1. Agronomy and Range Science, University of California, 1 Shields Avenue, Davis, California 95616, USA
2. Institute of Cell, Animal & Population Biology, University of Edinburgh, King's Buildings, West Mains Road, Edinburgh EH9 3JT, UK

Correspondence to: R. Ford Denison¹ Email: rfdenison@ucdavis.edu

Abstract

Explaining mutualistic cooperation between species remains one of the greatest problems for evolutionary biology^{1, 2, 3, 4}. Why do symbionts provide costly services to a host, indirectly benefiting competitors sharing the same individual host? Host monitoring of symbiont performance and the imposition of sanctions on 'cheats' could stabilize mutualism^{5, 6}. Here we show that soybeans penalize rhizobia that fail to fix N₂ inside their root nodules. We prevented a normally mutualistic rhizobium strain from cooperating (fixing N₂) by replacing air with an N₂-free atmosphere (Ar:O₂). A series of experiments at three spatial scales (whole plants, half root systems and individual nodules) demonstrated that forcing non-cooperation (analogous to cheating) decreased the reproductive success of rhizobia by about 50%. Non-invasive monitoring implicated decreased O₂ supply as a possible mechanism for sanctions against cheating rhizobia. More generally, such sanctions by one or both partners may be important in stabilizing a wide range of mutualistic symbioses.