Rickettsiales
The Rickettsiales, informally called rickettsias, are an order of small Alphaproteobacteria. They are obligate intracellular parasites, and some are notable pathogens, including Rickettsia, which causes a variety of diseases in humans, and Ehrlichia, which causes diseases in livestock. Another genus of well-known Rickettsiales is the Wolbachia, which infect about two-thirds of all arthropods and nearly all filarial nematodes. Genetic studies support the endosymbiotic theory according to which mitochondria and related organelles developed from members of this group. However, more recent examination has suggested that the mitochondria and Rickettsiales diverged independantly.
The Rickettsiales are difficult to culture, as they rely on living eukaryotic host cells for their survival.
Rickettsiales phylogeny
The Rickettsiales further consist of three known families, the Rickettsiaceae, the Midichloriaceae, and the Ehrlichiaceae. Most studies also support the inclusion of the Holosporaceae, but one study has challenged this view. In that alternative, the Holosporaceae are the sole representatives of their own order, the Holosporales, and as such not part of the Rickettsiales. Other lineages, not clearly part of any family, have been described, as well. Examples include Candidatus Arcanobacter lacustris and Rickettsiales bacterium Ac37b.Phylogenetic relationship between Rickettsiales and Pelagibacterales (SAR11)
The phylogenetic relationship between these two groups has yet to reach consensus in the scientific literature.Early reports suggested that they represented sister clades to each other. However, later studies suggested that this relationship is false and was due to a phylogenetic artefact, which artificially groups independent AT-rich and fast-evolving lineages together. Upon correcting for this artefact, the Pelagibacterales form a sister clade to the Hyphomicrobiales, Rhodobacterales and Caulobacterales instead.
Another study adheres to the sister relationship between the two clades. In their classification, the relation between the two orders is retained in the subclass, the Rickettsidae, which include the Rickettsiales, the Pelagibacteriales, and the extinct protomitochondrion.