Riddle Nicole C, Elgin Sarah C R
Department of Biology, Washington University, One Brookings Dr, St. Louis, MO 63130, USA.
Chromosome Res. 2006;14(4):405-16. doi: 10.1007/s10577-006-1061-6.
Historically, chromatin has been subdivided into heterochromatin, transcriptionally inactive regions that remain densely packaged throughout the cell cycle, and euchromatin, transcriptionally active regions that take on a diffuse appearance as the cell enters interphase. The banded portion of the small fourth chromosome (dot chromosome) of Drosophila melanogaster is unusual in exhibiting many characteristics of heterochromatic domains, and at the same time maintaining a gene density typical of euchromatin. Similar to genes embedded in pericentric heterochromatin, many of the dot chromosome genes have adapted to a heterochromatic environment. Little is known about the regulation of these genes and less about their evolution in a chromatin context. Interestingly, most of the genes from the D. melanogaster fourth chromosome remain clustered on a small chromosome throughout the genus Drosophila; yet the dot chromosome appears euchromatic in some species, such as D. virilis. Existing genomic sequence data allow an exploration of the underlying differences in DNA sequence organization between species. Here we review the available data describing the dot chromosome, which derives primarily from D. melanogaster. With its unusual and changing nature, the dot chromosome in the genus Drosophila provides a unique opportunity for the examination of transitions between chromatin states during evolution.
从历史上看，染色质已被细分为异染色质和常染色质，异染色质是在整个细胞周期中都保持紧密包装的转录非活性区域，常染色质是在细胞进入间期时呈现弥散外观的转录活性区域。果蝇的第四条小染色体（点状染色体）的带状部分很不寻常，它表现出许多异染色质结构域的特征，同时又保持着常染色质典型的基因密度。与嵌入着丝粒周围异染色质中的基因类似，许多点状染色体基因已经适应了异染色质环境。人们对这些基因的调控知之甚少，对它们在染色质环境中的进化了解更少。有趣的是，果蝇属中，来自黑腹果蝇第四条染色体的大多数基因在整个果蝇属中仍聚集在一条小染色体上；然而，点状染色体在某些物种中，如粗壮果蝇，看起来是常染色质的。现有的基因组序列数据有助于探索不同物种间DNA序列组织的潜在差异。在这里，我们回顾了主要来自黑腹果蝇的关于点状染色体的现有数据。果蝇属中的点状染色体具有独特且多变的性质，为研究进化过程中染色质状态的转变提供了一个独特的机会。
