Figure 3.

Conditions that favour plasmid mobility. (a) Plant hosts requiring different sets of symbiosis genes can exist in sympatry (e.g. clover and vetch nodulated by R. leguminosarum sv. trifolii and viciae, respectively). Plants may act as hotspots for selection on different sym genes with areas of no or low selection in between. (b) The same sym element can have different fitness/symbiotic qualities across different bacterial genotypes and in the presence of different co-infecting plasmids. Plasmid transfer therefore creates diversity of symbiotic function and plasmid fitness. (c) Co-infection of different sym elements may drive selection for mobility. Co-infecting sym elements could displace the existing sym element, explaining the lack of dual-sym plasmids. (Online version in colour.)