Interspecific conflict and the evolution of ineffective rhizobia

Microbial symbionts exhibit broad genotypic variation in their fitness effects on hosts, leaving hosts vulnerable to costly partnerships. Interspecific conflict and partner‐maladaptation are frameworks to explain this variation, with different implications for mutualism stability. We investigated the mutualist service of nitrogen fixation in a metapopulation of root‐nodule forming Bradyrhizobium symbionts in Acmispon hosts. We uncovered Bradyrhizobium genotypes that provide negligible mutualist services to hosts and had superior in planta fitness during clonal infections, consistent with cheater strains that destabilise mutualisms. Interspecific conflict was also confirmed at the metapopulation level – by a significant negative association between the fitness benefits provided by Bradyrhizobium genotypes and their local genotype frequencies – indicating that selection favours cheating rhizobia. Legumes have mechanisms to defend against rhizobia that fail to fix sufficient nitrogen, but ...

微生物共生体对宿主的适合度效应表现出广泛的基因型变异，导致宿主易受代价高昂的共生关系影响。种间冲突与伙伴不适应是解释这种变异的理论框架，二者对互利共生稳定性具有不同的意义。我们研究了在Acmispon宿主中形成根瘤的慢生根瘤菌（Bradyrhizobium）共生体集合种群中的固氮互利服务。我们发现，部分慢生根瘤菌基因型对宿主提供的互利服务可忽略不计，但在克隆感染期间具有更优的体内适合度，这与破坏互利共生稳定性的欺骗菌株特征一致。种间冲突在集合种群水平上也得到了证实——慢生根瘤菌基因型提供的适合度收益与其局部基因型频率之间存在显著负相关，表明选择倾向于欺骗性根瘤菌。豆科植物具有防御无法固定足够氮素的根瘤菌的机制，但...