EVOLUTION OF THERMAL DEPENDENCE OF GROWTH RATE OF ESCHERICHIA COLI POPULATIONS DURING 20,000 GENERATIONS IN A CONSTANT ENVIRONMENT

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Abstract
Twelve experimental populations of the bacterium Escherichia coli evolved for 20,000 generations in a defined medium at 37°C. We measured their maximum growth rates across a broad range of temperatures and at several evolutionary time points to quantify the extent to which they became thermal specialists with diminished performance at other temperatures. We also sought to determine whether antagonistic pleiotropy (genetic trade-offs) or mutation accumulation (drift decay) was primarily responsible for any thermal specialization. Populations showed consistent improvement in growth rate at moderate temperatures (27–39°C), but tended to have decreased growth rate at both low (20°C) and high (41–42°C) 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 defects in their DNA repair, but they did not subsequently undergo a gre...