Data for "Phenotypic robustness peaks at mildly cold, not optimal-for-growth, temperatures"

It has been suggested that benign thermal conditions maintain more genetic variation by mitigating the fitness effects of mutations, contributing to temperature-dependent evolutionary rates. Here we argue that temperatures maximizing phenotypic robustness do not coincide with those optimizing growth. This mismatch arises because warming simultaneously accelerates rate-limiting processes and reduces protein stability, potentially unmasking cryptic genetic variation at high temperatures. Using parallel experimental evolution lines of <i>Escherichia coli </i>and <i>Pseudomonas fluorescens</i>, we quantified phenotypic robustness across thermal gradients. Among genotype variation in maximum growth rate was minimized at mildly cold temperatures, where mean growth rates were reduced to 0.36-0.42 of optimal values in <i>E. coli</i> and 0.50-0.79 in <i>P. fluorescens</i>. Optimal-for-robustness temperatures were consistently lower than optimal-for-growth temperatures and did not align with evolution temperatures. These results suggest that environments cooler than growth optima may serve as reservoirs of genetic diversity, whereas global warming may erode phenotypic robustness by exposing cryptic variation.