Data from: Historical contingency in the evolution of antibiotic resistance after decades of relaxed selection

Populations often encounter changed environments that remove selection for
the maintenance of particular phenotypic traits. The resulting genetic
decay of those traits under relaxed selection reduces an organism’s
fitness in its prior environment. However, whether and how such decay
alters the subsequent evolvability of a population upon restoration of
selection for a previously diminished trait is not well understood. We
addressed this question using Escherichia coli strains from the long-term
evolution experiment (LTEE) that independently evolved for multiple
decades in the absence of antibiotics. We first confirmed that these
derived strains are typically more sensitive to various antibiotics than
their common ancestor. We then subjected the ancestral and derived strains
to various concentrations of these drugs to examine their potential to
evolve increased resistance. We found that evolvability was idiosyncratic
with respect to initial genotype; that is, the derived strains did not
generally compensate for their greater susceptibility by “catching up” to
the resistance level of the ancestor. Instead, the capacity to evolve
increased resistance was constrained in some backgrounds, implying that
evolvability depended upon prior mutations in a historically contingent
fashion. We further subjected a time-series of clones from one LTEE
population to tetracycline and determined that an evolutionary constraint
arose early in that population, corroborating the role of contingency. In
summary, relaxed selection not only can drive populations to increased
antibiotic susceptibility, but it can also affect the subsequent
evolvability of antibiotic resistance in an unpredictable manner. This
conclusion has potential implications for public health, and it
underscores the need to consider the genetic context of pathogens when
designing drug-treatment strategies.