Widespread fitness alignment in the legume–rhizobium symbiosis

Authors

  • Maren L. Friesen

    1. Center for Population Biology, University of California, Davis, One Shields Ave., Davis, CA 95616, USA
    2. Present address: Section of Molecular and Computational Biology, Department of Biology, University of Southern California, 1050 Childs Way, RRI 201-B Los Angeles, CA 90089, USA
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Author for correspondence:
Maren L. Friesen
Tel: +1 213 740 3065
Email: friesen@usc.edu

Summary

  • Although ‘cheaters’ potentially destabilize the legume–rhizobium mutualism, we lack a comprehensive review of host–symbiont 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 host’s 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.

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