Evolution of satellite plasmids can stabilize the maintenance of newly acquired accessory genes in bacteria

Plasmids play a principal role in the spread of antibiotic resistance and other traits by horizontal gene transfer in bacteria. However, newly acquired plasmids generally impose a fitness burden on a cell, and they are lost from a population rapidly if there is not selection to maintain a unique function encoded on the plasmid. Mutations that ameliorate this fitness cost can sometimes eventually stabilize a plasmid in a new host, but they typically do so by inactivating some of its novel accessory genes. In this study, we identified an additional evolutionary pathway that can prolong the maintenance of newly acquired genes encoded on a plasmid. We discovered that propagation of an RSF1010-based IncQ plasmid in Escherichia coli often generated satellite plasmids with spontaneous deletions of accessory genes and genes required for plasmid replication. These smaller plasmid variants are nonautonomous genetic parasites. Their presence in a cell drives down the copy number of full-length plasmids, which reduces the burden from the accessory genes without eliminating them entirely. The evolution of satellite plasmids may be favored relative to other plasmid fates because they give a more immediate fitness advantage to a cells progeny and because the organization of IncQ plasmids makes them particularly prone to certain deletions during replication. Satellite plasmids also evolved in Snodgrassella alvi colonizing the honey bee gut, suggesting that this mechanism may broadly contribute to the importance of IncQ plasmids as agents of bacterial gene transfer in nature.\n\nSignificance StatementPlasmids are multicopy DNA elements found in bacteria that replicate independently of a cells chromosome. The spread of plasmids carrying antibiotic-resistance genes to new bacterial pathogens is a challenge for treating life-threatening infections. Often plasmids or their accessory genes encoding unique functions are lost soon after transfer into a new cell because they impose a fitness burden. We report that a family of transmissible plasmids can rapidly evolve satellite plasmids that replicate as genetic parasites of the original plasmid. Satellite plasmid formation reduces the burden from the newly acquired genes, which may enable them to survive intact for longer after transfer into a new cell and thereby contribute to the spread of antibiotic resistance and other traits within bacterial populations.