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.

互利共生合作关系广泛存在，并在众多生态系统中发挥基础性作用。鉴于这类相互作用通常为专性依赖，参与合作的个体的达尔文适合度（Darwinian fitness）不仅取决于自身基因组所编码的遗传信息，同时也与其对应交互伙伴的性状与功能能力息息相关。因此，一个尚未解决的核心科学问题是：专性互利共生合作关系如何影响生物体适应环境变化的进化能力。本研究以两种可相互交换合成成本较高氨基酸的大肠杆菌（Escherichia coli）营养缺陷型基因型间的互利共生合作关系为模型，对该问题展开探讨。研究中将添加氨基酸的单培养体系与未添加氨基酸的共培养体系，分别置于逐步升高浓度的不同抗生素环境中进行培养。该选择实验结果表明，代谢上相互依赖的细菌，其适应环境胁迫的能力普遍弱于自主生长的菌株。此外，专性互利共生伙伴往往会重新获得代谢自主性，最终导致互利共生相互作用的瓦解。