EVOLUTIONARY ADAPTATION TO TEMPERATURE II. THERMAL NICHES OF EXPERIMENTAL LINES OF ESCHERICHIA COLI.

Groups of replicated lines of the bacterium Escherichia coli were propagated for 2,000 generations at constant 32, 37, or 42°C, or in an environment that alternated between 32 and 42°C. Here, we 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. In addition, no significant differences among groups were observed in rate of extinction at more extreme temperatures. Within the thermal niche, we measured the mean fitness of the evolved groups relative to their common ancestor. 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 to the ancestor was greatest (the thermal optimum) diverged by about 10°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, although tradeoffs were observed for a few of the lines. We conclude that adaptation in this system was quite temperature specific, but substantial divergence among groups in thermal optima had little effect on the limits of their thermal niches and did not necessarily involve tradeoffs in fitness at other temperatures.