中国农大在Plant Cell揭示小麦多倍化杂种优势形成机理

近日，中国农业科学院作物科学研究所毛龙研究团队与四川农业大学和诺禾致源生物信息公司合作，以新合成六倍体为材料，以系统翔实的数据揭示了不同倍性物种非对等杂交导致杂种优势形成的分子基础，为作物杂种优势形成机理提供了重要的证据。相关研究成果发表在最新一期的国际植物类顶尖杂志《植物细胞(The Plant Cell)》上。

作物杂种优势在生产上有着广泛的应用，但是对于它的形成机理依不同的研究体系而各不相同，迄今未能在理论上形成广泛接受的统一认知，更缺乏直接的实验证据。异源多倍体，如六倍体小麦，它的形成包括近缘物种间杂交和染色体加倍，从而把杂种优势固定在其中。利用多倍体形成过程来研究杂种优势的形成机理是目前国际上该领域的前沿课题。

毛龙研究团队用四倍体硬粒小麦Triticum turgidum (AABB)和二倍体粗山羊草Aegilops tauschii (DD)作为亲本进行杂交并经染色体加倍获得新合成的异源六倍体小麦。这种合成小麦在生长优势和适应性等方面均超过其亲本。该团队进而利用小麦A和D基因组序列草图，通过rna-seq和小分子RNA高通量测序技术，分析了一套新合成异源六倍体小麦4个连续世代及其亲本在幼苗、抽穗期和籽粒发育早期等3个阶段的基因和小分子RNA表达谱，发现合成六倍体小麦3个发育阶段的非加性表达蛋白质编码基因数目非常有限，并表现为抽穗期非加性表达基因与细胞生长显著关联。

更为重要的是，与非加性表达基因不同，亲本表达显性基因在子代差异基因中占有相当比例，并表现出四倍体亲本表达显性基因主要贡献于六倍体小麦发育和二倍体亲本表达显性基因主要贡献于六倍体小麦适应性的基因组亚功能化现象。这种在小麦多倍化过程中与杂种优势有关基因表达的新特点为研究领域首次报道。此外，靶向抗逆、抗病、开花等重要生物学过程的miRNA均表现为非加性表达，并很可能参与了亲本表达显性基因的表达调控。这项发现为植物多倍化杂种优势的形成提供了重要理论启示和借鉴作用，为进一步在新合成异源六倍体小麦中快速筛选亲本优异基因提供了新策略。

原文摘要：

mRNA and Small RNA Transcriptomes Reveal Insights into Dynamic Homoeolog Regulation of Allopolyploid Heterosis in Nascent Hexaploid Wheat

Aili Li, Dengcai Liu, Jun Wu, Xubo Zhao, Ming Hao, Shuaifeng geng, Jun Yan, Xiaoxue Jiang, Lianquan Zhang, Junyan Wu, Lingjie Yin,Rongzhi Zhang, Liang Wu, Youliang Zheng and Long Mao

Nascent allohexaploid wheat may represent the initial genetic state of common wheat (Triticum aestivum), which arose as a hybrid between Triticum turgidum(AABB) and Aegilops tauschii (DD) and by chromosome doubling and outcompeted its parents in growth vigor and adaptability. To better understand the molecular basis for this success, we performed mRNA and small RNA transcriptome analyses in nascent allohexaploid wheat and its following generations, their progenitors, and the natural allohexaploid cultivar Chinese Spring, with the assistance of recently published A and D genome sequences. We found that nonadditively expressed protein-coding genes were rare but relevant to growth vigor. Moreover, a high proportion of protein-coding genes exhibited parental expression level dominance, with genes for which the total homoeolog expression level in the progeny was similar to that in T. turgidum potentially participating in development and those with similar expression to that in Ae. tauschii involved in adaptation. In addition, a high proportion of microRNAs showed nonadditive expression upon polyploidization, potentially leading to differential expression of important target genes. Furthermore, increased small interfering RNA density was observed for transposable element–associated D homoeologs in the allohexaploid progeny, which may account for biased repression of D homoeologs. Together, our data provide insights into small RNA–mediated dynamic homoeolog regulation mechanisms that may contribute to heterosis in nascent hexaploid wheat.

作者：中国农业大学