Abstract
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 the 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‐ to 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, emphasising the importance of multiple mutualism effects on evolutionary trajectories.
Almost all species interact with multiple mutualisms, either simultaneously or sequentially, but the consequences of these multiple mutualisms for eco‐evolutionary dynamics remain unclear. We used a 2 × 2 experiment manipulating the presence and absence of mycorrhizal fungi and rhizobia to examine how they individually and jointly affected heritability and selection on their share host plant, Medicago truncatula, as well as fitness alignment between partners. We found pervasive non‐additive effects of interacting with multiple mutualisms on heritability and selection on plant traits and enhanced fitness alignment between mutualists, suggesting broad‐ranging evolutionary consequences of interacting with multiple mutualisms.
