View source for Biology:Epixenosomes

{{Short description|Genus of bacteria}}
{{Automatic taxobox
|image = TEM - Epixenosomes.jpg
|image_caption = [[Engineering:Transmission electron microscopy|Transmission electron micrograph]] of stage II epixenosomes.
|image_alt=Transmission electron micrograph of stage II epixenosomes.
|taxon = Epixenosoma
|authority = Bauer Aet al. 2005
| type_species = "''[[Biology:Candidatus|Ca.]]'' Epixenosoma ejectans"
| type_species_authority = Bauer Aet al. 2005
|subdivision_ranks = Species
|subdivision=
* "''[[Biology:Candidatus|Ca.]]'' Epixenosoma ejectans"
}}

'''Epixenosomes''', also known as '''[[Biology:Candidatus|Candidatus]] ''Epixenosoma''''', are a genus of [[Biology:Bacteria|bacteria]] in the phylum [[Biology:Verrucomicrobiota|Verrucomicrobiota]] that form a [[Earth:Symbiosis|symbiosis]] with marine ciliates of the genus ''[[Biology:Euplotidium|Euplotidium]]'', where they help to defend their ciliate hosts against predators.  It is a [[Biology:Monotypic taxon|monospecific]] genus, containing only the species '''Ca. ''Epixenosoma ejectans'''''.

==Description==
Epixenosomes possess two distinct developmental phases linked to the host cell cycle. Stage I epixenosomes are spherical, 0.5 μm in diameter and are surrounded by two [[Biology:Cell membrane|membranes]]. They divide by direct binary [[Biology:Fission|fission]]. Stage II epixenosomes are egg-shaped (2.2 μm in length and 1 μm in width) and show complex organization with different [[Biology:Cytoplasm|cytoplasm]]ic compartments, more complicated than the majority of prokaryotic organisms. They contain an extrusive apparatus within a [[Biology:Protein|protein]]aceous matrix, although apparently not membrane-bound, which differs from the remaining [[Biology:Cytoplasm|cytoplasm]]. A functional cell compartmentalization has also been evidenced.<ref name=":0">{{Cite journal|last=Rosati|first=G.|date=1999|title=Epixenosomes: symbionts of the hypotrich ciliate ''Euplotidium itoi''|journal=Symbiosis|volume=26|pages=1–23}}</ref>

Their phylogenetic position was originally unclear, because they appeared to have both prokaryote-like traits, such as binary fission, and [[Biology:Eukaryote|eukaryote]]-like traits, such as intracellular membranes.<ref name=":0" /> However, [[Biology:Molecular phylogenetics|molecular phylogenetics]] showed that they are actually bacteria from the phylum [[Biology:Verrucomicrobiota|Verrucomicrobiota]].<ref>{{Cite journal|last1=Petroni|first1=Giulio|last2=Spring|first2=Stefan|last3=Schleifer|first3=Karl-Heinz|last4=Verni|first4=Franco|last5=Rosati|first5=Giovanna|date=2000-02-15|title=Defensive extrusive ectosymbionts of ''Euplotidium'' (Ciliophora) that contain microtubule-like structures are bacteria related to Verrucomicrobia|journal=Proceedings of the National Academy of Sciences|language=en|volume=97|issue=4|pages=1813–1817|doi=10.1073/pnas.030438197|issn=0027-8424|pmid=10660683|pmc=26518|bibcode=2000PNAS...97.1813P|doi-access=free}}</ref>

==Defensive symbiosis with ''Euplotidium''==
Epixenosomes live on the dorsal surface of their hosts, marine ciliates in the genus ''Euplotidium''. The name "epixenosomes" comes from the ancient Greek {{lang|el|επι ξενον σομα}}, meaning "external alien body", referring to their extracellular position on the host. The extrusive apparatus ejects its contents when triggered; this process helps to defend the ciliate host against [[Earth:Predation|predators]]. Although ''Euplotidium'' can grow and reproduce without epixenosomes, those with epixenosomes have much higher survival when exposed to predators such as the ciliate ''Litonotus''.<ref>{{Cite journal|last1=Rosati|first1=Giovanna|last2=Petroni|first2=Giulio|last3=Quochi|first3=Silvia|last4=Modeo|first4=Letizia|last5=Verni|first5=Franco|date=1999-05-01|title=Epixenosomes: Peculiar Epibionts of the Hypotrich Ciliate Euplotidium Itoi Defend Their Host Against Predators|journal=Journal of Eukaryotic Microbiology|language=en|volume=46|issue=3|pages=278–282|doi=10.1111/j.1550-7408.1999.tb05125.x|s2cid=85293508|issn=1550-7408}}</ref>

External signals of unknown origin are detected by the membrane receptors located at the top of the organism. The consequent activation of the adenylate cyclase-[[Chemistry:Cyclic adenosine monophosphate|cAMP]] system triggers the ejection of the extrusive apparatus, with the formation of a hollow tube, about 40 μm long, terminating in a head mainly consisting of the apical portion of the epixenosome (the region containing [[Biology:DNA|DNA]]).<ref name=":0" />

The extrusive apparatus is surrounded by a basket of tubules. Experiments with antitubulin drugs and [[Biology:Immunocytochemistry|immunocytochemical]] analyses at the optical and electron microscopical level suggested that these tubules consist of [[Biology:Tubulin|tubulin]], which is a eukaryotic [[Biology:Protein|protein]].<ref name=":0" /> Although some free-living Verrucomicrobiota are reported to have tubulin genes,<ref>{{Cite journal|last1=Jenkins|first1=Cheryl|last2=Samudrala|first2=Ram|last3=Anderson|first3=Iain|last4=Hedlund|first4=Brian P.|last5=Petroni|first5=Giulio|last6=Michailova|first6=Natasha|last7=Pinel|first7=Nicolas|last8=Overbeek|first8=Ross|last9=Rosati|first9=Giovanna|date=2002-12-24|title=Genes for the cytoskeletal protein tubulin in the bacterial genus Prosthecobacter|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=99|issue=26|pages=17049–17054|doi=10.1073/pnas.012516899|issn=0027-8424|pmid=12486237|pmc=139267|bibcode=2002PNAS...9917049J|doi-access=free}}</ref> they have not been found to have tubular structures like those of epixenosomes.

==See also==
* [[Biology:R bodies|R bodies]]
* [[Biology:Kappa organism|Kappa organism]]
* [[Biology:Extrusome|Extrusome]] - organelles in some protists that have a similar function as epixenosomes
* [[Biology:List of bacterial orders|List of bacterial orders]]
* [[Biology:List of bacteria genera|List of bacteria genera]]

==References==
{{Reflist|30em}}

{{Bacteria classification|state=collapsed}}
{{Taxonbar|from=Q3730697|from2=Q51536694}}
[[Category:Candidatus taxa]]
[[Category:Cell biology]]
[[Category:Symbiosis]]
[[Category:Monotypic bacteria genera]]

{{Sourceattribution|Epixenosomes}}