Legume-rhizobium symbiosis with two different life histories

Symbiotic rhizobia that fix little or no nitrogen persist in natural populations despite legume sanctions. Cheating strategies available to rhizobia and the effects of host sanctions depend on the rhizobium life history, often related to nodule type. Bacteroids, the differentiated N₂-fixing form of rhizobia, often lose reproductive viability in nodules with indeterminate growth, while bacteroids from determinate nodules typically accumulate carbon resources for their own reproduction. Rhizobia infecting indeterminate nodules may produce rhizopines that their undifferentiated non-fixing kin occupying the same nodule can acquire as a source of carbon. If ineffective and effective rhizobia can jointly infect a single nodule (mixed strains), host sanctions at the nodule level would be less severe than for nodules occupied exclusively by ineffective rhizobia. Simulation models allowing for mixed strain nodules and host sanctions help us understand how rhizobia in determinate and indeterminate nodules differ in suppressing cheater strains. The models show that sanctions on determinate nodules with mixed strains could not eliminate cheaters from the population while they could eliminate cheaters in mixed indeterminate nodules over time. This model assumes that rhizopines are equally available to different strains in the same indeterminate nodule. Review of present literature suggests that legumes with indeterminate nodules are the ancestral type and determinate nodules arose independently at least twice. If nodule types are indeed correlated to rhizobial reproductive viability, further examination of the phylogenetic distribution of different nodule types is a valuable key to understanding the evolution of the mutualism.