Department of Life Science and Technology, Tokyo Institute of Technology, Yokohama, Japan.
Department of Biology, University of Florence, Italy.
Genome Biol Evol. 2018 Jan 1;10(1):157-165. doi: 10.1093/gbe/evx281.
Rod cells of many nocturnal mammals have a "non-standard" nuclear architecture, which is called the inverted nuclear architecture. Heterochromatin localizes to the central region of the nucleus. This leads to an efficient light transmission to the outer segments of photoreceptors. Rod cells of diurnal mammals have the conventional nuclear architecture. Owl monkeys (genus Aotus) are the only taxon of simian primates that has a nocturnal or cathemeral lifestyle, and this adaptation is widely thought to be secondary. Their rod cells were shown to exhibit an intermediate chromatin distribution: a spherical heterochromatin block was found in the central region of the nucleus although it was less complete than that of typical nocturnal mammals. We recently demonstrated that the primary DNA component of this heterochromatin block was OwlRep, a megasatellite DNA consisting of 187-bp-long repeat units. However, the origin of OwlRep was not known. Here we show that OwlRep was derived from HSAT6, a simple repeat sequence found in the centromere regions of human chromosomes. HSAT6 occurs widely in primates, suggesting that it was already present in the last common ancestor of extant primates. Notably, Strepsirrhini and Tarsiformes apparently carry a single HSAT6 copy, whereas many species of Simiiformes contain multiple copies. Comparison of nucleotide sequences of these copies revealed the entire process of the OwlRep formation. HSAT6, with or without flanking sequences, was segmentally duplicated in New World monkeys. Then, in the owl monkey linage after its divergence from other New World monkeys, a copy of HSAT6 was tandemly amplified, eventually forming a megasatellite DNA.
许多夜间活动的哺乳动物的杆状细胞具有一种“非标准”的核结构，称为倒位核结构。异染色质定位于核的中心区域。这导致光有效地传输到光感受器的外节。昼间活动的哺乳动物的杆状细胞具有常规的核结构。猫头鹰猴（属 Aotus）是灵长类动物中唯一具有夜间或昼夜活动生活方式的分类群，这种适应被广泛认为是次要的。它们的杆状细胞表现出中间的染色质分布：在核的中心区域发现了一个球形的异染色质块，尽管它不如典型的夜间活动的哺乳动物完整。我们最近证明，这个异染色质块的主要 DNA 成分是 OwlRep，一种由 187 个碱基对长的重复单元组成的大型卫星 DNA。然而，OwlRep 的起源尚不清楚。在这里，我们表明 OwlRep 是从 HSAT6 衍生而来的，HSAT6 是一种简单重复序列，存在于人类染色体的着丝粒区域。HSAT6 在灵长类动物中广泛存在，表明它已经存在于现存灵长类动物的最后共同祖先中。值得注意的是，食虫目和灵长目显然携带单个 HSAT6 拷贝，而许多类人猿物种则含有多个拷贝。对这些拷贝的核苷酸序列进行比较揭示了 OwlRep 形成的整个过程。HSAT6 及其侧翼序列在新世界猴中发生了片段性重复。然后，在猫头鹰猴谱系与其从其他新世界猴分支后，HSAT6 的一个拷贝发生了串联扩增，最终形成了一个大型卫星 DNA。
