Mutualism breakdown in the legume-rhizobium symbiosis.
Sachs, Joel*,, Simms, Ellen,
ABSTRACT- Cooperative interactions among species, known as mutualisms, are ubiquitous yet perplexing. Mutualisms are known to be vulnerable to the evolution of exploitation, in which individuals gain reproductive advantage by reaping but not reciprocating cooperative benefits. Recent work suggests that maladaptation between partners can also disrupt mutualisms. Such maladaptation comes about when natural selection shapes one cooperative partner species independent of the other. The emergence of exploitation and maladaptation can each promote mutualism breakdown, the loss of a cooperative traits in populations of one or both partners. Here, we examine mutualism breakdown in one of the most intensively studied cooperative systems on earth, the legume-rhizobium symbiosis. We describe the conditions favoring dissolution of mutualism between legumes and rhizobia and evaluate the empirical evidence from agricultural and natural populations. By studying the varied forces that potentially disrupt this mutualism we hope to offer a more complete understanding of the manifold evolutionary trajectories that exist, beyond mutual cooperation. Finally, we extend the principles considered here to other mutualisms. We conclude that the conditions that favor mutualism breakdown are commonplace, hence that the dissolution of mutualisms may be a widespread evolutionary end.
Key words: Legume-rhizobium, mutualism-breakdown, maladaptation, exploitation
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