Jakobids are an

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

of free-living,

heterotrophic A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...

, flagellar

eukaryote Eukaryotes () are organisms whose cells have a 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 three domains of life. Bacter ...

s in the supergroup

Excavata Excavata is a major supergroup of unicellular organisms belonging to the domain Eukaryota. It was first suggested by Simpson and Patterson in 1999 and introduced by Thomas Cavalier-Smith in 2002 as a formal taxon. It contains a variety of free- ...

. They are small (less than 15 μm), and can be found in aerobic and anaerobic environments. The order Jakobida, believed to be

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gr ...

, consists of only twenty species at present, and was classified as a group in 1993. There is ongoing research into the mitochondrial genomes of jakobids, which are unusually large and bacteria-like, evidence that jakobids may be important to the evolutionary history of

eukaryotes Eukaryotes () are organisms whose cells have a 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 three domains of life. Bacter ...

. Molecular phylogenetic evidence suggests strongly that jakobids are most closely related to Heterolobosea (Percolozoa) and

Euglenozoa Euglenozoa are a large group of flagellate Discoba. They include a variety of common free-living species, as well as a few important parasites, some of which infect humans. Euglenozoa are represented by three major clades, i.e., Kinetoplastea, D ...

Structure and Biology

Jakobids have two flagella, inserted in the anterior end of the cell, and, like other members of order

, have a ventral feeding groove and associated

cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is co ...

support. The posterior flagella has a dorsal vane and is aligned within the ventral groove, where it generates a current that the cell uses for food intake. The

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: * Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...

is generally in the anterior part of the cell and bears a

nucleolus The nucleolus (, plural: nucleoli ) is the largest structure in the nucleus of eukaryotic cells. It is best known as the site of ribosome biogenesis, which is the synthesis of ribosomes. The nucleolus also participates in the formation of sign ...

. Most known jakobids have one

mitochondrion 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 use ...

, again located anteriorly, and different genera have flattened, tubular, or absent

cristae A crista (; plural cristae) is a fold in the inner membrane of a mitochondrion. The name is from the Latin for ''crest'' or ''plume'', and it gives the inner membrane its characteristic wrinkled shape, providing a large amount of surface area fo ...

. Food

vacuoles A vacuole () is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic mo ...

are mostly located on the cell posterior, and in most jakobids the

endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ...

is distributed throughout the cell. The sessile, loricate Histionidae and occasionally free-swimming '' Jakoba libera'' ( Jakobidae) have extrusomes under the dorsal membrane that are theorized to be defensive structures.

Ecology

Jakobids are widely dispersed, having been found in soil, freshwater, and marine habitats, but generally not common. However,

environmental DNA Environmental DNA or eDNA is DNA that is collected from a variety of environmental samples such as soil, seawater, snow or air, rather than directly sampled from an individual organism. As various organisms interact with the environment, DNA ...

surveys suggest that Stygiellidae are abundant in anoxic marine habitats. Some are capable of surviving hypersaline and anoxic environments, though the Histionids have only been found in freshwater ecosystems, where they attach themselves to algae or zooplankton. Outside of obligate sessile species, many species of jakobids can attach temporarily to surfaces, using either of the two flagella or the cell body itself. All known jakobids are heterotrophic suspension feeders. Their primary prey is generally considered to be bacteria, though one species has been observed eating extremely small (< 1 µm) eukaryotic cells. Jakobids are generally slow swimmers, with low clearance rates relative to similar organisms. No study has suggested jakobids might be pathogenic or toxic.

Mitochondrial DNA

Since jakobids have no current commercial use, most research into jakobids has focused on their evolutionary significance. The

mitochondrial DNA Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mitochondrial D ...

of jakobids is the most bacteria-like of all known eukaryotic mitochondrial DNA, suggesting that jakobid mitochondrial genomes might approximate the ancestral mitochondrial genome. Jakobid mitochondrial DNA is substantially different from most other eukaryotes, especially in terms of the number of genes (nearly 100 in some species) and bacteria-like elements within their genomes. Nine of the genes have never been found in eukaryotic mitochondrial DNA. Uniquely, jakobid mitochondrial genomes code for bacteria-type

RNA polymerase In molecular biology, RNA polymerase (abbreviated RNAP or RNApol), or more specifically DNA-directed/dependent RNA polymerase (DdRP), is an enzyme that synthesizes RNA from a DNA template. Using the enzyme helicase, RNAP locally opens th ...

