PDF(Pubmed)
Abstract：
Heterochromatin comprises a significant component of many eukaryotic genomes. In comparison to euchromatin, heterochromatin is gene poor, transposon rich, and late replicating. It serves many important biological roles, from gene silencing to accurate chromosome segregation, yet little is known about the evolutionary constraints that shape heterochromatin. A complementary approach to the traditional one of directly studying heterochromatic DNA sequence is to study the evolution of proteins that bind and define heterochromatin. One of the best markers for heterochromatin is the heterochromatin protein 1 (HP1), which is an essential, nonhistone chromosomal protein. Here we investigate the molecular evolution of five HP1 paralogs present in Drosophila melanogaster. Three of these paralogs have ubiquitous expression patterns in adult Drosophila tissues, whereas HP1D/rhino and HP1E are expressed predominantly in ovaries and testes respectively. The HP1 paralogs also have distinct localization preferences in Drosophila cells. Thus, Rhino localizes to the heterochromatic compartment in Drosophila tissue culture cells, but in a pattern distinct from HP1A and lysine-9 dimethylated H3. Using molecular evolution and population genetic analyses, we find that rhino has been subject to positive selection in all three domains of the protein: the N-terminal chromo domain, the C-terminal chromo-shadow domain, and the hinge region that connects these two modules. Maximum likelihood analysis of rhino sequences from 20 species of Drosophila reveals that a small number of residues of the chromo and shadow domains have been subject to repeated positive selection. The rapid and positive selection of rhino is highly unusual for a gene encoding a chromosomal protein and suggests that rhino is involved in a genetic conflict that affects the germline, belying the notion that heterochromatin is simply a passive recipient of \"junk DNA\" in eukaryotic genomes.
摘要：
异染色质包含许多真核生物基因组的重要组分。与常染色质相比，异染色质基因贫乏，转座子丰富，和后期复制。它具有许多重要的生物学作用，从基因沉默到精确的染色体分离，然而，对形成异染色质的进化约束知之甚少。与传统的直接研究异色DNA序列的一种互补方法是研究结合和定义异染色质的蛋白质的进化。异染色质的最佳标记之一是异染色质蛋白1（HP1），这是必不可少的，非组蛋白染色体蛋白。在这里，我们研究了果蝇中存在的五个HP1同源物的分子进化。其中三个旁系同源物在成年果蝇组织中具有普遍存在的表达模式，而HP1D/犀牛和HP1E分别主要在卵巢和睾丸中表达。HP1旁系同源物在果蝇细胞中也具有不同的定位偏好。因此，犀牛定位于果蝇组织培养细胞的异色区室，但与HP1A和赖氨酸9二甲基化H3不同。利用分子进化和种群遗传分析，我们发现犀牛在蛋白质的所有三个结构域中都受到了正向选择：N末端色素结构域，C端色度阴影域，以及连接这两个模块的铰链区。来自20种果蝇的犀牛序列的最大似然分析表明，色度和阴影域的少量残基已经历了重复的阳性选择。对于编码染色体蛋白的基因来说，犀牛的快速和积极选择是非常不寻常的，这表明犀牛参与了影响种系的遗传冲突，否认异染色质只是真核生物基因组中“垃圾DNA”的被动接受者。
