The cooperative relationship between legumes (peas, beans, and their relatives) and the nitrogen-fixing bacteria (rhizobia) that live in nodules on their roots is a textbook example of mutualism. However, it has been unclear why this cooperation is maintained, given the evolutionary temptation to cheat the system. New research led by Ford Denison of the University of California, Davis, provides evidence that legumes maintain good behavior among their rhizobia by imposing sanctions against cheats.

In the legume rhizobium system, the bacteria fix atmospheric nitrogen for use by the host plant in return for accommodation in root nodules. But because nitrogen fixation is energetically expensive, any rhizobium strain that withholds this service should increase its own growth and fitness at the expense of cooperative strains and its host. "If free riders can obtain benefits while letting others pay the costs, that can undermine cooperation between species, just as it undermines cooperation...

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