BACKGROUND Sex chromosome evolution is a dynamic process that can proceed at varying rates across lineages. For example, different chromosomes can be sex-linked between closely related species, whereas other sex chromosomes have been conserved for > 100 million years. Cases of long-term sex chromosome conservation could be informative of factors that constrain sex chromosome evolution. Cytological similarities between the X chromosomes of the German cockroach (Blattella germanica) and most flies suggest that they may be homologous-possibly representing an extreme case of long-term conservation. RESULTS To test the hypothesis that the cockroach and fly X chromosomes are homologous, we analyzed whole-genome sequence data from cockroaches. We found evidence in both sequencing coverage and heterozygosity that a significant excess of the same genes are on both the cockroach and fly X chromosomes. We also present evidence that the candidate X-linked cockroach genes may be dosage compensated in hemizygous males. Consistent with this hypothesis, three regulators of transcription and chromatin on the fly X chromosome are conserved in the cockroach genome. CONCLUSIONS Our results support our hypothesis that the German cockroach shares the same X chromosome as most flies. This may represent the convergent evolution of the X chromosome in the lineages leading to cockroaches and flies. Alternatively, the common ancestor of most insects may have had an X chromosome that resembled the extant cockroach and fly X. Cockroaches and flies diverged ∼ 400 million years ago, which would be the longest documented conservation of a sex chromosome. Cockroaches and flies have different mechanisms of sex determination, raising the possibility that the X chromosome was conserved despite the evolution of the sex determination pathway.

中文翻译：

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尽管有4亿年的差异，德国蟑螂德国小Bl的X染色体与蝇X染色体同源。

背景技术性染色体进化是一个动态过程，可以跨谱系以不同的速率进行。例如，不同的染色体可以在密切相关的物种之间进行性别关联，而其他的性染色体已经保存了超过一亿年的历史。长期性染色体保护的案例可能会为限制性染色体进化的因素提供参考。德国蟑螂（Blattella germanica）和大多数苍蝇的X染色体之间的细胞学相似性表明，它们可能是同源的，可能代表了长期保存的极端情况。结果为了检验假说蟑螂和X染色体染色体同源的假说，我们分析了蟑螂的全基因组序列数据。我们在测序覆盖率和杂合性中均发现，蟑螂和X染色体上都显着过量存在相同的基因。我们还提供了证据，证明候选X连锁蟑螂基因可以在半合子男性中得到剂量补偿。与此假设相一致，蟑螂基因组中的蝇X染色体上的转录和染色质的三个调节子是保守的。结论我们的结果支持我们的假设，即德国蟑螂与大多数苍蝇具有相同的X染色体。这可能代表X染色体在导致蟑螂和苍蝇的世系中的趋同进化。另外，大多数昆虫的共同祖先可能有一条X染色体，类似于现存的蟑螂和苍蝇X。蟑螂和苍蝇在约4亿年前就散布了，这将是最长的文献记载的性染色体保护。蟑螂和苍蝇具有不同的性别决定机制，这增加了尽管性别决定途径进化了X染色体仍被保守的可能性。