on how the diversity in sex chromosomes across taxa highlights an equal diversity in each stage of sex chromosome evolution.

中文翻译：

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性染色体进化：规则的许多例外。

对许多非模型物种的基因组分析发现了令人难以置信的性染色体系统多样性，从而有可能对描述性染色体进化各个阶段的进化理论进行丰富的实验。经典理论预测，性染色体起源于一对同源常染色体，并且通过倒转来解决性冲突会抑制它们之间的重组。Y染色体基因含量的最终降解导致需要在异配子性行为中进行剂量补偿。性染色体理论也暗示了一个线性过程，从性染色体起源开始，发展到异态。尽管独立进化的性染色体系统展现出许多收敛的基因组模式，并且有许多案例研究支持了这些理论预测，新兴数据为这些长期存在的理论提供了许多有趣的例外，并表明性染色体的显着多样性与它们进化中的类似多样性相匹配。例如，很明显，性染色体对并不总是来源于同源常染色体。此外，性染色体对之间重组抑制的原因和机制仍不清楚，可能是重组抑制的传播是一个比以前认为更为渐进的过程。同样清楚的是，可以通过多种方式实现剂量补偿，并且在不同系统中显示出一系列功效。最后，在许多系统中，性染色体的显着转换以及性染色体发散速率的变化，这表明，关于性染色体进化不可避免的线性的假设并不总是得到经验支持，并且性染色体进化的生死周期的驱动因素仍有待阐明。在这里，我们关注于整个分类群中性染色体的多样性如何突出性染色体进化的每个阶段的均等多样性。