Plasmid transfer is biased towards close kin in bacteria from natural populations.

Plasmids play a key role in microbial ecology and evolution, yet the determinants of plasmid transfer rates are poorly understood. Here we investigate the importance of genetic similarity between naturally co-occurring Escherichia coli isolates in the transfer of two plasmids (narrow-host-range R1 and broad-host-range RP4). We uncovered extensive variability, spanning over five orders of magnitude, in the ability of isolates to donate and receive plasmids. Overall, transfer was strongly biased towards clone-mates, but not correlated to genetic distance between donors and recipients. Transfer was limited by the presence of a functional restriction-modification system in recipients, thus bias towards kin might be explained by sharing of identical restriction systems. Such conjugation within lineages sets the stage for longer-term pair-wise coevolutionary interactions between plasmids and bacterial hosts.