Constraints in temperature adaptation reinforce differences in thermal niche between mesophilic and psychrotolerant Bacillus cereus group species

Experimental evolution has demonstrated that mesophilic microbes readily
adapt to increases in temperature. However, many microbes are
psychrotolerant and resistant to cold, which is associated with
physiological specializations, suggesting constraints in thermal
adaptation. We hypothesized that constraints would limit adaption
differently in a mesophilic species (B. thuringiensis) compared to its
psychrotolerant relative B. mycoides – with adaptation at cooler
temperatures and adaptation at higher temperatures being constrained in
each species respectively. To test this hypothesis, we imposed 140
generations of selection at temperatures at and below the optimum for
productivity for both species, conducting competition fitness assays,
thermal profiling via thermal performance curves and SNP variant analysis
to determine the changes that occurred during experimental
evolution.  The data and code enclosed is that used to create
each figure in the paper and the raw data generated in this study; the
only exception is the genetic sequencing data, which is available in NCBI
via the Bioproject PRJNA826440. The fitness and thermal performance of
evolved bacteria showed ancestral thermal niche plays a role in thermal
adaptation over this timescale, in support of our hypothesis of adaptive
constraints. Temperature-dependent trade-offs appeared common
in B. mycoides, with fitness gains associated with decreases in
operational niche width; fitness gains at one temperature caused a
decrease in the range of temperatures that the bacterium showed
appreciable growth. Genome resequencing showed that variation in mutation
supply and selection strength could not explain temperature-dependent
responses to selection. Importantly, metabolic theory only held true for
mesophilic B. thuringiensis, showing abundant but less studied
psychrotolerant species could follow different adaptive
trajectories.