Evolution of thermal dependence of growth rate of Escherichia coli populations during 20,000 generations in a constant environment

Twelve experimental populations of the bacterium Escherichia coli evolved f or 20,000 generations in a defined medium at 37 degreesC. We measured their maximum growth rates across a broad range of temperatures and at several e volutionary time points to quantify the extent to which they became thermal specialists: with diminished performance at other temperatures. We also so ught to determine whether antagonistic pleiotropy (genetic trade-offs) or m utation accumulation (drift decay) was primarily responsible for any therma l specialization. Populations showed consistent improvement in growth rate at moderate temperatures (27-39 degreesC), but tended to h;ive decreased gr owth rate at both low (20 degreesC) and high (41-42 degreesC) temperatures. Most loss occurred early in the experiment, when adaptation was most rapid . This dynamic is predicted by antagonistic pleiotropy but not by mutation accumulation. Several populations evolved high mutation rates due to defect s: in their DNA repair, but they did not subsequently undergo a greater dec rease in growth rate at thermal extremes than populations that retained low mutation rates, contrary to the acceleration of decay predicted by mutatio n accumulation. Antagonistic pleiotropy therefore is more likely to be resp onsible for the evolution of thermal specialization observed in maximum gro wth rate.