Data from: Adding biotic complexity alters the metabolic benefits of mutualism

Mutualism is ubiquitous in nature and plays an integral role in most communities. To predict the eco-evolutionary dynamics of mutualism it is critical to extend classic pair-wise analysis to include additional species. We investigated the effect of adding a third species to a pair-wise mutualism in a spatially structured environment. We tested the hypotheses that selection for costly excretions in a focal population i) decreases when an exploiter is added ii) increases when a third mutualist is added relative to the pair-wise scenario. We assayed the selection acting on Salmonella enterica when it exchanges methionine for carbon in an obligate mutualism with an auxotrophic Escherichia coli. A third bacterium, Methylobacterium extorquens, was then added and acted either as an exploiter of the carbon or third obligate mutualist depending on the nitrogen source. In the tri-partite mutualism M. extorquens provided nitrogen to the other species. Contrary to our expectations, adding an exploiter increased selection for methionine excretion in S. enterica. Conversely, selection for cooperation was lower in the tri-partite mutualism relative to the pair-wise system. Genome-scale metabolic models helped identify the mechanisms underlying these changes in selection. Our results highlight the utility of connecting metabolic mechanisms and eco-evolutionary dynamics.

互利共生（mutualism）在自然界中无处不在，对绝大多数群落均发挥着不可或缺的作用。若要预测互利共生的生态进化动力学（eco-evolutionary dynamics），将经典的两两互作分析拓展至包含更多物种的场景至关重要。本研究在空间结构化环境中，探究了向两两互利共生体系中引入第三个物种所产生的影响。我们验证了两项假说：相较于两两互作体系，焦点种群针对代价性分泌产物的演化选择压力，i）在引入利用者（exploiter）时会降低；ii）在引入第三类互利共生者时会升高。我们针对肠炎沙门氏菌（Salmonella enterica）与营养缺陷型大肠杆菌（Escherichia coli）之间的专性互利共生（obligate mutualism）体系展开分析：二者以碳源交换甲硫氨酸，借此测定了作用于肠炎沙门氏菌的选择压力。随后我们引入第三种细菌——扭脱甲基杆菌（Methylobacterium extorquens），该菌可依据培养环境的氮源（nitrogen source）类型，分别扮演碳源利用者或第三类专性互利共生者的角色。在三组分互利共生（tri-partite mutualism）体系中，扭脱甲基杆菌会为另外两个物种提供氮源。与我们的预期相悖的是，引入利用者反而提升了肠炎沙门氏菌针对甲硫氨酸分泌的选择压力。与之相反，相较于两两互作体系，三组分互利共生体系中针对合作行为的选择压力更低。基因组规模代谢模型（Genome-scale metabolic models）助力我们解析了上述选择压力变化背后的潜在机制。本研究结果凸显了将代谢机制与生态进化动力学相结合的研究价值。