Center for Information Biology, National Institute of Genetics, Mishima, Shizuoka, Japan.
Mol Biol Evol. 2014 Mar;31(3):614-24. doi: 10.1093/molbev/mst239. Epub 2013 Dec 3.
Sex chromosome dosage compensation (DC) is widely accepted in various organisms. This concept is mostly supported by comparisons of gene expression between chromosomes and between sexes. However, genes on the X chromosome and autosomes are mostly not homologous, and the average gene expression level on these chromosomes may not be the same even under DC, which complicates comparisons between chromosomes. Many genes with sex-biased expression also make comparisons between sexes difficult. To overcome these issues, we investigated DC by comparing the expression of neo-X-linked genes in Drosophila pseudoobscura with those of their autosomal orthologs in other Drosophila species. The ratio of the former to the latter in males would be 1 under DC, whereas it becomes 0.5 without DC. We found that the ratio was ∼0.85 for adult whole bodies, indicating that the DC is incomplete on the neo-X chromosome in adults as a whole. The ratio (∼0.90) was also significantly less than 1 for adult bodies without gonads, whereas it was ∼1.0 for adult heads. These results indicate that DC varies among tissues. Our sliding-window analysis of the ratio also revealed that the upregulation of neo-X-linked genes in males occurred chromosome wide in all tissues analyzed, indicating global upregulation mechanisms. However, we found that gene functions also affected the levels of DC. Furthermore, most of the genes recently moved to the X were already under DC at the larval stage but not at the adult stage. These results suggest that DC in Drosophila species operates in a tissue/stage-dependent manner.
性染色体剂量补偿(DC)在各种生物中被广泛接受。这个概念主要是通过比较染色体之间和性别之间的基因表达来支持的。然而,X 染色体和常染色体上的基因大多不是同源的,即使在 DC 下,这些染色体上的平均基因表达水平也可能不同,这使得染色体之间的比较变得复杂。许多具有性别偏向表达的基因也使得性别之间的比较变得困难。为了克服这些问题,我们通过比较果蝇拟暗斑亚种中新 X 连锁基因在雄性中的表达与其在其他果蝇物种中的常染色体同源基因的表达,来研究 DC。在 DC 下,前者与后者的比值应为 1,而在没有 DC 的情况下,该比值变为 0.5。我们发现,对于成年整体,该比值约为 0.85,这表明在成年整体中,新 X 染色体上的 DC 不完全。对于没有性腺的成年体,该比值(约 0.90)也明显小于 1,而对于成年头部,该比值约为 1.0。这些结果表明 DC 在组织之间存在差异。我们对该比值的滑动窗口分析还表明,在所有分析的组织中,雄性中新 X 连锁基因的上调发生在整个染色体上,表明存在全局上调机制。然而,我们发现基因功能也会影响 DC 的水平。此外,最近转移到 X 染色体上的大多数基因在幼虫期已经受到 DC 的调控,但在成虫期没有。这些结果表明,果蝇物种中的 DC 以组织/阶段依赖的方式运作。