Lazarus effects: the frequency and genetic causes of Escherichia coli population recovery under lethal heat stress

Sometimes populations may crash and yet be capable of adapting and recovering before being lost completely. Such recoveries have been observed incidentally in evolution experiments using Escherichia coli, and this phenomenon has been termed the “Lazarus effect.” To investigate how often it occurs and the genetic changes that drive it, we evolved ~300 populations of E. coli at lethally high temperatures (43.0°) for five days and sequenced the genomes of populations that recovered. Our results reveal that the Lazarus effect is uncommon, but frequent enough, at ~9% of populations, to potentially be a source of evolutionary innovation. Furthermore, our sequencing efforts uncover a set of mutations adaptive to lethal 43.0° that are mostly distinct from those that are beneficial at a high but nonlethal temperature (42.2°). The short-term nature of our experiment enabled us to identify mutations within two operons—the heat shock hslUV operon and the RNA polymerase rpoBC operon—that drive adaptation to lethal temperature. In our experiment, recovered populations fix mutations in only one operon or the other, but not both, indicating that epistatic interactions between beneficial mutations are important even at the earliest stages of adaptation. Lastly, mutations in hslUV exhibit little antagonistic pleiotropy and likely arise neutrally prior to subjection to lethal temperature, whereas rpoBC mutations cause strong fitness tradeoffs at lower temperatures. We find that rpoBC mutants provide greater fitness benefits than hslUV mutants at lethal temperature, but they are less prevalent than hslUV mutations, perhaps because of their antagonistic effects.