Multiple mutualism effects generate synergistic selection and strengthen fitness alignment in the interaction between legumes, rhizobia, and mycorrhizal fungi

Nearly all organisms participate in multiple mutualisms, and complementarity within these complex interactions can result in synergistic fitness effects. However, it remains largely untested how multiple mutualisms impact eco-evolutionary dynamics in interacting species. We tested how multiple microbial mutualists-- N-fixing bacteria and mycorrrhizal fungi-- affected selection and heritability of traits in their shared host plant (Medicago truncatula), as well as fitness alignment between partners. Our results demonstrate for the first time that multiple mutualisms synergistically affect selection and heritability of host traits and enhance fitness alignment between mutualists. Specifically, we found interaction with multiple microbial symbionts doubled the strength of natural selection on a plant architectural trait, resulted in 2-3-fold higher heritability of plant reproductive success, and more than doubled fitness alignment between N-fixing bacteria and plants. These findings show synergism generated by multiple mutualisms extends to key components of microevolutionary change, emphasizing the importance of multiple mutualism effects on evolutionary trajectories.

Methods
The details on data collection are available in the associated paper in Ecology Letters. In brief, 213 M. truncatula genotypes were grown in a factorial experiment with four microbial environments environments: no microbes (NM,NR), rhizobia alone (NM, R), mycorrhizal fungi alone (M, NR), and both microbes (R,M). We measured a plant fitness proxy (pod production), a rhizobia fitness proxy (nodule number), and several plant traits related to plant architecture (branch number) and investment (allocation to roots vs shoot biomass).