Widespread Negative Frequency-Dependent Selection Maintains Diversity in the Legume-Rhizobia Symbiosis: Balancing nodulation may explain the paradox of rhizobium diversity

The evolutionary origin and ecological maintenance of biodiversity is a central problem in biology. For diversity to be stable through time, each genotype or species must have an advantage when rare. This negative frequency-dependence prevents deterministic extinction and mitigates the stochastic loss of diversity (1–4). However, models of mutualism typically generate positive frequency-dependence that reduces diversity (5–8). Here, we report empirical evidence for negative frequency-dependence in the legume-rhizobium mutualism within a single host generation, a phenomenon that we term balancing nodulation. Balancing nodulation increases rare rhizobia across all 13 legume genera investigated to date, at high and low inoculum densities, and with minimal genetic differentiation between rhizobia strains. While the mechanism generating this phenomenon is currently unknown, balancing nodulation could actively maintain variation in the rhizobia-legume symbiosis.