Evolution of Plasmid Mobility: Origin and Fate of Conjugative and Nonconjugative Plasmids

Conjugation drives the horizontal transfer of adaptive traits across prokaryotes. One-fourth of the plasmids encode the functions necessary to conjugate autonomously, the others being eventually mobilizable by conjugation. To understand the evolution of plasmid mobility, we studied plasmid size, gene repertoires, and conjugation-related genes. Plasmid gene repertoires were found to vary rapidly in relation to the evolutionary rate of relaxases, for example, most pairs of plasmids with 95% identical relaxases have fewer than 50% of homologs. Among 249 recent transitions of mobility type, we observed a clear excess of plasmids losing the capacity to conjugate. These transitions are associated with even greater changes in gene repertoires, possibly mediated by transposable elements, including pseudogenization of the conjugation locus, exchange of replicases reducing the problem of incompatibility, and extensive loss of other genes. At the microevolutionary scale of plasmid taxonomy, transitions of mobility type sometimes result in the creation of novel taxonomic units. Interestingly, most transitions from conjugative to mobilizable plasmids seem to be lost in the long term. This suggests a source-sink dynamic, where conjugative plasmids generate nonconjugative plasmids that tend to be poorly adapted and are frequently lost. Still, in some cases, these relaxases seem to have evolved to become efficient at plasmid mobilization in trans, possibly by hijacking multiple conjugative systems. This resulted in specialized relaxases of mobilizable plasmids. In conclusion, the evolution of plasmid mobility is frequent, shapes the patterns of gene flow in bacteria, the dynamics of gene repertoires, and the ecology of plasmids.