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23-531: 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

46-466: A convenient "artificial key" according to his Systema Sexuale , largely based on the arrangement of flowers. In botany, classes are now rarely discussed. Since the first publication of the APG system in 1998, which proposed a taxonomy of the flowering plants up to the level of orders, many sources have preferred to treat ranks higher than orders as informal clades . Where formal ranks have been assigned,

69-458: A case of convergent evolution that would result in an artefactual clustering. However, several studies disagree. Furthermore, it has been found that the GC-content of ribosomal RNA (the traditional phylogenetic marker for prokaryotes) little reflects the GC-content of the genome. One example of this atypical decorrelation of ribosomal GC-content with phylogeny is that members of

92-536: A few studies have been reported on natural genetic transformation in the Alphaproteobacteria , this process has been described in Agrobacterium tumefaciens , Methylobacterium organophilum , and Bradyrhizobium japonicum . Natural genetic transformation is a sexual process involving DNA transfer from one bacterial cell to another through the intervening medium, and the integration of

115-412: A general definition of a class is available, it has historically been conceived as embracing taxa that combine a distinct grade of organization—i.e. a 'level of complexity', measured in terms of how differentiated their organ systems are into distinct regions or sub-organs—with a distinct type of construction, which is to say a particular layout of organ systems. This said, the composition of each class

138-482: 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 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

161-789: 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). Rickettsiales genomes are undergoing reductive evolution and are typically small (generally < 1,5 Mbp), AT-rich (generally < 40% GC) with

184-767: A widely distributed and may constitute over 10% of the open ocean microbial community. There is some disagreement on the phylogeny of the orders , especially for the location of the Pelagibacterales , but overall there is some consensus. The discord stems from the large difference in gene content ( e.g. genome streamlining in Pelagibacter ubique ) and the large difference in GC-content between members of several orders. Specifically, Pelagibacterales , Rickettsiales and Holosporales contain species with AT-rich genomes. It has been argued that it could be

207-571: Is based on whole-genome analysis. Subclass names are based on Ferla et al . (2013).   Magnetococcales   Mariprofundales   Rickettsiales (including mitochondria )  " Pelagibacterales "   Sphingomonadales   Rhodospirillales   Rhodothalassiales   Iodidimonadales   Kordiimonadales   Emcibacterales   Sneathiellales   Hyphomicrobiales   Rhodobacterales   Micropepsales  " Parvularculales "   Caulobacterales Spirochaetota Although only

230-542: Is composed by a large diversity of magnetotactic bacteria , but only one is described, Magnetococcus marinus . The Rickettsidae is composed of the intracellular Rickettsiales and the free-living Pelagibacterales . The Caulobacteridae is composed of the Holosporales , Rhodospirillales , Sphingomonadales , Rhodobacterales , Caulobacterales , Kiloniellales , Kordiimonadales , Parvularculales and Sneathiellales . Comparative analyses of

253-431: Is ultimately determined by the subjective judgment of taxonomists . In the first edition of his Systema Naturae (1735), Carl Linnaeus divided all three of his kingdoms of nature ( minerals , plants , and animals ) into classes. Only in the animal kingdom are Linnaeus's classes similar to the classes used today; his classes and orders of plants were never intended to represent natural groups, but rather to provide

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276-425: The Holosporales have a much higher ribosomal GC-content than members of the Pelagibacterales and Rickettsiales , even though they are more closely related to species with high genomic GC-contents than to members of the latter two orders. The Class Alphaproteobacteria is divided into three subclasses Magnetococcidae , Rickettsidae and Caulobacteridae . The basal group is Magnetococcidae , which

299-646: The Alphaproteobacteria . The Alphaproteobacteria are highly diverse and possess few commonalities, but nevertheless share a common ancestor. Like all Proteobacteria , its members are gram-negative , although some of its intracellular parasitic members lack peptidoglycan and are consequently gram variable. The Alphaproteobacteria are a diverse taxon and comprise several phototrophic genera, several genera metabolising C1-compounds ( e.g. , Methylobacterium spp.), symbionts of plants ( e.g. , Rhizobium spp.), endosymbionts of arthropods ( Wolbachia ) and intracellular pathogens ( e.g. Rickettsia ). Moreover,

