The Rickettsiales, informally called rickettsias, are an order of small

Alphaproteobacteria Alphaproteobacteria is a class of bacteria in the phylum Pseudomonadota (formerly Proteobacteria). The Magnetococcales and Mariprofundales are considered basal or sister to the Alphaproteobacteria. The Alphaproteobacteria are highly diverse and ...

. They are obligate intracellular parasites, and some are notable pathogens, including ''

Rickettsia ''Rickettsia'' is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that may occur in the forms of cocci (0.1 μm in diameter), bacilli (1–4 μm long), or threads (up to about 10 μm long). The term "ricke ...

'', which causes a variety of diseases in humans, and '' Ehrlichia'', which causes diseases in livestock. Another genus of well-known Rickettsiales is the ''

Wolbachia ''Wolbachia'' is a genus of intracellular bacteria that infects mainly arthropod species, including a high proportion of insects, and also some nematodes. It is one of the most common parasitic microbes, and is possibly the most common reproducti ...

'', which infect about two-thirds of all arthropods and nearly all filarial nematodes.

Gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

tic studies support the

endosymbiotic theory Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possib ...

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 used ...

and related

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

s developed from members of this group.Thomas S. (2016). Rickettsiales:Biology, Molecular Biology, Epidemiology, and Vaccine Development. pp.529. Springer• 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 "''Candidatus'' Midichloriaceae" is a family of bacteria, included in the order Rickettsiales. No member of this family has been cultured in the laboratory, so the entire family has been given the status ''candidatus In prokaryote nomenclature, ...

, and the

Ehrlichiaceae The Ehrlichiaceae are a family of bacteria, included in the order Rickettsiales The Rickettsiales, informally called rickettsias, are an order of small Alphaproteobacteria. They are obligate intracellular parasites, and some are notable path ...

. Most studies also support the inclusion of the

Holosporaceae The Holosporaceae are a family of bacteria. The member ''Holospora'' is an intracellular parasite found in the unicellular protozoa ''Paramecium '' ''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic, unicellular cil ...

, 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 The ''Hyphomicrobiales'' are an order of Gram-negative Alphaproteobacteria. The rhizobia, which fix nitrogen and are symbiotic with plant roots, appear in several different families. The four families ''Nitrobacteraceae'', ''Hyphomicrobiaceae' ...

, Rhodobacterales and

Caulobacterales ''Caulobacterales'' is an order of gram-negative pseudomonadota Pseudomonadota (synonym Proteobacteria) is a major phylum of Gram-negative bacteria. The renaming of phyla in 2021 remains controversial among microbiologists, many of whom contin ...

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 The proto-mitochondrion is the hypothetical ancestral bacterial endosymbiont from which all mitochondria in eukaryotes are thought to descend, after an episode of symbiogenesis which created the aerobic eukaryotes. Phylogeny The phylogenetic ...

(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 allelic drift or the Wright effect, is the change in the frequency of an existing gene variant (allele) in a population due to random chance. Genetic drift may cause gene variants to disappear completely and there ...

and

Muller's ratchet In evolutionary genetics, Muller's ratchet (named after Hermann Joseph Muller, by analogy with a ratchet effect) is a process through which, in the absence of recombination (especially in an asexual population), an accumulation of irreversible d ...

. 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 unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). H ...

(the eukaryotic host cell is a natural barrier).

References

Alphaproteobacteria {{alphaproteobacteria-stub