Arizona Genomics Institute, School of Plant Sciences, Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona 85721, USA.
Plant Cell. 2011 Aug;23(8):2821-30. doi: 10.1105/tpc.111.085605. Epub 2011 Aug 19.
Recombination is strongly suppressed in centromeric regions. In chromosomal regions with suppressed recombination, deleterious mutations can easily accumulate and cause degeneration of genes and genomes. Surprisingly, the centromere of chromosome8 (Cen8) of rice (Oryza sativa) contains several transcribed genes. However, it remains unclear as to what selective forces drive the evolution and existence of transcribed genes in Cen8. Sequencing of orthologous Cen8 regions from two additional Oryza species, Oryza glaberrima and Oryza brachyantha, which diverged from O. sativa 1 and 10 million years ago, respectively, revealed a set of seven transcribed Cen8 genes conserved across all three species. Chromatin immunoprecipitation analysis with the centromere-specific histone CENH3 confirmed that the sequenced orthologous regions are part of the functional centromere. All seven Cen8 genes have undergone purifying selection, representing a striking phenomenon of active gene survival within a recombination-free zone over a long evolutionary time. The coding sequences of the Cen8 genes showed sequence divergence and mutation rates that were significantly reduced from those of genes located on the chromosome arms. This suggests that Oryza has a mechanism to maintain the fidelity and functionality of Cen8 genes, even when embedded in a sea of repetitive sequences and transposable elements.
重组在着丝粒区域受到强烈抑制。在重组受到抑制的染色体区域，有害突变很容易积累，导致基因和基因组退化。令人惊讶的是，水稻（Oryza sativa）第 8 号染色体（Cen8）的着丝粒含有几个转录基因。然而，驱动 Cen8 中转录基因进化和存在的选择压力仍不清楚。对来自另外两个水稻物种——Oryza glaberrima 和 Oryza brachyantha 的同源 Cen8 区域进行测序，这两个物种分别在 100 万年前和 1000 万年前与 O. sativa 分化，结果发现了一组在这三个物种中都保守的七个转录 Cen8 基因。用着丝粒特异性组蛋白 CENH3 进行的染色质免疫沉淀分析证实，测序的同源区域是功能着丝粒的一部分。这七个 Cen8 基因都经历了纯化选择，这代表了在一个长期的进化过程中，在一个无重组区域中活跃基因存活的惊人现象。Cen8 基因的编码序列显示出序列分歧和突变率明显低于染色体臂上基因的序列分歧和突变率。这表明，即使嵌入在重复序列和转座元件的海洋中，水稻也有一种机制来维持 Cen8 基因的保真度和功能。
