Rapid compensatory evolution promotes the survival of conjugative plasmids.

Conjugative plasmids play a vital role in bacterial adaptation through horizontal gene transfer. Explaining how plasmids persist in host populations however is difficult, given the high costs often associated with plasmid carriage. Compensatory evolution to ameliorate this cost can rescue plasmids from extinction. In a recently published study we showed that compensatory evolution repeatedly targeted the same bacterial regulatory system, GacA/GacS, in populations of plasmid-carrying bacteria evolving across a range of selective environments. Mutations in these genes arose rapidly and completely eliminated the cost of plasmid carriage. Here we extend our analysis using an individual based model to explore the dynamics of compensatory evolution in this system. We show that mutations which ameliorate the cost of plasmid carriage can prevent both the loss of plasmids from the population and the fixation of accessory traits on the bacterial chromosome. We discuss how dependent the outcome of compensatory evolution is on the strength and availability of such mutations and the rate at which beneficial accessory traits integrate on the host chromosome.

接合质粒通过水平基因转移在细菌适应过程中起着至关重要的作用。然而，鉴于携带质粒往往伴随着高昂的代价，解释质粒如何在宿主群体中持续存在是困难的。补偿性进化以减轻这种代价，可以使质粒免于灭绝。在最近发表的一项研究中，我们表明，在一系列选择环境中进化的携带质粒的细菌群体中，补偿性进化反复针对相同的细菌调控系统GacA/GacS。这些基因中的突变迅速出现，并完全消除了携带质粒的代价。在此，我们使用基于个体的模型扩展我们的分析，以探索该系统中补偿性进化的动态。我们表明，减轻质粒携带代价的突变既可以防止质粒从群体中丢失，也可以防止附属性状在细菌染色体上固定。我们讨论补偿性进化的结果在多大程度上取决于此类突变的强度和可用性以及有益附属性状在宿主染色体上整合的速率。