Epixenosomes, also known as

Candidatus In prokaryote nomenclature, ''Candidatus'' (Latin for candidate of Roman office) is used to name prokaryotic phyla that are well characterized but yet-uncultured. Contemporary sequencing approaches, such as 16S sequencing or metagenomics, provide m ...

''Epixenosoma'' are a genus of

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

in the phylum

Verrucomicrobiota Verrucomicrobiota is a phylum of Gram-negative bacteria that contains only a few described species. The species identified have been isolated from fresh water, marine and soil environments and human faeces. A number of as-yet uncultivated speci ...

that form a symbiosis with marine

ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a differen ...

s of the genus '' Euplotidium'', where they help to defend their ciliate hosts against predators. It is a

monospecific In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispec ...

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 membranes. They divide by direct binary fission. Stage II epixenosomes are egg-shaped (2.2 μm in length and 1 μm in width) and show complex organization with different

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...

ic compartments, more complicated than the majority of

prokaryotic A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...

organisms. They contain an extrusive apparatus within a

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

aceous matrix, although apparently not membrane-bound, which differs from the remaining

. A functional cell compartmentalization has also been evidenced. Their

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

position was originally unclear, because they appeared to have both prokaryote-like traits, such as binary fission, and eukaryote-like traits, such as intracellular membranes. However,

molecular phylogenetics Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...

showed that they are actually bacteria from the phylum

Defensive symbiosis with ''Euplotidium''

Epixenosomes live on the

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal c ...

surface of their hosts, marine ciliates in the genus ''Euplotidium''. The name "epixenosomes" comes from the ancient Greek , 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

predators Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...

. Although ''Euplotidium'' can grow and reproduce without epixenosomes, those with epixenosomes have much higher survival when exposed to predators such as the ciliate '' Litonotus''. 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 Adenylate cyclase (EC 4.6.1.1, also commonly known as adenyl cyclase and adenylyl cyclase, abbreviated AC) is an enzyme with systematic name ATP diphosphate-lyase (cyclizing; 3′,5′-cyclic-AMP-forming). It catalyzes the following reaction: :A ...

cAMP Camp may refer to: Outdoor accommodation and recreation * Campsite or campground, a recreational outdoor sleeping and eating site * a temporary settlement for nomads * Camp, a term used in New England, Northern Ontario and New Brunswick to descri ...

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 DNA). The extrusive apparatus is surrounded by a basket of tubules. Experiments with antitubulin drugs and immunocytochemical analyses at the optical and electron microscopical level suggested that these tubules consist of

tubulin Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular proteins, or one of the member proteins of that superfamily. α- and β-tubulins polymerize into microtubules, a major component of the eukaryotic cytoske ...

, which is a

eukaryotic Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...

. Although some free-living Verrucomicrobiota are reported to have tubulin genes, they have not been found to have tubular structures like those of epixenosomes.

References

{{Taxonbar, from=Q3730697, from2=Q51536694 Candidatus taxa Verrucomicrobiota Cell biology Symbiosis Monotypic bacteria genera

Description

Defensive symbiosis with ''Euplotidium''

See also

References