Differences in the fitness effects of traded resources shape traits and persistence in multi-mutualist communities

Mutualistic interactions, where species reciprocally benefit from each
other, are crucial for ecosystem stability and biodiversity. These
interactions often involve species that experience different fitness
effects for the traded resources or services. Because mutualisms rely on
positive feedback between partners, such asymmetries can strongly
influence evolutionary outcomes. Differences in fitness effects create
divergent selective pressures, shaping trait evolution and determining the
persistence of mutualisms. The strength of these effects can also vary
depending on the availability of traded resources from other sources.
Despite their importance, the evolutionary role of fitness asymmetries in
mutualism has received little attention, beyond recognizing that some
species may be more dependent on their partners than others. This study
investigates how asymmetry in the fitness effects of a traded resource
influences the persistence and phenotypic trait evolution of species in
multi-mutualist guilds. To test this, we constructed synthetic
multi-mutualist communities by combining reproductively isolated and
genetically modified strains of Saccharomyces cerevisiae that engage in a
nutritional mutualism by trading adenine and lysine. One guild of four
strains cannot produce lysine but overproduces adenine while the other
guild cannot produce adenine but overproduces lysine. Lysine overproducers
survive periods of low adenine better than adenine overproducers survive
low lysine. Over a four-week evolution experiment we observed that strain
persistence was strongly influenced by the availability of external
resources. Communities in media containing traded resources supported the
survival of all strains, whereas obligate conditions led to a significant
extinction, especially for adenine overproducers. We observed distinct
evolutionary trajectories of traits under obligate versus supplemented
conditions. Phenotypic assays revealed that costs and benefits evolved
differently depending on the essentiality of the traded resource and
nutrient supplementation. These results demonstrate that asymmetries in
the fitness effects of traded resources can influence evolutionary
outcomes, species persistence, and community stability in multi-mutualist
communities.