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. ...

质粒（Plasmids）通过在不同谱系间共享具有重要生态意义的性状，加速细菌的适应性进化。然而，由于质粒携带存在相关代价，解释其在细菌种群中的稳定性颇具挑战。既往的理论与实验研究表明，脉冲式正选择（pulsed positive selection）可通过偏好基因流动性及促进补偿性进化（compensatory evolution）以降低质粒代价，从而解释质粒的稳定性。本研究测试了脉冲式正选择的频率如何影响汞抗性质粒pQBR103在荧光假单胞菌SBW25（Pseudomonas fluorescens SBW25）实验种群中的动态变化。质粒动态随Hg²⁺正选择的频率而异：在无Hg²⁺的条件下，质粒频率降至较低水平；而Hg²⁺选择脉冲则使质粒迅速扩散至高流行率。旨在降低质粒携带代价的补偿性进化在所有Hg²⁺选择制度范围内均广泛存在，包括持续及脉冲式Hg²⁺选择。...