Abstract
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 toward 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 nonlethal stressors.