The origins of uncooperative rhizobia

Mutualisms are thought to be destabilized by exploitative mutants that receive benefits from partners without reciprocation. Nonetheless, there is surprisingly little evidence for the spread of exploitation in mutualist populations. In particular mutualisms, non-beneficial partners are commonplace and this raises the question of whether exploitation is invading as an adaptive strategy. Here, we highlight the legume rhizobium mutualism as a key test case. Rhizobial bacteria fix nitrogen in legume roots in exchange for carbon from their hosts. However, non-beneficial rhizobia are widespread, including non-fixing and non-nodulating strains. Recent research has shown that legumes can punish some uncooperative rhizobia and substantially reduce their fitness, but these sanctions must not be universally effective. Important questions about uncooperative rhizobia remain unresolved. (1) Is it adaptive for rhizobia to be uncooperative with hosts? (2) Do uncooperative rhizobia evolve from cooperative ancestors? (3) What are the mechanisms of rhizobial exploitation? We describe experimental approaches and testable hypotheses that address these gaps in our knowledge.