Mechanisms Maintaining Cooperation in Rhizobial Populations

物种之间的合作，或互惠互利，无处不在，但由于作弊者的问题而令人困惑。作弊者不需要回报就能得到好处。是什么阻止了他们打破互惠互利呢？已经提出了两种解决方案。根据合作伙伴选择，挑剔的合作伙伴奖励互惠互利的人，惩罚作弊的人。在伴侣忠诚的情况下，对长期互助者的欺骗会反作用于伤害出轨者。很少有研究对这些理论进行实证检验。在一种重要的生态互惠关系中，根瘤根瘤细菌将大气中的氮转化为植物可利用的形式，以换取豆科植物宿主通过光合作用产生的资源。然而，一些根瘤菌感染宿主而不提供好处，而一些根瘤菌则完全忽视宿主。人们对阻止这些细菌传播的选择性力量知之甚少。这个项目将研究选择如何促进根瘤菌方面的合作。调查和操作实验将：(1)表征土壤、根和根瘤中的根瘤菌种群；(2)记录根瘤菌在感染过程中的种群遗传变化；(3)测试从衰老的根瘤中释放合作根瘤菌；(4)检查几种豆科植物的伴侣选择。植物和动物，包括人类和我们的食物来源，严重依赖有益的微生物。然而，互惠共生的微生物可能会变得寄生。珊瑚漂白等现象可能可以用互惠微生物对环境的敏感性来解释。了解推动这些变化的因素以及如何保持合作有助于我们了解如何限制人类病原体的毒力进化，提高作物和牲畜生产力，以及预测和管理对环境变化的反应。研究成果将用于丰富课堂学习和公开讲座。来自不同背景的学生将学习研究方法，研究生和博士后学者将学习指导本科生。

Cooperation among species, or mutualism, is ubiquitous but perplexing because of the problem of cheaters. Cheaters receive benefits without reciprocating. What prevents them from breaking down a mutualism? Two solutions have been proposed. Under partner choice, choosy partners reward beneficial mutualists and punish cheaters. Under partner fidelity, cheating on a long-term mutualist feeds back to harm the cheater. Scant work has tested these theories empirically. In an ecologically important mutualism, root-nodulating rhizobial bacteria convert atmospheric nitrogen to a plant-available form in exchange for resources produced through photosynthesis by the legume host. However, some rhizobia infect hosts without providing benefit and some ignore hosts altogether. Little is known about the selective forces that prevent these bacteria from spreading. This project will study how selection can promote cooperation in rhizobia. Surveys and manipulative experiments will: (1) characterize rhizobial populations in soil, on roots, and in nodules, (2) document population genetic changes in nodulated rhizobia during infection, (3) test for release of cooperative rhizobia from senescing nodules, and (4) examine partner choice in several legume species.Plants and animals, including humans and our food sources, are critically dependent on beneficial microbes. However, mutualistic microbes can become parasitic. Phenomena such as coral bleaching might be explained by environmental sensitivity of mutualistic microbes. Understanding what factors drive these shifts and how cooperation is maintained helps us understand how to limit the evolution of virulence in human pathogens, improve crop and livestock productivity, and predict and manage responses to environmental change. Research results will be used to enrich classroom learning and public lectures. Students from diverse backgrounds will learn research methods, and graduate students and postdoctoral scholars will learn to mentor undergraduate students.