First adaptive step in a microbial community is dominated by mutualism-enhancing mutations. Saccharomyces cerevisiae strain:GSY6699

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.

当生态群落中的类群发生快速演化时，耦合的生态-演化动态会变得极为复杂，难以通过常规方法解析。本研究采用条形码谱系追踪（barcode lineage tracking）技术，解析由莱茵衣藻（Chlamydomonas reinhardtii）与酿酒酵母（Saccharomyces cerevisiae）构建的实验性兼性互利群落中的生态-演化耦合关系。研究发现，与莱茵衣藻的互作会改变酿酒酵母中数百个新生适应性突变的适合度效应，且不会产生适合度权衡，但这仍会使酵母种群内克隆竞争的遗传与表型结果发生显著变化。具体而言，那些在群落中受到强烈选择但在单培养体系中无此效应的突变，一旦固定便会多效性增强酵母与衣藻之间的互利共生关系。本研究结果表明，即便生态伙伴施加的选择压力发生微小变化，也能够引导生态-演化轨迹走向新型生态状态。