Evolutionary rescue occurs when adaptation restores population growth against a lethal stressor. Here we studied evolutionary rescue by conducting experiments with Escherichia coli at the lethal temperature of 43.0 °C, to determine the adaptive mutations that drive rescue and to investigate their effects on fitness and gene expression. From hundreds of populations, we observed that ∼9% were rescued by genetic adaptations. We sequenced 26 populations and identified 29 distinct mutations. Of these populations, 21 had a mutation in the hslVU or rpoBC operon, suggesting that mutations in either operon could drive rescue. We isolated seven strains of E. coli carrying a putative rescue mutation in either the hslVU or rpoBC operon to investigate the mutations’ effects. The single rescue mutations increased E. coli’s relative fitness by an average of 24% at 42.2 °C, but they decreased fitness by 3% at 37.0 °C, illustrating that antagonistic pleiotropy likely affected the establishment of rescue in our system. Gene expression analysis revealed only 40 genes were upregulated across all seven mutations, and these were enriched for functions in translational and flagellar production. As with previous experiments with high temperature adaptation, the rescue mutations tended to restore gene expression towards the unstressed state, but they also caused a higher proportion of novel gene expression patterns. Overall, we find that rescue is infrequent, that it is facilitated by a limited number of mutational targets, and that rescue mutations may have qualitatively different effects than mutations that arise from evolution to non-lethal stressors.

中文翻译：

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驱动进化拯救到大肠杆菌致死温度的遗传突变。

当适应恢复人口增长以对抗致命的压力时，就会发生进化救援。在这里，我们通过在 43.0 °C 的致死温度下对大肠杆菌进行实验来研究进化救援，以确定驱动救援的适应性突变并研究它们对适应性和基因表达的影响。从数百个种群中，我们观察到约 9% 的种群被遗传适应所拯救。我们对 26 个群体进行了测序，并确定了 29 个不同的突变。在这些人群中，有 21 个在hslVU或rpoBC操纵子中发生了突变，这表明任一操纵子中的突变都可以推动救援。我们分离出7株的大肠杆菌无论是在携带一个假定救援突变hslVU或rpoBC操纵子来研究突变的影响。单个拯救突变增加了大肠杆菌的相对适应度在 42.2°C 时平均降低了 24%，但在 37.0°C 时它们的适应度降低了 3%，说明拮抗多效性可能影响了我们系统中救援的建立。基因表达分析显示，所有七个突变中只有 40 个基因上调，并且这些基因在翻译和鞭毛产生方面的功能得到了丰富。与之前的高温适应实验一样，拯救突变倾向于将基因表达恢复到无压力状态，但它们也导致更高比例的新基因表达模式。总的来说，我们发现拯救并不常见，它是由有限数量的突变目标促进的，并且拯救突变可能具有与从进化到非致命压力源的突变在性质上不同的影响。