title: Evolution and diversity of Rickettsia bacteria.
abstract: BACKGROUND
  Rickettsia 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 Rickettsia 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 Rickettsia from arthropods, and constructed a multi-gene phylogeny of the entire genus which includes these new strains.
  
  
  RESULTS
  We show that Rickettsia are primarily arthropod-associated bacteria, and identify several novel groups within the genus. Rickettsia do not co-speciate with their hosts but host shifts most often occur between related arthropods. Rickettsia have evolved adaptations including transmission through vertebrates and killing males in some arthropod hosts. We uncovered one case of horizontal gene transfer among Rickettsia, 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.
  
  
  CONCLUSIONS
  Approximately 150 million years ago, Rickettsia 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 Rickettsia 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 Rickettsia will help in the future to elucidate the mechanisms of pathogenicity, transmission and virulence.
authors.affiliation: Institute of Evolutionary Biology, University of Edinburgh, Edinburgh, EH9 3JT, UK. lucy.weinert@ed.ac.uk
authors.forename: Lucy A
authors.initials: LA
authors.lastname: Weinert
authors.forename: John H
authors.initials: JH
authors.lastname: Werren
authors.forename: Alexandre
authors.initials: A
authors.lastname: Aebi
authors.forename: Graham N
authors.initials: GN
authors.lastname: Stone
authors.forename: Francis M
authors.initials: FM
authors.lastname: Jiggins
mesh_terms: D000818:Animals; D001181:Arthropods / Q000382:microbiology; D001483:Base Sequence; D005798:Genes, Bacterial / Q000235:genetics; D014644:Genetic Variation; D008297:Male; D008969:Molecular Sequence Data; D010802:Phylogeny; D011995:Recombination, Genetic; D012281:Rickettsia / Q000235:genetics / Q000502:physiology*; D016415:Sequence Alignment