Evolutionary contingency's impact on laboratory evolution of Escherichia coli under fluctuating environments

The adaptation of biological organisms to fluctuating environments is one major determinant of their structural and dynamical complexity. Organisms have evolved devoted adaptations to ensure the robust performance of physiological functions under environmental fluctuations. To further our understanding of particular adaptation strategies to different environmental fluctuations, we perform laboratory evolution experiments of Escherichia coli evolved under three temperature fluctuation regimes alternating between 15C and 43C. Two of these regimes are determined by the population9s growth, while the third regime switches stochastically. To address evolutionary contingencies, the experiments are performed on two lineages departing from different genetic backgrounds. The two lineages display distinct evolutionary trajectories, demonstrating dependency on the starting strain9s genetic background. Several genes exhibit a high degree of parallelism, suggesting their potential adaptive nature. The growth increase of the representative clones from each final population relative to their ancestor at 15C and 43C demonstrated no correlation between both temperatures, insinuating an absence of a strong trade-off between these two temperatures. Some strains grew less at 15C unless exposed to a 43C epoch, indicating some degree of internalization of the structure of the environment fluctuations. The phenotypic response of the evolved clones at 15C and 43C was assessed by a phenotype array method. The resulting responses reveal a general tendency to move closer to the phenotypic response of our starting strains at the optimum of 37C. This observation expands the documented restorative responses, even when facing complex environmental conditions.