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.

Gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

tic studies support the endosymbiotic theory according to which

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

and related

organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...

s developed from members of this group. 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 (see the schematic tree below). 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 (Rickettsiales and Pelagibacterales have both properties) 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 (see schematic tree). 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 (mitochondria themselves are not bacteria, but organelles).

Reductive evolution

Rickettsiales genomes are undergoing reductive evolution and are typically small (generally < 1.5 Mbp), AT-rich (generally < 40% GC) with a low coding density (generally < 85%) and a relatively high number of pseudogenes. Reduction in genome size, % GC and coding density and genes are generally attributed to

genetic drift Genetic drift, also known as random genetic drift, allelic drift or the Wright effect, is the change in the Allele frequency, frequency of an existing gene variant (allele) in a population due to random chance. Genetic drift may cause gene va ...

and Muller's ratchet. Genetic drift is enhanced in Rickettsiales genomes due to low population sizes (given their endosymbiotic nature) and frequent population bottlenecks. Similarly, Muller's ratchet is activated through the lack of recombination and

horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...

(the eukaryotic host cell is a natural barrier).

References

Alphaproteobacteria {{alphaproteobacteria-stub