Guo Xiang, Han Fangpu
State Key Laboratory of Plant Cell and Chromosome Engineering, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China University of Chinese Academy of Sciences, Beijing 100049, China.
State Key Laboratory of Plant Cell and Chromosome Engineering, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China
Plant Cell. 2014 Nov;26(11):4311-27. doi: 10.1105/tpc.114.129841. Epub 2014 Nov 18.
rRNA genes consist of long tandem repeats clustered on chromosomes, and their products are important functional components of the ribosome. In common wheat (Triticum aestivum), rDNA loci from the A and D genomes were largely lost during the evolutionary process. This biased DNA elimination may be related to asymmetric transcription and epigenetic modifications caused by the polyploid formation. Here, we observed both sets of parental nucleolus organizing regions (NORs) were expressed after hybridization, but asymmetric silencing of one parental NOR was immediately induced by chromosome doubling, and reversing the ploidy status could not reactivate silenced NORs. Furthermore, increased CHG and CHH DNA methylation on promoters was accompanied by asymmetric silencing of NORs. Enrichment of H3K27me3 and H3K9me2 modifications was also observed to be a direct response to increased DNA methylation and transcriptional inactivation of NOR loci. Both A and D genome NOR loci with these modifications started to disappear in the S4 generation and were completely eliminated by the S7 generation in synthetic tetraploid wheat. Our results indicated that asymmetric epigenetic modification and elimination of rDNA sequences between different donor genomes may lead to stable allopolyploid wheat with increased differentiation and diversity.
核糖体RNA（rRNA）基因由成簇分布于染色体上的长串联重复序列组成，其产物是核糖体的重要功能成分。在普通小麦（Triticum aestivum）中，A和D基因组的核糖体DNA（rDNA）位点在进化过程中大量丢失。这种有偏向性的DNA消除可能与多倍体形成导致的不对称转录和表观遗传修饰有关。在此，我们观察到杂交后双亲的核仁组织区（NORs）均有表达，但染色体加倍会立即诱导其中一个亲本NOR的不对称沉默，而且逆转倍性状态并不能重新激活沉默的NORs。此外，NORs的不对称沉默伴随着启动子区域CHG和CHH DNA甲基化水平的升高。还观察到H3K27me3和H3K9me2修饰的富集是对NOR位点DNA甲基化增加和转录失活的直接响应。具有这些修饰的A和D基因组NOR位点在人工合成四倍体小麦的S4代开始消失，并在S7代被完全消除。我们的结果表明，不同供体基因组之间rDNA序列的不对称表观遗传修饰和消除可能导致具有更高分化和多样性的稳定异源多倍体小麦。
