EVOLUTIONARY ADAPTATION TO TEMPERATURE. VI. PHENOTYPIC ACCLIMATION AND ITS EVOLUTION IN ESCHERICHIA COLI.

Acclimation refers to reversible, nongenetic changes in phenotype that are induced by specific environmental conditions. Acclimation is generally assumed to improve function in the environment that induces it (the beneficial acclimation hypothesis). In this study, we experimentally tested this assumption by measuring relative fitness of the bacterium Escherichia coli acclimated to different thermal environments. The beneficial acclimation hypothesis predicts that bacteria acclimated to the temperature of competition should have greater fitness than do bacteria acclimated to any other temperature. The benefit predicted by the hypothesis was found in only seven of 12 comparisons; in the other comparisons, either no statistically demonstrable benefit was observed or a detrimental effect of acclimation was demonstrated. For example, in a lineage evolutionarily adapted to 37°C, bacteria acclimated to 37°C have a higher fitness at 32°C than do bacteria acclimated to 32°C, a result exactly contrary to prediction; acclimation to 27°C or 40°C prior to competition at those temperatures confers no benefit over 37°C acclimated forms. Consequently, the beneficial acclimation hypothesis must be rejected as a general prediction of the inevitable result of phenotypic adjustments associated with new environments. However, the hypothesis is supported in many instances when the acclimation and competition temperatures coincide with the historical temperature at which the bacterial populations have evolved. For example, when the evolutionary temperature of the population was 37°C, bacteria acclimated to 37°C had superior fitness at 37°C to those acclimated to 32°C; similarly, bacteria evolutionarily adapted to 32°C had a higher fitness during competition at 32°C than they did when acclimated to 37°C. The more surprising results are that when the bacteria are acclimated to their historical evolutionary temperature, they are frequently competitively superior even at other temperatures. For example, bacteria that have evolved at either 20°C or 32°C and are acclimated to their respective evolutionary temperatures have a greater fitness at 37°C than when they are acclimated to 37°C. Thus, acclimation to evolutionary temperature may, as a correlated consequence, enhance performance not only in the evolutionary environment, but also in a variety of other thermal environments.