First adaptive step in a microbial community is dominated by mutualism-enhancing mutations

When members of an ecological community evolve rapidly, the coupled eco-evolutionary dynamics are complex and difficult to disentangle with conventional methods. We use barcode lineage tracking to dissect the eco-evolutionary coupling in an experimental facultatively mutualistic community formed by alga Chlamydomonas reinhardtii and yeast Saccharomyces cerevisiae. We find that interactions with alga alter the fitness effects of hundreds of nascent adaptive mutations in yeast without imposing fitness trade-offs, which nevertheless causes large shifts in the genetic and phenotypic outcomes of clonal competition within the yeast population. Specifically, mutations that are strongly selected in the community but not in monoculture pleiotropically strengthen the yeast-alga mutualism once they become fixed. These results demonstrate how even subtle shifts in selection pressures exerted by ecological partners can steer eco-evolutionary trajectories towards novel ecological states.