Obligate mutualistic cooperation limits evolvability

Cooperative mutualisms are widespread and play fundamental roles in many ecosystems. Given that these interactions are often obligate, the Darwinian fitness of the participating individuals is not only determined by the information encoded in their own genomes, but also the traits and capabilities of their corresponding interaction partners. Thus, a major outstanding question is how obligate cooperative mutualisms affect the ability of organisms to adapt evolutionarily to changing environmental conditions. Here we address this issue using a mutualistic cooperation between two auxotrophic genotypes of Escherichia coli that reciprocally exchanged costly amino acids. Amino acid-supplemented monocultures and unsupplemented cocultures were exposed to stepwise increasing concentrations of different antibiotics. This selection experiment reveals that metabolically interdependent bacteria are generally less able to adapt to environmental stress than autonomously growing strains. Moreover, obligate cooperative mutualists frequently regain metabolic autonomy, resulting in a collapse of the mutualistic interaction.合作共生关系（cooperative mutualism）广泛分布于各类生态系统中，并在其中发挥基础性作用。鉴于这类互作通常具有专性（obligate），参与互作个体的达尔文适合度（Darwinian fitness）不仅由自身基因组所编码的遗传信息决定，同时还与其对应互作伙伴的性状与功能密切相关。因此，一个尚未解决的核心科学问题是：专性合作共生关系如何影响生物体适应环境变化的进化能力。本研究以两株能够相互交换昂贵氨基酸的大肠杆菌（Escherichia coli）营养缺陷型（auxotrophic）菌株间的合作共生关系为模型，对上述问题展开探究。我们将添加了氨基酸的单培养体系与未添加氨基酸的共培养体系，分别暴露于逐步升高浓度的多种抗生素环境中。本次选择实验结果显示：与自主生长的菌株相比，代谢上相互依赖的细菌通常更难适应环境胁迫。此外，专性合作共生体往往会重新获得代谢自主性，最终导致共生互作关系的崩溃。