Conjugation genes are common throughout the genus Rickettsia and are transmitted horizontallyWeinert, LA, Welch JJ & Jiggins FM
(2009) Proc Roy Soc B 276, 3619-3627.
Rickettsia are endosymbionts of arthropods, some of which are vectored to vertebrates where they cause disease. Recently it has been found that some Rickettsia strains harbour conjugative plasmids and others encode some conjugative machinery within the bacterial genome. We have investigated the distribution of these conjugation genes in a phylogenetically diverse collection of Rickettsia isolated from arthropods. We found that these genes are common throughout the genus, and in stark contrast to other genes in the genome, conjugation genes are frequently horizontally transmitted between strains. In addition, there is no evidence to suggest that these genes are preferentially transferred between phylogenetically related strains, which is surprising given that closely related strains infect similar host species. As well as detecting patterns of horizontal transmission between diverse Rickettsia species, these findings have implications for the evolution of pathogenicity, the evolution of Rickettsia genomes and the genetic transformation of intracellular bacteria.