Plasmids have a key role in bacterial ecology and evolution because they mobilize accessory genes by horizontal gene transfer. However, recent studies have revealed that the evolutionary impact of plasmids goes above and beyond their being mere gene delivery platforms. Plasmids are usually kept at multiple copies per cell, producing islands of polyploidy in the bacterial genome. As a consequence, the evolution of plasmid-encoded genes is governed by a set of rules different from those affecting chromosomal genes, and these rules are shaped by unusual concepts in bacterial genetics, such as genetic dominance, heteroplasmy or segregational drift. In this Review, we discuss recent advances that underscore the importance of plasmids in bacterial ecology and evolution beyond horizontal gene transfer. We focus on new evidence that suggests that plasmids might accelerate bacterial evolution, mainly by promoting the evolution of plasmid-encoded genes, but also by enhancing the adaptation of their host chromosome. Finally, we integrate the most relevant theoretical and empirical studies providing a global understanding of the forces that govern plasmid-mediated evolution in bacteria.

质粒在细菌生态学和进化中具有关键作用，因为它们通过水平基因转移来调动辅助基因。然而，最近的研究表明，质粒的进化影响超出了它们仅仅是基因传递平台的范围。质粒通常以每个细胞的多个拷贝保存，在细菌基因组中产生多倍体岛。因此，质粒编码基因的进化受一组不同于影响染色体基因的规则的支配，这些规则是由细菌遗传学中不寻常的概念塑造的，例如遗传优势、异质性或分离漂移。在这篇综述中，我们讨论了强调质粒在细菌生态学和进化中超越水平基因转移的重要性的最新进展。我们专注于表明质粒可能加速细菌进化的新证据，主要是通过促进质粒编码基因的进化，但也通过增强宿主染色体的适应性。最后，我们整合了最相关的理论和实证研究，提供了对控制质粒介导的细菌进化的力量的全球理解。