Grillo Michael A, Stinchcombe John R, Heath Katy D
University of Illinois, Department of Plant Biology, Urbana, Illinois 61801, USA
Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario, Canada M5S 3B2.
Am J Bot. 2016 Oct;103(10):1763-1770. doi: 10.3732/ajb.1600090. Epub 2016 Sep 26.
Resource mutualisms such as the symbiosis between legumes and nitrogen-fixing rhizobia are context dependent and are sensitive to various aspects of the environment, including nitrogen (N) addition. Mutualist hosts such as legumes are also thought to use mechanisms such as partner choice to discriminate among potential symbionts that vary in partner quality (fitness benefits conferred to hosts) and thus impose selection on rhizobium populations. Together, context dependency and partner choice might help explain why the legume-rhizobium mutualism responds evolutionarily to N addition, since plant-mediated selection that shifts in response to N might be expected to favor different rhizobium strains in different N environments.
We test for the influence of context dependency on partner choice in the model legume, Medicago truncatula, using a factorial experiments with three plant families across three N levels with a mixed inoculation of three rhizobia strains.
Neither the relative frequencies of rhizobium strains occupying host nodules, nor the size of those nodules, differed in response to N level.
Despite the lack of context dependence, plant genotypes respond very differently to mixed populations of rhizobia, suggesting that these traits are genetically variable and thus could evolve in response to longer-term increases in N.
资源互利共生关系，如豆科植物与固氮根瘤菌之间的共生关系，取决于环境背景，并且对环境的各个方面都很敏感，包括添加氮（N）。豆科植物等互利共生宿主也被认为会利用诸如伙伴选择等机制，来区分潜在共生体，这些共生体在伙伴质量（赋予宿主的适应性益处）方面存在差异，从而对根瘤菌种群施加选择。环境依赖性和伙伴选择共同作用，可能有助于解释为什么豆科植物 - 根瘤菌共生关系在进化上会对添加氮作出反应，因为预计随着氮的变化而改变的植物介导选择，在不同的氮环境中会有利于不同的根瘤菌菌株。
我们使用析因实验，在三个氮水平下对三个植物家系进行混合接种三种根瘤菌菌株，以此来测试环境依赖性对模式豆科植物蒺藜苜蓿伙伴选择的影响。
占据宿主根瘤的根瘤菌菌株的相对频率，以及这些根瘤的大小，均未因氮水平的变化而有所不同。
尽管缺乏环境依赖性，但植物基因型对根瘤菌混合群体的反应差异很大，这表明这些性状具有遗传变异性，因此可能会随着氮的长期增加而进化。