, as opposed to typical eukaryotic mitochondrial

, referred to as “phage-type”, which appears to be viral in origin. This does not necessarily mean that jakobids are basal to the phylogeny of eukaryotes. While jakobid mitochondria have genetic features that seem to have developed from bacteria, and apparently lack phage-type RNA, it is possible that other eukaryotic clades lost their bacterial features independently. Several proposed possibilities might explain the bacterial features of jakobid mitochondrial DNA. One is that jakobids diverged very early from the rest of the eukaryotes. This hypothesis depends on whether or not jakobids are indeed basal to all living eukaryotes, but there is no evidence yet to support that suggestion. Another hypothesis is that the phage-type RNA polymerase moved from one eukaryote group to another via lateral gene transfer, replacing the bacteria-type enzyme, and simply did not reach the jakobids. This would not depend on jakobids being basal to eukaryotes as a whole, but has not been widely studied. A third possibility is the reverse of the others, suggesting that the phage-type RNA polymerase is the basal one. Under this scenario, jakobids acquired their bacteria-type RNA polymerase much more recently and that then spread via lateral gene transfer. However, the gene arrangement of jakobid mitochondrial DNA suggests an ancestral origin of bacteria-type RNA polymerase over a more-recent divergence. One of the proposed scenarios suggests that the common ancestor of eukaryotes had two mitochondrial RNA polymerases, both phage-type and bacteria-type, and jakobids lost their phage-type polymerase while the rest of the eukaryotes lost the bacteria-type, possibly several times. Such a model eliminates the need for jakobids to be truly basal. One study proposed that the phage-type and bacteria-type polymerases, when present in the same mitochondrion, served different functions, much in the way that the organelles of land plants have two different RNA polymerase enzymes that transcribe different genes.

Taxonomy

Jakobida contains five families consisting of mostly free-swimming genera: Jakobidae, Moramonadidae, Andaluciidae, and Stygiellidae. The sixth family, Histionidae, is largely populated by sessile loricate genera, and includes the first jakobids ever described. Jakobids are a

group, and are most closely related to the

and Heterolobosea. * Class Jakobea Cavalier-Smith 1999 ** Order Jakobida Cavalier-Smith 1993 *** Suborder Ophirinina Yabuki et al. 2018 **** Family Ophirinidae Yabuki et al. 2018 ***** Genus '' Ophirina'' Yabuki et al. 2018 ****** Species '' Ophirina amphinema'' Yabuki et al. 2018 *** Suborder Andaluciina Cavalier-Smith 2013 **** Family Andaluciidae Cavalier-Smith 2013 ***** Genus ''

Andalucia Andalusia (, ; es, Andalucía ) is the southernmost autonomous community in Peninsular Spain. It is the most populous and the second-largest autonomous community in the country. It is officially recognised as a "historical nationality". The ...

'' Lara et al. 2006 ****** Species '' Andalucia godoyi'' Lara et al. 2006 **** Family Stygiellidae Pánek, Táborský & Čepička 2015 ***** Genus '' Velundella'' Pánek, Táborský & Čepička 2015 ****** Species '' V. nauta'' Pánek, Táborský & Čepička 2015 ****** Species '' V. trypanoides'' Pánek, Táborský & Čepička 2015 ***** Genus ''

Stygiella ''Stygiella'' /ˌstɪ.d͡ʒiˈɛ.lə/ is a genus of free-living marine flagellates belonging to the family Stygiellidae in the jakobids (excavata). The genus currently includes four species, all of which are secondary obligate anaerobes. The ...

