Department of Microbiology and Plant Pathology, University of California, Riverside, CA, USA.
Department of Botany and Plant Sciences, University of California, Riverside, CA, USA.
Proc Biol Sci. 2020 Jan 29;287(1919):20192549. doi: 10.1098/rspb.2019.2549.
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 spp that associate with the native legume . We quantified mutualism traits of 85 isolates gathered from a 700 km transect in California spanning 10 sampled populations. We clonally inoculated each isolate onto hosts and quantified nodulation capacity and net effects of infection, including host growth and isotopic nitrogen concentration. Six isolates from five populations were categorized as ineffective because they formed nodules but did not enhance host growth via nitrogen fixation. Six additional isolates from three populations failed to form root nodules. Phylogenetic reconstruction inferred two types of mutualism breakdown, including three to four independent losses of effectiveness and five losses of nodulation capacity on . The evolutionary and genomic drivers of these mutualism breakdown events remain poorly understood.
细菌共生体为真核生物带来了主要的适应益处，重塑了宿主表型及其与环境的相互作用。然而，微生物共生体种群预计会产生突变体，这些突变体在为宿主提供昂贵服务的同时，还保持着利用宿主资源的能力。在这里，我们研究了与本地豆科植物 共生的根瘤菌属 共生固氮的共生服务在一个复合种群中的表现。我们从加利福尼亚州 700 公里长的横断线上的 10 个采样种群中收集了 85 个 分离株，并对其进行了量化分析。我们将每个 分离株无性繁殖接种到 宿主上，并量化了其结瘤能力和感染的净效应，包括宿主生长和同位素氮浓度。五个种群中的六个 分离株被归类为无效，因为它们形成了根瘤，但没有通过固氮来促进宿主生长。另外六个 分离株来自三个种群，无法形成根瘤。系统发育重建推断出两种共生关系破裂的类型，包括三个到四个独立的有效性丧失和五个在 上的结瘤能力丧失。这些共生关系破裂事件的进化和基因组驱动因素仍知之甚少。
