Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts, United States of America ; Systems Biology Graduate Program, Harvard University, Cambridge, Massachusetts, United States of America.
Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts, United States of America ; Faculty of Arts and Sciences Center for Systems Biology, Harvard University, Cambridge, Massachusetts, United States of America.
PLoS Biol. 2014 Feb 18;12(2):e1001789. doi: 10.1371/journal.pbio.1001789. eCollection 2014 Feb.
Evolutionary adaptation to a constant environment is often accompanied by specialization and a reduction of fitness in other environments. We assayed the ability of the Lenski Escherichia coli populations to grow on a range of carbon sources after 50,000 generations of adaptation on glucose. Using direct measurements of growth rates, we demonstrated that declines in performance were much less widespread than suggested by previous results from Biolog assays of cellular respiration. Surprisingly, there were many performance increases on a variety of substrates. In addition to the now famous example of citrate, we observed several other novel gains of function for organic acids that the ancestral strain only marginally utilized. Quantitative growth data also showed that strains with a higher mutation rate exhibited significantly more declines, suggesting that most metabolic erosion was driven by mutation accumulation and not by physiological tradeoffs. These reductions in growth by mutator strains were ameliorated by growth at lower temperature, consistent with the hypothesis that this metabolic erosion is largely caused by destabilizing mutations to the associated enzymes. We further hypothesized that reductions in growth rate would be greatest for substrates used most differently from glucose, and we used flux balance analysis to formulate this question quantitatively. To our surprise, we found no significant relationship between decreases in growth and dissimilarity to glucose metabolism. Taken as a whole, these data suggest that in a single resource environment, specialization does not mainly result as an inevitable consequence of adaptive tradeoffs, but rather due to the gradual accumulation of disabling mutations in unused portions of the genome.
进化对恒定环境的适应通常伴随着专业化和在其他环境中适应能力的降低。我们检测了 Lenski 大肠杆菌种群在经过 50000 代葡萄糖适应后，利用一系列碳源生长的能力。通过直接测量生长速率，我们证明与先前 Biolog 细胞呼吸测定的结果相比，性能下降的范围要小得多。令人惊讶的是，在各种基质上有许多性能的提高。除了现在著名的柠檬酸例子外，我们还观察到了其他几种有机酸盐的新功能获得，而原始菌株只能勉强利用这些有机酸盐。定量生长数据还表明，具有更高突变率的菌株表现出明显更多的下降，这表明大多数代谢侵蚀是由突变积累驱动的，而不是由生理权衡驱动的。这些突变菌株的生长减少可以通过在较低温度下生长来缓解，这与代谢侵蚀主要是由相关酶的不稳定突变引起的假设是一致的。我们进一步假设，生长速率的降低对于与葡萄糖代谢最不同的底物最大，我们使用通量平衡分析来定量地提出这个问题。令我们惊讶的是，我们发现生长速率的降低与葡萄糖代谢的不相似性之间没有显著关系。总的来说，这些数据表明，在单一的资源环境中，专业化并不主要是适应性权衡的必然结果，而是由于基因组中未使用部分的失活突变逐渐积累的结果。
