title: Partner choice in nitrogen-fixation mutualisms of legumes and rhizobia.
abstract: Mutualistic interactions are widespread and obligatory for many organisms, yet their evolutionary persistence in the face of cheating is theoretically puzzling. Nutrient-acquisition symbioses between plants and soil microbes are critically important to plant evolution and ecosystem function, yet we know almost nothing about the evolutionary dynamics and mechanisms of persistence of these ancient mutualisms. Partner-choice and partner-fidelity are mechanisms for dealing with cheaters, and can theoretically allow mutualisms to persist despite cheaters.Many models of cooperative behavior assume pairwise interactions, while most plant-microbe nutrient-acquisition symbioses involve a single plant interacting with numerous microbes. Market models, in contrast, are well suited to mutualisms in which single plants attempt to conduct mutually beneficial resource exchange with multiple individuals. Market models assume that one partner chooses to trade with a subset of individuals selected from a market of potential partners. Hence, determining whether partner-choice occurs in plant-microbe mutualisms is critical to understanding the evolutionary persistence and dynamics of these symbioses. The nitrogen-fixation/carbon-fixation mutualism between leguminous plants and rhizobial bacteria is widespread, ancient, and important for ecosystem function and human nutrition. It also involves single plants interacting simultaneously with several to many bacterial partners, including ineffective ("cheating") strains. We review the existing literature and find that this mutualism displays several elements of partner-choice, and may match the requirements of the market paradigm. We conclude by identifying profitable questions for future research.
authors.affiliation: Department of Integrative Biology, 3060 Valley Life Sciences Building #3140, University of California, Berkeley, California 94720-3140 and Plant Conservation Research Center, University of California Botanical Garden, 200 Centennial Drive #5045, Berkeley, California 94720-5045.
authors.forename: Ellen L
authors.initials: EL
authors.lastname: Simms
authors.forename: D Lee
authors.initials: DL
authors.lastname: Taylor