Evolution and diversity of <i>Rickettsia</i> bacteria

<b>Background: </b> <i>Rickettsia</i> are intracellular symbionts of eukaryotes that are best known for infecting and causing serious diseases in humans and other mammals. All known vertebrate-associated Rickettsia are vectored by arthropods as part of their life-cycle, and many other <i>Rickettsia</i> are found exclusively in arthropods with no known secondary host. However, little is known about the biology of these latter strains. Here, we have identified 20 new strains of <i>Rickettsia</i> from arthropods, and constructed a multi-gene phylogeny of the entire genus which includes these new strains.<br> <b>Results: </b> We show that <i>Rickettsia</i> are primarily arthropod-associated bacteria, and identify several novel groups within the genus. <i>Rickettsia</i> do not co-speciate with their hosts but host shifts most often occur between related arthropods. <i>Rickettsia</i> have evolved adaptations including transmission through vertebrates and killing males in some arthropod hosts. We uncovered one case of horizontal gene transfer among <i>Rickettsia</i>, where a strain is a chimera from two distantly related groups, but multi-gene analysis indicates that different parts of the genome tend to share the same phylogeny.<br> <b>Conclusion: </b> Approximately 150 million years ago, <i>Rickettsia</i> split into two main clades, one of which primarily infects arthropods, and the other infects a diverse range of protists, other eukaryotes and arthropods. There was then a rapid radiation about 50 million years ago, which coincided with the evolution of life history adaptations in a few branches of the phylogeny. Even though <i>Rickettsia</i> are thought to be primarily transmitted vertically, host associations are short lived with frequent switching to new host lineages. Recombination throughout the genus is generally uncommon, although there is evidence of horizontal gene transfer. A better understanding of the evolution of <i>Rickettsia</i> will help in the future to elucidate the mechanisms of pathogenicity, transmission and virulence.