title: Evolutionary dynamics of nitrogen fixation in the legume-rhizobia symbiosis.
abstract: The stabilization of host-symbiont mutualism against the emergence of parasitic individuals is pivotal to the evolution of cooperation. One of the most famous symbioses occurs between legumes and their colonizing rhizobia, in which rhizobia extract nutrients (or benefits) from legume plants while supplying them with nitrogen resources produced by nitrogen fixation (or costs). Natural environments, however, are widely populated by ineffective rhizobia that extract benefits without paying costs and thus proliferate more efficiently than nitrogen-fixing cooperators. How and why this mutualism becomes stabilized and evolutionarily persists has been extensively discussed. To better understand the evolutionary dynamics of this symbiosis system, we construct a simple model based on the continuous snowdrift game with multiple interacting players. We investigate the model using adaptive dynamics and numerical simulations. We find that symbiotic evolution depends on the cost-benefit balance, and that cheaters widely emerge when the cost and benefit are similar in strength. In this scenario, the persistence of the symbiotic system is compatible with the presence of cheaters. This result suggests that the symbiotic relationship is robust to the emergence of cheaters, and may explain the prevalence of cheating rhizobia in nature. In addition, various stabilizing mechanisms, such as partner fidelity feedback, partner choice, and host sanction, can reinforce the symbiotic relationship by affecting the fitness of symbionts in various ways. This result suggests that the symbiotic relationship is cooperatively stabilized by various mechanisms. In addition, mixed nodule populations are thought to encourage cheater emergence, but our model predicts that, in certain situations, cheaters can disappear from such populations. These findings provide a theoretical basis of the evolutionary dynamics of legume-rhizobia symbioses, which is extendable to other single-host, multiple-colonizer systems.
authors.affiliation: Division of Symbiotic Systems, National Institute for Basic Biology, National Institute for Natural Sciences, Okazaki, Japan.
authors.forename: Hironori
authors.initials: H
authors.lastname: Fujita
authors.affiliation: Department of General Systems Studies, Graduate School of Arts and Sciences, University of Tokyo, Komaba, Meguro-ku, Tokyo, Japan.
authors.forename: Seishiro
authors.initials: S
authors.lastname: Aoki
authors.affiliation: Division of Symbiotic Systems, National Institute for Basic Biology, National Institute for Natural Sciences, Okazaki, Japan; Department of Basic Biology, School of Life Science, Graduate University for Advanced Studies (SOKENDAI), Okazaki, Japan.
authors.forename: Masayoshi
authors.initials: M
authors.lastname: Kawaguchi
mesh_terms: D005075:Biological Evolution; D007887:Fabaceae / Q000235:genetics* / Q000378:metabolism; D008962:Models, Theoretical; D009584:Nitrogen / Q000378:metabolism; D009586:Nitrogen Fixation; D012230:Rhizobiaceae / Q000235:genetics* / Q000378:metabolism; D013559:Symbiosis / Q000235:genetics*
chemical_list: D009584:Nitrogen