Single mutation makes Escherichia coli an insect mutualist

We report an experimental system in which Escherichia coli evolves into an insect mutualist. When the essential gut symbiont of the stinkbug Plautia stali was replaced by E. coli, a few survivor insects exhibited specific localization and vertical transmission of E. coli. Through trans-generational maintenance with P. stali, several hyper-mutating E. coli lines independently evolved host’s high adult emergence and improved body color. Such “mutualistic” E. coli lines exhibited independent mutations disrupting the carbon catabolite repression (CCR) global transcriptional regulator. Each of the mutations reproduced the mutualistic phenotypes when introduced into wild-type E. coli, confirming that the single CCR mutations instantly make E. coli an insect mutualist. Our discovery uncovers that evolution of elaborate mutualism can proceed more easily and rapidly than conventionally envisaged.本研究报道了一套实验体系，在此体系中大肠杆菌（Escherichia coli）可演化成为昆虫共生体。当斑腹刺益蝽（Plautia stali）的必需肠道共生菌被大肠杆菌替代后，少量存活的昆虫体内出现了大肠杆菌的特异性定殖与垂直传播现象。通过与斑腹刺益蝽进行跨代共培养，数株高突变率的大肠杆菌品系独立演化出了可提升宿主成虫羽化率、优化宿主体色的性状。这类“共生型”大肠杆菌品系均携带了破坏碳分解代谢物阻遏（CCR）全局转录调控因子的独立突变。将上述任一突变导入野生型大肠杆菌后，均可重现该共生表型，证实单个CCR相关突变即可使大肠杆菌快速转变为昆虫共生体。本研究发现表明，复杂精细的共生关系的演化过程可能比传统认知中更加简便快捷。