Mongold Judith A, Bennett Albert F, Lenski Richard E
Center for Microbial Ecology, Michigan State University, East Lansing, Michigan, 48824.
Department of Ecology and Evolutionary Biology, University of California, Irvine, California, 92697.
Evolution. 1999 Apr;53(2):386-394. doi: 10.1111/j.1558-5646.1999.tb03774.x.
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 coli 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°C; all of the progenitors had upper thermal limits between about 40°C and 42°C. Two of the progenitors were from populations that had previously adapted to 32°C, two were from populations adapted to 37°C, and two were from populations adapted to 41-42°C. All 90 populations were screened for mutants that could survive and grow at 44°C. We obtained three thermotolerant mutants, all derived from progenitors previously adapted to 41-42°C. In an earlier study, we serendipitously found one other thermotolerant mutant derived from a population that had previously adapted to 32°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°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°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.
哪些因素会影响种群适应超出其当前耐受极限的极端环境的能力？我们通过将大肠杆菌的实验种群暴露于致死性高温来研究这个问题。我们提出了以下问题：（1）通过这种选择性筛选，我们是否能够获得具有扩展高温极限的耐热突变体；（2）获得耐热突变体的倾向是否取决于祖代基因型的先前选择历史；以及（3）与它们在祖先热生态位内的祖代相比，这些突变体的适应性特性如何。具体而言，我们将由六个祖代中的每一个建立的15个独立种群置于44°C环境中；所有祖代的高温极限都在约40°C至42°C之间。其中两个祖代来自先前适应32°C的种群，两个来自适应37°C的种群，还有两个来自适应41 - 42°C的种群。对所有90个种群进行筛选，以寻找能够在44°C下存活和生长的突变体。我们获得了三个耐热突变体，它们均源自先前适应41 - 42°C的祖代。在早期的一项研究中，我们偶然发现了另一个源自先前适应32°C种群的耐热突变体。因此，在升高但非致死温度下的先前选择可能使生物体更容易进化出更极端的耐热性，但这并非绝对必要条件。显然有可能获得耐受更极端温度的突变体，那么为什么它们在先前41 - 42°C（接近热生态位上限）的选择过程中没有变得普遍呢？为了解决这个问题，我们测量了耐热突变体在刚好处于祖先生态位内的高温下的适应性。这四个耐热突变体中没有一个相对于其祖代具有优势，即使在非常接近热生态位上限的情况下也是如此；事实上，所有突变体在41°C左右都表现出明显的适应性损失。因此，在高温但非致死温度下提高竞争适应性的遗传适应与允许耐受其他致死温度的遗传适应是不同的。
