Plasmids may carry genes coding for beneficial traits and thus contribute to adaptation of bacterial populations to environmental stress. Conjugative plasmids can horizontally transfer between cells, which a priori facilitates the spread of adaptive alleles. However, if the potential recipient cell is already colonized by another incompatible plasmid, successful transfer may be prevented. Competition between plasmids can thus limit horizontal transfer. Previous modeling has indeed shown that evolutionary rescue by a conjugative plasmid is hampered by incompatible resident plasmids in the population. If the rescue plasmid is a mutant variant of the resident plasmid, both plasmids transfer at the same rates. A high conjugation rate then has two, potentially opposing, effects – a direct positive effect on spread of the rescue plasmid and an increase in the fraction of resident plasmid cells. This raises the question whether a high conjugation rate always benefits evolutionary rescue. In this article, we systematically analyze three models of increasing complexity to disentangle the benefits and limits of increasing horizontal gene transfer in the presence of plasmid competition and plasmid costs. We find that the net effect can be positive or negative and that the optimal transfer rate is thus not always the highest one. These results can contribute to our understanding of the many facets of plasmid-driven adaptation and the wide range of transfer rates observed in nature.

质粒可能携带编码有益性状的基因，从而有助于细菌种群适应环境压力。接合质粒可以在细胞之间水平转移，这先验地促进了适应性等位基因的传播。然而，如果潜在的受体细胞已经被另一个不相容的质粒定殖，则可能会阻止成功的转移。因此，质粒之间的竞争可以限制水平转移。先前的模型确实表明，接合质粒的进化救援受到群体中不相容的常驻质粒的阻碍。如果救援质粒是驻留质粒的突变变体，则两个质粒以相同的速率转移。高接合率会产生两种可能相反的效果——对救援质粒传播的直接积极影响和驻留质粒细胞比例的增加。这就提出了一个问题：高接合率是否总是有利于进化救援。在本文中，我们系统地分析了三种复杂性不断增加的模型，以阐明在存在质粒竞争和质粒成本的情况下增加水平基因转移的好处和限制。我们发现净效应可能是正的，也可能是负的，因此最佳传输速率并不总是最高的。这些结果有助于我们理解质粒驱动的适应的许多方面以及自然界中观察到的广泛的转移率。