Dimensions: Collaborative Research: Elucidating the drivers of mutualism variation in host-symbiont metapopulations

共生细菌改变了植物和动物与环境的相互作用。众所周知，这些细菌具有促进宿主生长和耐受性的能力，但共生体对宿主健康和适应性的影响差异很大。对于形成这种变化并推动共生生物传播的力量，人们知之甚少，共生生物相互作用，但却不能使宿主受益。在这里，研究人员利用加州原生豆科植物和固氮慢生根瘤菌之间的关系来研究共生变化的驱动因素。该项目将采用相互作用的植物、细菌和土壤的环境采样、遗传方法和温室实验。这项研究将确定细菌为宿主提供的益处的大小，哪些细菌基因促进了宿主的益处或利用，以及宿主如何应对和保护自己免受无效共生体的侵害。该项目将培养本科生和研究生以及两名博士后。这些研究人员将对当地农民进行关于植物与微生物相互作用的教育，他们还将生成和整理一个植物和细菌变种的集合，这些变种将免费提供给其他研究人员。该项目之所以重要，是因为它将提供有关影响共生关系的参数的信息，并帮助指导如何更好地利用微生物来提高农业系统的生产力，促进人类和地球的健康。维持微生物共生的多样性是自相矛盾的。传统理论预测共生体的功能缺乏变化，因为宿主物种奖励有益的共生体，并强烈选择无效的伙伴。在这里，一个新的框架被测试来解释共生变异的维持。计划中的工作超越了主导的理论范式，通过研究根瘤菌群及其原生豆科宿主的生态、时空和基因组驱动因素。与其他寄主一样，豆科植物也表现出寄主控制性状：豆科植物可以在根瘤形成过程中对无效根瘤菌进行鉴别，并可以降低无效根瘤菌在根瘤内的生长速度。尽管宿主控制明显有效，但在自然和农业土壤中通常发现无效的根瘤菌。在这个关键的模式系统中，提出了四个目标来解释共生变异的维持。采用实验接种结合遗传和选择分析的方法，研究了豆科植物寄主群体对无效根瘤菌的容忍度差异，无效根瘤菌入侵的空间和系统发育起源，以及无效根瘤菌进化的基因组基础。iv)无效根瘤菌的推断入侵将通过实验在现实条件下测试适应度预测进行概括。这项研究挑战了传统的共生理论，强调了遗传变异、时空动态和适应性权衡的重要性。

Symbiotic bacteria transform how plants and animals interact with their environment. These bacteria are well known for their ability to enhance host growth and tolerance to stress, but symbioses vary greatly in their effects on host health and fitness. Little is understood about the forces that shape this variation and drive the spread of symbionts that interact, but fail to benefit the host. Here, researchers use the relationship between native California legumes and nitrogen-fixing Bradyrhizobium bacteria to study the drivers of variation in symbioses. The project will employ environmental sampling of interacting plants, bacteria, and soil, genetic approaches, and greenhouse experiments. The research will determine the magnitude of benefits the bacteria provide to the host, what bacterial genes facilitate benefit or exploitation of the host, and how the host responds and defends itself against ineffective symbionts. The project will train undergraduate and graduate students as well as two postdoctoral fellows. The researchers will educate local farmers on plant-microbe interactions, and they will generate and curate a collection of plant and bacterial variants that will be made freely available to other researchers. The project is important because it will provide information on the parameters that influence symbiosis and help guide how microbes can be better deployed to increase productivity of agricultural systems and promote health of humans and the planet. The maintenance of diversity in microbial symbioses is paradoxical. Conventional theory predicts a lack of variation in symbiont function because host species reward beneficial symbionts and intensely select against ineffective partners. Here, a novel framework is tested to explain the maintenance of symbiont variation. The planned work transcends the dominant theoretical paradigm by investigating ecological, spatiotemporal, and genomic drivers in a metapopulation of rhizobial bacteria and their native legume hosts. Like other hosts, legumes exhibit host control traits: legumes can discriminate against ineffective rhizobia during nodule formation, and can reduce within-nodule growth rates of ineffective rhizobia. Despite the apparent efficiency of host control, ineffective rhizobia are commonly uncovered in natural and agricultural soils. Four aims are proposed to explain the maintenance of symbiont variation in this key model system. Experimental inoculations coupled with genetic and selection analysis are used to investigate i) variation among legume host populations in their capacity to sanction ineffective rhizobia, ii) the spatial and phylogenetic origins of invasions by ineffective rhizobia, and iii) the genomic basis for the evolution of ineffective rhizobia. iv) Inferred invasions of ineffective rhizobia will be recapitulated using experiments that test fitness predictions under realistic conditions. This work challenges conventional mutualism theory and highlights the importance of genetic variation, spatio-temporal dynamics, and fitness tradeoffs.