322-1116: 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.   Magnetococcus marinus   Holosporales   Hyphomicrobiales , Rhodobacteraceae ,   Rhodospirillales , Sphingomonadales , etc .   Pelagibacter   Subgroups Ib, II, IIIa, IIIb, IV and V   Proto-mitochondria   Neorickettsia   Wolbachia   Anaplasma   Ehrlichia   Midichloria   Orientia   Rickettsia The phylogenetic relationship between these two groups has yet to reach consensus in

345-597: The endosymbiotic theory according to which mitochondria and related organelles developed from members of this group. The Rickettsiales are difficult to culture, as they rely on living eukaryotic host cells for their survival. The Rickettsiales further consist of three known families, the Rickettsiaceae , the Midichloriaceae , and the Ehrlichiaceae . Most studies also support the inclusion of

368-613: The sequenced genomes have also led to discovery of many conserved insertion-deletions (indels) in widely distributed proteins and whole proteins (i.e. signature proteins ) that are distinctive characteristics of either all Alphaproteobacteria , or their different main orders (viz. Rhizobiales , Rhodobacterales , Rhodospirillales , Rickettsiales , Sphingomonadales and Caulobacterales ) and families (viz. Rickettsiaceae , Anaplasmataceae , Rhodospirillaceae , Acetobacteraceae , Bradyrhiozobiaceae , Brucellaceae and Bartonellaceae ). These molecular signatures provide novel means for

391-482: The circumscription of these taxonomic groups and for identification/assignment of new species into these groups. Phylogenetic analyses and conserved indels in large numbers of other proteins provide evidence that Alphaproteobacteria have branched off later than most other phyla and classes of Bacteria except Betaproteobacteria and Gammaproteobacteria . The phylogeny of Alphaproteobacteria has constantly been revisited and updated. There are some debates for

414-512: The class is sister to the protomitochondrion , the bacterium that was engulfed by the eukaryotic ancestor and gave rise to the mitochondria , which are organelles in eukaryotic cells (See endosymbiotic theory ). A species of technological interest is Rhizobium radiobacter (formerly Agrobacterium tumefaciens ): scientists often use this species to transfer foreign DNA into plant genomes. Aerobic anoxygenic phototrophic bacteria , such as Pelagibacter ubique , are alphaproteobacteria that are

437-428: The donor sequence into the recipient genome by homologous recombination . Class (biology) The class as a distinct rank of biological classification having its own distinctive name – and not just called a top-level genus (genus summum) – was first introduced by French botanist Joseph Pitton de Tournefort in the classification of plants that appeared in his Eléments de botanique of 1694. Insofar as

460-413: The inclusion of Magnetococcidae in Alphaproteobacteria . For example, an independent proteobacterial class (" Candidatus Etaproteobacteria") for Magnetococcidae has been proposed. A recent phylogenomic study suggests the placement of the protomitochondrial clade between Magnetococcidae and all other alphaproteobacterial taxa, which suggests an early divergence of the protomitochondrial lineage from

483-523: The lack of recombination and horizontal gene transfer (the eukaryotic host cell is a natural barrier). This Alphaproteobacteria -related article is a stub . You can help Misplaced Pages by expanding it . Alphaproteobacteria Alphaproteobacteria or α-proteobacteria , also called α-Purple bacteria in earlier literature, is a class of bacteria in the phylum Pseudomonadota (formerly "Proteobacteria"). The Magnetococcales and Mariprofundales are considered basal or sister to

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506-577: The rest of alphaproteobacteria, except for Magnetococcidae . This phylogeny also suggests that the protomitochondrial lineage does not necessarily have a close relationship to Rickettsidae . The following taxa have been assigned to the Alphaproteobacteria , but have not been assigned to one or more intervening taxonomic ranks: The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN). The phylogeny

529-477: 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

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