Recurrent mutualism breakdown events in a legume rhizobia metapopulation

Bacterial mutualists generate major fitness benefits for eukaryotes, reshaping the host phenotype and its interactions with the environment. Yet microbial mutualist populations are predicted to generate mutants that defect from providing costly services to hosts while maintaining the capacity to exploit host resources. Here, we examined the mutualist service of symbiotic nitrogen fixation in a metapopulation of root-nodulating Bradyrhizobium spp. that associate with the native legume Acmispon strigosus. We quantified mutualism traits of 85 Bradyrhizobium isolates gathered from a 700km transect in California spanning ten sampled A. strigosus populations. We clonally inoculated each Bradyrhizobium isolate onto Acmispon strigosus hosts and quantified nodulation capacity and net effects of infection, including host growth and isotopic nitrogen concentration. Six Bradyrhizobium isolates from five populations were categorized as ineffective because they formed nodules but did not enhance host...

细菌互利共生菌可赋予真核生物显著的适合度收益，重塑宿主表型及其与环境的互作关系。然而，微生物互利共生种群理论上会产生突变体：这类突变体不再向宿主提供高成本的共生服务，却仍能利用宿主资源。本研究针对与本土豆科植物Acmispon strigosus共生的结瘤慢生根瘤菌属（Bradyrhizobium）集合种群（metapopulation），探究了其共生固氮（symbiotic nitrogen fixation）这一共生服务功能。我们从横跨加利福尼亚州700公里样带的10个采样Acmispon strigosus种群中，采集得到85株慢生根瘤菌分离株，并量化了各菌株的共生性状。我们将每株慢生根瘤菌分离株无性接种至Acmispon strigosus宿主上，量化其结瘤能力与感染的净效应，包括宿主生长量与同位素氮浓度。最终，来自5个种群的6株慢生根瘤菌分离株被归类为无效共生菌株：它们虽可形成根瘤，但无法促进宿主……