遗传所863项目发表Plant Cell文章

　　

　　普通小麦是异源六倍体，染色体组分别为A组（来自乌拉尔图小麦）、D组（来自粗山羊草）和B组（未确定但可能来自与S组相关的山羊草）。普通小麦形成经历两次杂交和两次染色体组加倍过程。在这个过程中，染色体组加倍伴随基因组冲击而发生修饰或重组等。

　　中国科学院遗传与发育生物学研究所韩方普研究组长期从事小麦远缘杂交与染色体工程工作，利用人工合成的四倍体小麦通过非编码RNA如rDNA研究表明：双亲的rDNA基因在杂种F1均表达，但染色体组加倍当代A组发生rDNA沉默并发生启动子区DNA甲基化升高。长期进化过程中小麦的rDNA发生严重的变化：A组位点大部分丢失，D组位点拷贝数减少并失去功能，B组的一对染色体rDNA位点发生位置变化。

    人工合成的部分组合产生的四倍体小麦在高代发生A组rDNA位点消失，符合小麦进化的第一次染色体加倍而长期进化丢失A组的部分或全部rDNA 位点。而B组的rDNA位点除一对染色体发生位置变化，均保持活跃转录状态。双单倍体状态下双亲的rDNA均表达，加倍当代表现出基因组偏向沉默，但用细胞质诱导获得普通小麦单倍体发现沉默的rDNA并不能因倍性改变而恢复活性。同时，利用人工合成小麦及其远缘杂交材料，详细研究非编码RNA在麦类着丝粒的变异与新着丝粒的形成作用及变化机制。

　　韩方普研究组博士生郭翔完成这一工作，部分结果发表在The Plant Cell (DOI:10.1105/tpc.114.129841)。该工作得到科技部“863”项目及植物细胞与染色体工程国家重点实验室支持。

原文摘要：

Asymmetric Epigenetic Modification and Elimination of rDNA Sequences by Polyploidization in Wheat

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.