PDF(Pubmed)
Abstract：
The genes encoding ribosomal RNA are highly conserved across life and in almost all eukaryotes are present in large tandem repeat arrays called the rDNA. rDNA repeat unit size is conserved across most eukaryotes but has expanded dramatically in mammals, principally through the expansion of the intergenic spacer region that separates adjacent rRNA coding regions. Here, we used long-read sequence data from representatives of the major amniote lineages to determine where in amniote evolution rDNA unit size increased. We find that amniote rDNA unit sizes fall into two narrow size classes: \"normal\" (∼11-20 kb) in all amniotes except monotreme, marsupial, and eutherian mammals, which have \"large\" (∼35-45 kb) sizes. We confirm that increases in intergenic spacer length explain much of this mammalian size increase. However, in stark contrast to the uniformity of mammalian rDNA unit size, mammalian intergenic spacers differ greatly in sequence. These results suggest a large increase in intergenic spacer size occurred in a mammalian ancestor and has been maintained despite substantial sequence changes over the course of mammalian evolution. This points to a previously unrecognized constraint on the length of the intergenic spacer, a region that was thought to be largely neutral. We finish by speculating on possible causes of this constraint.
摘要：
编码核糖体RNA的基因在整个生命中高度保守，并且在几乎所有的真核生物中都存在于称为rDNA的大型串联重复序列阵列中。rDNA重复单元大小在大多数真核生物中是保守的，但在哺乳动物中却急剧扩张，主要通过分离相邻rRNA编码区的基因间间隔区的扩展。在这里，我们使用来自主要羊水谱系代表的长读序列数据来确定羊水进化中rDNA单位大小增加的位置。我们发现，羊膜动物rDNA单位大小分为两个狭窄的大小类别：除单调外，所有羊膜动物中的“正常”（〜11-20kb），有袋动物和异形哺乳动物，具有“大”(35-45kb)尺寸。我们证实，基因间间隔区长度的增加解释了这种哺乳动物大小的增加，但是，与哺乳动物rDNA单位大小的一致性形成鲜明对比，哺乳动物基因间间隔区序列差异很大。这些结果表明，在哺乳动物祖先中发生了基因间间隔区大小的大量增加，尽管在哺乳动物进化过程中序列发生了实质性变化，但仍得以维持。这指向先前未认识到的对基因间间隔区长度的约束,一个被认为基本上是中立的地区。我们通过推测这种约束的可能原因来完成。
