Mechanisms Maintaining Cooperation in Rhizobial Populations

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

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.