Plasmids play a key role in the ecology and evolution of bacterial populations as they frequently carry genes conferring traits such as antibiotic resistance and pathogenesis. The conjugative systems that they often encode allow for their horizontal transmission to other bacterial cells. Conjugation of the A. tumefaciens‘ virulence plasmid is tightly regulated such that it only occurs when a quorum sensing system is activated in the presence of an infected host. We are working to understand the adaptive significance of this gene regulation.