Evolutionary Rescue to Lethal Temperatures in Escherichia coli

Evolutionary rescue occurs when adaptation by natural selection drives population survival in the face of lethal or rapidly deteriorating conditions. Here, we performed evolution experiments with Escherichia coli at a lethal temperature to determine the adaptive mutations that drive rescue and to investigate their effects on fitness and gene expression. We identified a rescue frequency of 8.8 percent and performed complete genome sequencing on the 26 populations that successfully underwent rescue. We identified 29 distinct point mutations or small indels within 20 different genic and intergenic regions. Of these populations, 21 populations had a mutation in either the hslVU or rpoBC operon, suggesting that mutating either operon could drive rescue. We isolated seven mutant strains of E. coli carrying a mutation in either the hslVU or rpoBC operon to investigate how these mutations affect the cell. Competition experiments against the ancestral genotype illustrated that the single rescue mutations increased relative fitness by an average of 24 percent at high temperature, but decreased fitness by 3 percent at the ancestral optimum temperature. To investigate the potential mechanism of rescue we performed RNA sequencing and found that the rescue mutations caused significant differential expression in 900 or more genes relative to the ancestral genotype at high temperature. Rescue mutations tended to restore gene expression towards an unstressed state, but also caused significant novel expression phenotypes. Overall, our results suggest that a single mutation can drive rescue to lethal temperature in E. coli, but may confer antagonistic pleiotropic effects. We also identified a high proportion of genes with novel expression phenotypes suggesting that mutations driving rescue may differ in their consequences compared to mutations arising from evolution to non-lethal conditions.