Wheat blast, a devastating disease having spread recently from South America to Asia and Africa, is caused by Pyricularia oryzae (synonym of Magnaporthe oryzae) pathotype Triticum, which first emerged in Brazil in 1985. Rmg8 and Rmg7, genes for resistance to wheat blast found in common wheat and tetraploid wheat, respectively, recognize the same avirulence gene, AVR-Rmg8. Here we show that an ancestral resistance gene, which had obtained an ability to recognize AVR-Rmg8 before the differentiation of Triticum and Aegilops, has expanded its target pathogens. Molecular cloning revealed that Rmg7 was an allele of Pm4, a gene for resistance to wheat powdery mildew on 2AL, whereas Rmg8 was its homoeologue on 2BL ineffective against wheat powdery mildew. Rmg8 variants with the ability to recognize AVR-Rmg8 were distributed not only in Triticum spp. but also in Aegilops speltoides, Aegilops umbellulata and Aegilops comosa. This result suggests that the origin of resistance gene(s) recognizing AVR-Rmg8 dates back to the time before differentiation of A, B, S, U and M genomes, that is, ~5 Myr before the emergence of its current target, the wheat blast fungus. Phylogenetic analyses suggested that, in the evolutionary process thereafter, some of their variants gained the ability to recognize the wheat powdery mildew fungus and evolved into genes controlling dual resistance to wheat powdery mildew and wheat blast.

麦瘟是一种最近从南美洲传播到亚洲和非洲的毁灭性疾病，是由米瘟病菌（ Magnaporthe oryzae ）致病型小麦引起的，该病种于 1985 年在巴西首次出现。 Rmg8和Rmg7是发现的抗麦瘟基因普通小麦和四倍体小麦分别识别相同的无毒基因AVR-Rmg8 。在这里，我们表明，在小麦和山羊草分化之前就获得了识别AVR-Rmg8的能力的祖先抗性基因已经扩大了其目标病原体。分子克隆显示， Rmg7是2AL上抗小麦白粉病基因Pm4的等位基因，而Rmg8是2BL上的同源基因，对小麦白粉病无效。具有识别AVR-Rmg8能力的Rmg8变体不仅分布在小麦属中。但也存在于speltoides 、 Ae gilops umbellulata和Ae gilops comosa中。这一结果表明，识别AVR-Rmg8的抗性基因的起源可以追溯到 A、B、S、U 和 M 基因组分化之前的时间，即在其当前靶标（即 AVR-Rmg8）出现之前约 5 Myr。麦瘟菌。系统发育分析表明，在其后的进化过程中，它们的一些变体获得了识别小麦白粉病真菌的能力，并进化为控制小麦白粉病和麦瘟病双重抗性的基因。