Genomic signatures of UV resistance evolution in Escherichia coli depend on the growth phase during exposure

Physiological states can determine the ability of organisms to handle
stress. Does this mean that the same selection pressure will lead to
different evolutionary outcomes, depending on the organisms’ physiological
state? If yes, what will be the genomic signatures of such adaptation(s)?
We used experimental evolution in Escherichia coli followed by
whole-genome whole-population sequencing to investigate these questions.
The sensitivity of Escherichia coli to ultraviolet (UV) radiation depends
on the growth phase during which it experiences the radiation. We evolved
replicate E. coli populations under two different conditions of UV
exposures, namely exposure during the lag and the exponential growth
phases. Initially, the UV sensitivity of the ancestor was greater during
the exponential phase than the lag phase. However, at the end of 100
cycles of exposure, UV resistance evolved to similar extents in both
treatments. Genome analysis showed that mutations in genes involved in DNA
repair, cell membrane structure and RNA polymerase were common in both
treatments. However, different functional groups were found mutated in
populations experiencing lag and exponential UV treatment. In the former,
genes involved in transcriptional and translational regulations and
cellular transport were mutated, whereas the latter treatment showed
mutations in genes involved in signal transduction and cell adhesion.
Interestingly, the treatments showed no phenotypic differences in a number
of novel environments. Taken together, these results suggest that
selection pressures at different physiological stages can lead to
differences in the genomic signatures of adaptation, which need not
necessarily translate into observable phenotypic differences.