Department of Botany and Plant Pathology, Oregon State University, Corvallis, Oregon, United States of America.
PLoS Pathog. 2013 Feb;9(2):e1003204. doi: 10.1371/journal.ppat.1003204. Epub 2013 Feb 28.
Two diametric paradigms have been proposed to model the molecular co-evolution of microbial mutualists and their eukaryotic hosts. In one, mutualist and host exhibit an antagonistic arms race and each partner evolves rapidly to maximize their own fitness from the interaction at potential expense of the other. In the opposing model, conflicts between mutualist and host are largely resolved and the interaction is characterized by evolutionary stasis. We tested these opposing frameworks in two lineages of mutualistic rhizobia, Sinorhizobium fredii and Bradyrhizobium japonicum. To examine genes demonstrably important for host-interactions we coupled the mining of genome sequences to a comprehensive functional screen for type III effector genes, which are necessary for many Gram-negative pathogens to infect their hosts. We demonstrate that the rhizobial type III effector genes exhibit a surprisingly high degree of conservation in content and sequence that is in contrast to those of a well characterized plant pathogenic species. This type III effector gene conservation is particularly striking in the context of the relatively high genome-wide diversity of rhizobia. The evolution of rhizobial type III effectors is inconsistent with the molecular arms race paradigm. Instead, our results reveal that these loci are relatively static in rhizobial lineages and suggest that fitness conflicts between rhizobia mutualists and their host plants have been largely resolved.
已经提出了两种截然相反的范例来模拟微生物共生体与其真核宿主的分子协同进化。在一种范例中，共生体和宿主表现出一种拮抗的军备竞赛，每个伙伴都迅速进化，以最大限度地从相互作用中提高自身的适应性，而潜在地牺牲对方的适应性。在相反的范例中，共生体和宿主之间的冲突基本得到解决，相互作用的特点是进化稳定。我们在两个共生根瘤菌谱系（中华根瘤菌和日本根瘤菌）中检验了这些相反的框架。为了研究对宿主相互作用至关重要的基因，我们将基因组序列的挖掘与 III 型效应基因的全面功能筛选相结合，III 型效应基因是许多革兰氏阴性病原体感染宿主所必需的。我们证明，根瘤菌 III 型效应基因在内容和序列上具有惊人的高度保守性，这与一个特征明确的植物病原物种形成鲜明对比。在根瘤菌相对较高的全基因组多样性的背景下，这种 III 型效应基因的保守性尤为显著。根瘤菌 III 型效应子的进化与分子军备竞赛范例不一致。相反，我们的结果表明，这些基因座在根瘤菌谱系中相对稳定，表明根瘤菌共生体与其宿主植物之间的适应性冲突已基本得到解决。
