“Rickettsiales”的意思、由来-开放百科全书

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 are the Wolbachia, which infect approximately two-thirds of all arthropods and nearly all filarial nematodes.^[2] Genetic studies support the endosymbiotic theory according to which mitochondria and related organelles developed from members of this group.^[3]

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.^[4] Here, the Holosporaceae are the sole representatives of its 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 ^[4] 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.^[5]^[6] However, later studies suggested that this relationship is false and was due to a phylogenetic artefact, which arteficially groups independent AT-rich and fast-evolving lineages (Rickettsiales and Pelagibacterales have both properties) together.^[7]^[8] Upon correcting for this artefact, the Pelagibacterales form a sister clade to the Rhizobiales, Rhodobacterales and Caulobacterales instead.

Another study ^[4] adheres to the sister relationship between the two clades (see schematic tree). In their classification, the relation between the two order is retained in the subclass, the Rickettsidae, which include the Rickettsiales, the Pelagibacteriales and the extinct protomitochondrion (mitochondria themselves are not bacteria, but organelles).^[9]

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.^[10] 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 the lack of recombination and horizontal gene transfer (the eukaryotic host cell is a natural barrier).

References

1. ^{{cite book | last = Garrity | first = George | title = Bergey's Manual of Systematic Bacteriology | publisher = Springer | year = 2005 | isbn = 978-0-387-24145-6 }}
2. ^{{cite journal |vauthors=Werren JH, Baldo L, Clark ME |title=Wolbachia: master manipulators of invertebrate biology |journal=Nat. Rev. Microbiol. |volume=6 |issue=10 |pages=741–51 |year=2008 |pmid=18794912 |doi=10.1038/nrmicro1969 |url=}}
3. ^Thomas S. (2016). Rickettsiales:Biology, Molecular Biology, Epidemiology, and Vaccine Development. pp.529. Springer•{{ISBN|978-3-319-46857-0}}. ,
4. ^{{cite journal |vauthors=Martijn J, Schulz F, Zaremba-Niedzwiedzka K, et al. |title=Single-cell genomics of a rare environmental alphaproteobacterium provides unique insights into Rickettsiaceae evolution |journal=ISME J |volume=9 |issue=11 |pages=2373–85 |year=2015 |pmid=25848874 |pmc=4497978 |doi=10.1038/ismej.2015.46 |url=}}
5. ^{{cite journal |vauthors=Williams KP, Sobral BW, Dickerman AW |title=A robust species tree for the alphaproteobacteria |journal=J. Bacteriol. |volume=189 |issue=13 |pages=4578–86 |year=2007 |pmid=17483224 |pmc=1913456 |doi=10.1128/JB.00269-07 |url=}}
6. ^{{cite journal |vauthors=Thrash JC, Boyd A, Huggett MJ, et al. |title=Phylogenomic evidence for a common ancestor of mitochondria and the SAR11 clade |journal=Sci Rep |volume=1 |issue= |pages=13 |year=2011 |pmid=22355532 |pmc=3216501 |doi=10.1038/srep00013 |url=|bibcode=2011NatSR...1E..13T }}
7. ^{{cite journal |vauthors=Viklund J, Ettema TJ, Andersson SG |title=Independent genome reduction and phylogenetic reclassification of the oceanic SAR11 clade |journal=Mol. Biol. Evol. |volume=29 |issue=2 |pages=599–615 |year=2012 |pmid=21900598 |doi=10.1093/molbev/msr203 |url=}}
8. ^{{cite journal |vauthors=Rodríguez-Ezpeleta N, Embley TM |title=The SAR11 group of alpha-proteobacteria is not related to the origin of mitochondria |journal=PLoS ONE |volume=7 |issue=1 |pages=e30520 |year=2012 |pmid=22291975 |pmc=3264578 |doi=10.1371/journal.pone.0030520 |url=|bibcode=2012PLoSO...730520R }}
9. ^¹²{{Cite journal | pmid = 24349502 | pmc = 3859672 | year = 2013 | author1 = Ferla | first1 = M. P. | title = New rRNA gene-based phylogenies of the Alphaproteobacteria provide perspective on major groups, mitochondrial ancestry and phylogenetic instability | journal = PLoS ONE | volume = 8 | issue = 12 | pages = e83383 | last2 = Thrash | first2 = J. C. | last3 = Giovannoni | first3 = S. J. | last4 = Patrick | first4 = W. M. | doi = 10.1371/journal.pone.0083383| bibcode = 2013PLoSO...883383F }}
10. ^{{cite journal |vauthors=Darby AC, Cho NH, Fuxelius HH, Westberg J, Andersson SG |title=Intracellular pathogens go extreme: genome evolution in the Rickettsiales |journal=Trends Genet. |volume=23 |issue=10 |pages=511–20 |year=2007 |pmid=17822801 |doi=10.1016/j.tig.2007.08.002 |url=}}