Data from: Plasmid stability is enhanced by higher-frequency pulses of positive selection

Plasmids accelerate bacterial adaptation by sharing ecologically important traits between lineages. However, explaining plasmid stability in bacterial populations is challenging due to their associated costs. Previous theoretical and experimental studies suggest that pulsed positive selection may explain plasmid stability by favouring gene mobility and promoting compensatory evolution to ameliorate plasmid cost. Here we test how the frequency of pulsed positive selection affected the dynamics of a mercury resistance plasmid, pQBR103, in experimental populations of Pseudomonas fluorescens SBW25. Plasmid dynamics varied according to the frequency of Hg2+ positive selection: in the absence of Hg2+ plasmids declined to low frequency whereas pulses of Hg2+ selection allowed plasmids to sweep to high prevalence. Compensatory evolution to ameliorate the cost of plasmid carriage was widespread across the entire range of Hg2+ selection regimes, including both constant and pulsed Hg2+ selection. Consistent with theoretical predictions, gene mobility via conjugation appeared to play a greater role in promoting plasmid stability under low-frequency pulses of Hg2+ selection. However, upon removal of Hg2+ selection, plasmids which had evolved under low-frequency pulse selective regimes declined over time. Our findings suggest that temporally variable selection environments, such as those created during antibiotic treatments, may help to explain the stability of mobile plasmid-encoded resistance.

质粒通过在不同谱系间共享具有生态重要性的性状，加速细菌的适应性进化。然而，由于质粒会带来适应性代价，解释细菌种群中质粒的稳定性极具挑战。此前的理论与实验研究表明，脉冲式正选择或许可以解释质粒的稳定性：这类选择会青睐基因移动性，并推动补偿进化以减轻质粒携带的适应性代价。本研究针对脉冲式正选择的频率如何影响抗汞质粒pQBR103在荧光假单胞菌（Pseudomonas fluorescens）SBW25实验种群中的动态变化展开了测试。质粒动态随汞离子（Hg2+）正选择的频率发生变化：在无汞离子选择的条件下，质粒的频率会降至极低水平；而汞离子选择的脉冲处理则可使质粒扩散至高频率。针对质粒携带代价的补偿进化在所有汞离子选择模式下均广泛存在，包括恒定汞离子选择与脉冲式汞离子选择。与理论预测一致，通过接合作用实现的基因移动性，在低频汞离子选择脉冲条件下对促进质粒稳定性的作用更为显著。然而，当移除汞离子选择压力后，在低频脉冲选择环境下进化出的质粒会随时间推移逐渐丢失。本研究结果表明，时间可变的选择环境（比如抗生素治疗过程中形成的选择环境）或可解释可移动质粒编码的抗性性状的稳定性。