Microbial mutualists provide substantial benefits to hosts that feed back to enhance fitness of the associated microbes. In many systems, beneficial microbes colonize symbiotic organs, specialized host structures that house symbionts and mediate resources exchanged between parties. Mutualisms are characterized by net benefits exchanged among members of different species, however inequalities in the magnitude of these exchanges could result in evolutionary conflict, destabilizing the mutualism. We investigated joint fitness effects of root nodule formation, the symbiotic organ of legumes that house nitrogen-fixing rhizobia in planta. We quantified host and symbiont fitness parameters dependent on the number of nodules formed using near isogenic Lotus japonicus and Mesorhizobium loti mutants, respectively. Empirically estimated fitness functions suggest that legume and rhizobia fitness is aligned as the number of nodules formed increases from zero until the host optimum is reached, a point where aligned fitness interests shift to diverging fitness interests between host and symbiont. However, fitness conflict was only inferred when analyzing wildtype hosts along with their mutants dysregulated for control over nodule formation. These data demonstrate that to avoid conflict, hosts must tightly regulate investment into symbiotic organs maximizing their benefit to cost ratio of associating with microbes.

Biomass Harvest Data Sheet

Host Phenotype data collected at the time of harvest.

Culturing Data Sheet

Estimated viable rhizobia per nodule. Proportion data and corresponding cell estimates are only applicable for coinoculum rows.

Histology Sheet

Histological measurements of host nodules infected with MAFF at 5wpi. Area units are squared micrometers.

d15N Sheet

Elemental and isotopic analysis of leaf tissue for plants harvested at 5wpi.