Multiple Mutualist Effects generate synergistic selection and strengthen fitness alignment in a tripartite interaction between legumes, rhizobia, and mycorrhizal fungi

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