Temporal partner heterogeneity constrains the evolution of mutualistic benefits

Evolutionary theory predicts that coevolutionary specialization between mutualistic lineages is beneficial, increasing the evolution of cooperative benefits. However, many mutualists interact with temporally variable partners, which can limit opportunities for sustained reciprocal selection. Few empirical studies have directly examined how this partner heterogeneity affects mutualistic evolution, largely because tracking partner dynamics over evolutionary timescales is challenging in natural systems. We circumvent this constraint via experimental evolution with an engineered resource sharing mutualism in Baker’s yeast (Saccharomyces cerevisiae), which allows precise control of partner heterogeneity across generations. We compared low temporal partner heterogeneity (consistent pairings) with high temporal partner heterogeneity (temporally rotating partners) across 54 rounds of selection (∼378–426 generations). Low partner heterogeneity produced the strongest mutualistic benefits (7.2% increased growth over controls), while high partner heterogeneity yielded only modest benefits (2.2% increase). Mutualistic benefits in the low partner heterogeneity treatment were consistent with increased resource exchange. Time-shift experiments indicate that coevolution may have enhanced the evolution of mutualistic benefits but was not strictly necessary: benefits also evolved through one-sided adaptation. Genomic analyses revealed parallel evolution in amino acid metabolism and starvation response genes. Surprisingly, low-heterogeneity lineages evolved as generalists rather than specialists, cooperating equally well with all partners tested. Overall, these results demonstrate that sustained interactions between specific partner lineages facilitate the rapid evolution of mutualistic benefits but are not strictly required.