What factors influence the ability of populations to adapt to extreme environments that lie outside their current tolerance limits? We investigated this question by exposing experimental populations of the bacterium Escherichia coil to lethally high temperatures. We asked: (1) whether we could obtain thermotolerant mutants with an extended upper thermal limit by this selective screen; (2) whether the propensity to obtain thermotolerant mutants depended on the prior selective history of the progenitor genotypes; and (3) how the fitness properties of these mutants compared to those of their progenitors within the ancestral thermal niche. Specifically, we subjected 15 independent populations founded from each of six progenitors to 44 degrees C; all of the progenitors had upper thermal limits between about 40 degrees C and 42 degrees C. Two of the progenitors were from populations that had previously adapted to 32 degrees C, two were from populations adapted to 37 degrees C, and two were from populations adapted to 41-42 degrees C. All 90 populations were screened for mutants that could survive and grow at 44 degrees C. We obtained three thermotolerant mutants, all derived from progenitors previously adapted to 41-42 degrees C. In an earlier study,we serendipitously found one other thermotolerant mutant derived from a population that had previously adapted to 32 degrees C. Thus, prior selection at an elevated but nonlethal temperature may predispose organisms to evolve more extreme thermotolerance, but this is not an absolute requirement. It is evidently possible to obtain mutants that tolerate more extreme temperatures, so why did they not become prevalent during prior selection at 41-42 degrees C, near the upper limit of the thermal niche? To address this question, we measured the fitness of the thermotolerant mutants at high temperatures just within the ancestral niche. None of the four thermotolerant mutants had an advantage relative to their progenitor even very near the upper limit of the thermal niche; in fact, all of the mutants showed a noticeable loss of fitness around 41 degrees C. Thus, the genetic adaptations that improve competitive fitness at high but nonlethal temperatures are distinct from those that permit tolerance of otherwise lethal temperatures.
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