Rickettsiales


The Rickettsiales, informally called rickettsias, are an order of small Alphaproteobacteria that are endosymbionts of eukaryotic cells. 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.
The Rickettsiales are difficult to cultivate, because they rely on eukaryotic host cells for their survival.

Rickettsiales phylogeny

The Rickettsiales further consist of three known families, the Rickettsiaceae, the Midichloriaceae, and the Anaplasmataceae. Most studies also support the inclusion of the Holosporaceae, but one study has challenged this view. Here, 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 Rhizobiales, 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.

Reductive evolution

Rickettsiales genomes are undergoing reductive evolution and are typically small, AT-rich with a low coding density and a relatively high number of pseudogenes. Reduction in genome size, % GC and coding density and genes are generally attributed to genetic drift and Muller's ratchet. Genetic drift is enhanced in Rickettsiales genomes due to low population sizes and frequent population bottlenecks. Similarly, Muller's ratchet is activated through the lack of recombination and horizontal gene transfer.