Data from: Adaptation of Escherichia coli to glucose promotes evolvability in lactose

The selective history of a population can influence its subsequent evolution, an effect known as historical contingency. We previously observed that five of six replicate populations that were evolved in a glucose-limited environment for 2,000 generations, then switched to lactose for 1,000 generations, had higher fitness increases in lactose than populations started directly from the ancestor. To test if selection in glucose systematically increased lactose evolvability, we started 12 replay populations—six from a population subsample and six from a single randomly selected clone—from each of the six glucose-evolved founder populations. These replay populations and 18 ancestral populations were evolved for 1,000 generations in a lactose-limited environment. We found that replay populations were initially slightly less fit in lactose than the ancestor, but were more evolvable, in that they increased in fitness at a faster rate and to higher levels. This result indicates that evolution in the glucose environment resulted in genetic changes that increased the potential of genotypes to adapt to lactose. Genome sequencing identified four genes—iclR, nadR, spoT and rbs—that were mutated in most glucose-evolved clones and are candidates for mediating increased evolvability. Our results demonstrate short-term selective costs during selection in one environment can lead to changes in evolvability that confer longer-term benefits.

种群的选择历史可影响其后续进化，这一效应被称为历史偶然性（historical contingency）。我们此前观察到，6个在葡萄糖限制环境中传代2000代的重复种群，随后转接至乳糖限制环境中传代1000代后，其在乳糖环境中的适合度提升幅度高于直接由祖先种群起始的种群。为检验葡萄糖环境中的选择是否系统性提升了种群的乳糖可进化性（evolvability），我们从6个经葡萄糖环境进化的奠基者种群中各构建了12个重演种群——其中6个来自种群亚样本，另外6个来自单株随机选取的克隆菌株。将这些重演种群与18个祖先种群一同置于乳糖限制环境中传代1000代。我们发现，重演种群在初始阶段的乳糖环境适合度略低于祖先种群，但具备更高的可进化性：其适合度提升速率更快，最终达到的适合度水平也更高。这一结果表明，葡萄糖环境中的进化过程引发了遗传改变，提升了基因型适应乳糖环境的潜能。基因组测序鉴定出4个基因——iclR、nadR、spoT与rbs——在多数经葡萄糖环境进化的克隆菌株中发生了突变，这些基因是介导可进化性提升的候选基因。我们的研究结果表明，在某一环境中选择时产生的短期选择代价，可引发可进化性的改变，进而赋予种群长期的适应优势。