Evolutionary adaptation to temperature. II. Thermal niches of experimental lines of Escherichia coli

Abstract

Groups of replicated lines of the bacterium Escherichia coli were propagated for 2000 generations at constant 32, 37, or 42°C, or in an environment that alternated between 32 and 42°C. Here, the authors examine the performance of each group across a temperature range of 12-44°C measuring the temperatures over which each line can maintain itself in serial dilution culture (the thermal niche). Thermal niche was not affected by selection history: average lower and upper limits remained about 19 and 42°C for all groups. No significant differences among groups were observed in rate of extinction at more extreme temperatures. Increases in mean fitness were temperature specific, with the largest increase for each group occurring near its selected temperature. Thus, the temperature at which mean fitness relative ot the ancestor was greatest (the thermal optimum) diverged by c10°C for the groups selected at constant 32°C versus constant 42°C. Tradeoffs in relative fitness (decrements relative to the ancestor elsewhere within the thermal niche) did not necessarily accompany fitness improvements. (See also 92L/06764). -from Authors