Genotypes exhibiting an increased mutation rate, called hypermutators, can propagate in microbial populations because they can have an advantage due to the higher supply of beneficial mutations needed for adaptation. Although this is a frequently observed phenomenon in natural and laboratory populations, little is known about the influence of parameters such as the degree of maladaptation, stress intensity and the genetic architecture for adaptation on the emergence of hypermutators. To address this knowledge gap, we measured the emergence of hypermutators over ~1000 generations in experimental Escherichia coli populations exposed to different levels of osmotic or antibiotic stress. Our stress types were chosen based on the assumption that the genetic architecture for adaptation differs between them. Indeed, we show that the size of the genetic basis for adaptation is larger for osmotic stress compared to antibiotic stress. During our experiment, we observed an increased emergence of hypermutators in populations exposed to osmotic stress but not in those exposed to antibiotic stress, indicating that hypermutator emergence rates are stress-type-dependent. These results support our hypothesis that hypermutator emergence is linked to the size of the genetic basis for adaptation. In addition, we identified other parameters that covaried with stress type (stress level and IS transposition rates) that might have contributed to an increased hypermutator provision and selection. Our results provide a first comparison of hypermutator emergence rates under varying stress conditions and point towards complex interactions of multiple stress-related factors on the evolution of mutation rates.

• mutation_list_g1000.csv: whole genome sequencing of evolved populations and analysed using Breseq
• Four datasets with growth kinetics parameters of ancestral and evolved strains in different antibiotic (GM) or salt concentrations. Raw data were obtained with a TECAN plate reader and growth kinetic parameters were calculated with the grofit package in R:
  • GM_allparameters_ancestor.csv
  • GM_allparameters_evolved.csv
  • salt_allparameters_ancestor_corrected.csv
  • salt_allparameters_evolved.csv
• Compiled_OD_measures.csv: OD measurements of experimental populations throughout the experiment after 24h of growth
• Timing of IS10 insertions - PCR results.xlsx: results for the PCR assay ("PCR" = performed on all populations; "int PCR" = performed on populations with a positive PCR)

All CSV files except Timing of IS10 insertions can be opened with Open Office.