Widespread fitness alignment in the legume-rhizobium symbiosis

Although cheaters potentially destabilize the legumerhizobium mutualism, we lack a comprehensive review of hostsymbiont fitness correlations. Studies measuring rhizobium relative or absolute fitness and host benefit are surveyed. Mutant studies are tallied for evidence of pleiotropy; studies of natural strains are analyzed with meta-analysis. Of 80 rhizobium mutations, 19 decrease both partners fitness, four increase both, two increase host fitness but decrease symbiont fitness and none increase symbiont fitness at the hosts expense. The pooled correlation between rhizobium nodulation competitiveness and plant aboveground biomass is 0.65 across five experiments that compete natural strains against a reference, whereas, across 14 experiments that compete rhizobia against soil populations or each other, the pooled correlation is 0.24. Pooled correlations between aboveground biomass and nodule number and nodule biomass are 0.76 and 0.83. Positive correlations between legume and rhizobium fitness imply that most ineffective rhizobia are defective rather than defectors; this extends to natural variants, with only one significant fitness conflict. Most studies involve non-coevolved associations, indicating that fitness alignment is the default state. Rhizobium mutations that increase both host and symbiont fitness suggest that some plants maladaptively restrict symbiosis with novel strains.