Contrasting effects of historical contingency on phenotypic and genomic trajectories during a two-step evolution experiment with bacteria. historical contingency in Escherichia coli

Background: The impact of historical contingency, i.e. the past evolutionary history of a population, on further adaptation is mostly unknown at both the phenotypic and genomic levels. We addressed this question using a two-step evolution experiment. First, replicate populations of Escherichia coli were propagated in four different environmental conditions for 1000 generations. Then, all replicate populations were transferred and propagated for further 1000 generations to a single new environment. Results: Using this two-step experimental evolution strategy, we investigated, at both the phenotypic and genomic levels, whether and how adaptation in the initial historical environments impacted evolutionary trajectories in a new environment. We showed that both the growth rate and fitness of the evolved populations obtained after the second step of evolution were contingent upon past evolutionary history. In contrast however, the genes that were modified during the second step of evolution were independent from the previous history of the populations. Conclusions: Our work suggests that historical contingency affects phenotypic adaptation to a new environment. This was however not reflected at the genomic level implying complex relationships between environmental factors and the genotype-to- phenotype map.

研究背景：历史偶然性（historical contingency），即种群的过往进化历史，对后续适应性演化的影响，在表型（phenotypic）与基因组（genomic）层面大多仍未明确。本研究采用两步演化实验策略解答该问题：首先将大肠杆菌（Escherichia coli）的重复种群置于四种不同环境条件中继代培养1000代；随后将所有重复种群转接至单一全新环境中，继续传代培养1000代。
研究结果：借助该两步实验演化策略，我们从表型与基因组层面探究了初始历史环境中的适应性演化是否以及如何影响种群在全新环境中的演化轨迹。研究发现，经第二步演化后获得的进化种群，其生长速率与适合度（fitness）均受过往演化历史的制约。但与之相对，第二步演化过程中发生修饰的基因，则与种群的既往演化历史无关。
研究结论：本研究结果表明，历史偶然性会影响种群对全新环境的表型适应性，但该效应并未在基因组层面体现，这暗示环境因子与基因型-表型映射（genotype-to-phenotype map）之间存在复杂的关联机制。