'' Pánek, Táborský & Čepička 2015 non Bruand 1853 ****** Species '' S. incarcerata'' (Bernard, Simpson & Patterson 2000) Pánek, Táborský & Čepička 2015 'Jakoba incarcerata'' Bernard, Simpson & Patterson 2000; ''Andalucia incarcerata'' (Bernard, Simpson & Patterson 2000) Lara et al. 2006] ****** Species ''Stygiella agilis, S. agilis'' Pánek, Táborský & Čepička 2015 ****** Species ''Stygiella cryptica, S. cryptica'' Pánek, Táborský & Čepička 2015 ****** Species '' S. adhaerens'' Pánek, Táborský & Čepička 2015 *** Suborder Histonina Cavalier-Smith 1993 **** Species ?'' Jakoba echidna'' O'Kelly 1991 **** Family Moramonadidae Strassert et al. 2016 ***** Genus '' Moramonas'' Strassert et al. 2016 ****** Species '' Moramonas marocensis'' Strassert et al. 2016 ***** Genus '' Seculamonas'' Marx et al. 2003 nomen nudum ****** Species '' Seculamonas ecuadoriensis'' Marx et al. 2003 nomen nudum **** Family Jakobidae Patterson 1990 ***** Genus '' Jakoba'' Patterson 1990 ****** Species '' Jakoba bahamiensis'' Burger & Lang (indeitum) ****** Species '' Jakoba libera'' (Ruinen 1938) Patterson 1990 'Cryptobia libera'' Ruinen 1938**** Family Histionidae Flavin & Nerad 1993 ***** Genus '' Histiona'' Voigt 1902 'Zachariasia''_Voigt_1901_non_Lemmermann_1895.html" ;"title="Zachariasia.html" ;"title="' 'Zachariasia''_Voigt_1901_non_Lemmermann_1895">Zachariasia.html"_;"title="'Zachariasia">'Zachariasia''_Voigt_1901_non_Lemmermann_1895******_Species_?'' 'Zachariasia''_Voigt_1901_non_Lemmermann_1895">Zachariasia.html"_;"title="'Zachariasia">'Zachariasia''_Voigt_1901_non_Lemmermann_1895******_Species_?''Histiona_planctonica">H._planctonica''_Scourfield_1937 ******_Species_'' 'Zachariasia''_Voigt_1901_non_Lemmermann_1895">Zachariasia.html"_;"title="'Zachariasia">'Zachariasia''_Voigt_1901_non_Lemmermann_1895******_Species_?''Histiona_planctonica">H._planctonica''_Scourfield_1937 ******_Species_''Histiona_aroides">H._aroides''_Pascher_1943 ******_Species_''Histiona_velifera.html" ;"title="Histiona_aroides.html" ;"title="Histiona_planctonica.html" ;"title="Zachariasia">'Zachariasia'' Voigt 1901 non Lemmermann 1895">Zachariasia.html" ;"title="'Zachariasia">'Zachariasia'' Voigt 1901 non Lemmermann 1895****** Species ?''Histiona planctonica">H. planctonica'' Scourfield 1937 ****** Species ''Histiona aroides">H. aroides'' Pascher 1943 ****** Species ''Histiona velifera">H. velifera'' (Voigt 1901) Pascher 1943 [''Zachariasia velifera'' Voigt 1901; ''Histiona zachariasii'' Voigt 1901 nom. illeg.] ***** Genus ''Reclinomonas'' Flavin & Nerad 1993 ****** Species ''Reclinomonas americana, R. americana'' Flavin & Nerad 1993 ****** Species ''Reclinomonas campanula, R. campanula'' (Penard 1921) Flavin & Nerad 1993 'Histiona campanula'' Penard 1921; ''Stenocodon campanula'' (Penard 1921) Pascher 1942***** Genus '' Stenocodon'' Pascher 1942 ****** Species '' Stenocodon epiplankton'' Pascher 1942 ***** Genus '' Stomatochone'' Pascher 1942 ****** Species '' S. infundibuliformis'' Pascher 1942 ****** Species '' S. cochlear'' Pascher 1942 ****** Species '' S. excavata'' Pascher 1942 ****** Species '' S. epiplankton'' Pascher 1942

References

{{Taxonbar, from=Q1135350

Structure and Biology

Ecology

Mitochondrial DNA

Taxonomy

See also

References