Pahua V J, Stokes P J N, Hollowell A C, Regus J U, Gano-Cohen K A, Wendlandt C E, Quides K W, Lyu J Y, Sachs J L
Department of Evolution Ecology and Organismal Biology, University of California, Riverside, CA, USA.
Department of Botany and Plant Sciences, University of California, Riverside, CA, USA.
J Evol Biol. 2018 Apr;31(4):599-610. doi: 10.1111/jeb.13249. Epub 2018 Feb 26.
Legumes can preferentially select beneficial rhizobial symbionts and sanction ineffective strains that fail to fix nitrogen. Yet paradoxically, rhizobial populations vary from highly beneficial to ineffective in natural and agricultural soils. Classic models of symbiosis focus on the single dimension of symbiont cost-benefit to sympatric hosts, but fail to explain the widespread persistence of ineffective rhizobia. Here, we test a novel framework predicting that spatio-temporal and community dynamics can maintain ineffective strains in rhizobial populations. We used clonal and multistrain inoculations and quantitative culturing to investigate the relative fitness of four focal Bradyrhizobium strains varying from effective to ineffective on Acmispon strigosus. We found that an ineffective Bradyrhizobium strain can be sanctioned by its native A. strigosus host across the host's range, forming fewer and smaller nodules compared to beneficial strains. But the same ineffective Bradyrhizobium strain exhibits a nearly opposite pattern on the broadly sympatric host Acmispon wrangelianus, forming large nodules in both clonal and multistrain inoculations. These data suggest that community-level effects could favour the persistence of ineffective rhizobia and contribute to variation in symbiotic nitrogen fixation.
豆科植物能够优先选择有益的根瘤菌共生体，并制裁那些无法固氮的无效菌株。然而矛盾的是，在自然土壤和农业土壤中，根瘤菌种群从高度有益到无效各不相同。经典的共生模型侧重于共生体对同域宿主成本效益的单一维度，但无法解释无效根瘤菌的广泛存在。在这里，我们测试了一个新的框架，该框架预测时空和群落动态可以维持根瘤菌种群中的无效菌株。我们使用克隆接种和多菌株接种以及定量培养来研究四种在硬毛链荚豆上从有效到无效的慢生根瘤菌菌株的相对适合度。我们发现，一种无效的慢生根瘤菌菌株可以在其原生宿主硬毛链荚豆的整个分布范围内受到制裁，与有益菌株相比，形成的根瘤更少更小。但同样的无效慢生根瘤菌菌株在广泛同域的宿主瓦氏链荚豆上表现出几乎相反的模式，在克隆接种和多菌株接种中都形成大根瘤。这些数据表明，群落水平的影响可能有利于无效根瘤菌的持续存在，并导致共生固氮的差异。
