Opisthokont
Encyclopedia
The opisthokonts or "Fungi/Metazoa group" are a broad group of eukaryote
s, including both the animal
and fungus
kingdoms
, together with the eukaryotic microorganism
s that are sometimes grouped in the paraphyletic phylum
Choanozoa
(previously assigned to the protist
"kingdom"). Both genetic
and ultrastructural
studies strongly support that opisthokonts form a monophyletic group.
cells, such as most animal sperm
and chytrid spores, propel themselves with a single posterior flagellum. This gives the group its name. In contrast, flagellate cells in other eukaryote groups propel themselves with one or more anterior flagella.
in 1987, who used the informal name opisthokonta (the formal name has been used for the chytrids), and was confirmed by later genetic studies.
Early phylogenies placed them near the plant
s and other groups that have mitochondria
with flat cristae, but this character varies.
Cavalier-Smith and Stechmann argue that the uniciliate eukaryotes such as opisthokonts and Amoebozoa
, collectively called unikont
s, split off from the other biciliate eukaryotes, called bikont
s, shortly after they evolved
.
or Nucletmycea (fungi and all organisms more closely related to fungi than to animals) and Holozoa
(animals and all organisms more closely related to animals than to fungi); no opisthokonts basal to the Holomycota/Holozoa split have yet been identified. Holomycota and Holozoa are composed of the following groups.
The paraphyletic taxon Choanozoa
includes either non-animal holozoans, or non-animal, non-fungi opisthokonts.
The choanoflagellates have a circular mitochondrial DNA genome with long intergenic regions. This is four times as large and contains two times as many protein genes as do animal mitochondrial mitochondria.
Corallochytrium
seem likely to be more closely related to the fungi than to the animals on the basis of the presence of ergosterol
in their membranes and being capable of synthesis of lysine
via the AAA pathway.
The ichthyosporeans have a two amino acid deletion in their elongation factor
1 α gene that is considered characteristic of fungi.
The ichthyosporean mitochondrial genome is >200 kilobase pairs in length and consists of several hundred linear chromosomes that share elaborate terminal-specific sequence patterns.
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s, including both the animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
and fungus
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
kingdoms
Kingdom (biology)
In biology, kingdom is a taxonomic rank, which is either the highest rank or in the more recent three-domain system, the rank below domain. Kingdoms are divided into smaller groups called phyla or divisions in botany...
, together with the eukaryotic microorganism
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
s that are sometimes grouped in the paraphyletic phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
Choanozoa
Choanozoa
Choanozoa is the name of a phylum of protists that belongs to the line of opisthokonts....
(previously assigned to the protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
"kingdom"). Both genetic
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
and ultrastructural
Ultrastructure
Ultrastructure is the detailed structure of a biological specimen, such as a cell, tissue, or organ, that can be observed by electron microscopy...
studies strongly support that opisthokonts form a monophyletic group.
Flagella
One common characteristic of opisthokonts is that flagellateFlagellate
Flagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants...
cells, such as most animal sperm
Spermatozoon
A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...
and chytrid spores, propel themselves with a single posterior flagellum. This gives the group its name. In contrast, flagellate cells in other eukaryote groups propel themselves with one or more anterior flagella.
History
The close relationship between animals and fungi was suggested by Cavalier-SmithThomas Cavalier-Smith
Professor Thomas Cavalier-Smith , FRS, FRSC, NERC Professorial Fellow, is a Professor of Evolutionary Biology in the Department of Zoology, at the University of Oxford...
in 1987, who used the informal name opisthokonta (the formal name has been used for the chytrids), and was confirmed by later genetic studies.
Early phylogenies placed them near the plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s and other groups that have mitochondria
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
with flat cristae, but this character varies.
Cavalier-Smith and Stechmann argue that the uniciliate eukaryotes such as opisthokonts and Amoebozoa
Amoebozoa
The Amoebozoa are a major group of amoeboid protozoa, including the majority that move by means ofinternal cytoplasmic flow. Their pseudopodia are characteristically blunt and finger-like,...
, collectively called unikont
Unikont
Unikonts are members of the Unikonta, a taxonomic group proposed by Thomas Cavalier-Smith.It includes amoebozoa, opisthokonts, and Apusozoa.-Clade:...
s, split off from the other biciliate eukaryotes, called bikont
Bikont
A Bikont is a eukaryotic cell with two flagella, as its name suggests. It is a division of eukaryotes.-Enzymes:Another shared trait of bikonts is the fusion of two genes into a single unit: the genes for thymidylate synthase and dihydrofolate reductase encode a singleprotein with two...
s, shortly after they evolved
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
.
Taxonomy
Opisthokonts are divided into HolomycotaHolomycota
Holomycota is a grouping that includes nucleariida and fungi.Another name for this grouping is "Nucletmycea"....
or Nucletmycea (fungi and all organisms more closely related to fungi than to animals) and Holozoa
Holozoa
Holozoa is a group of organisms that includes animals and their closest single-celled relatives, but excludes fungi. Holozoa is also an old name for the tunicate genus Distaplia....
(animals and all organisms more closely related to animals than to fungi); no opisthokonts basal to the Holomycota/Holozoa split have yet been identified. Holomycota and Holozoa are composed of the following groups.
- Holomycota
- Fungi
- including chytrids (previously thought to be protistProtistProtists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
s) - including microsporidiaMicrosporidiaThe microsporidia constitute a phylum of spore-forming unicellular parasites. They were once thought to be protists but are now known to be fungi. Loosely 1500 of the probably more than one million species are named now. Microsporidia are restricted to animal hosts, and all major groups of animals...
(previously thought to be protists) - including Hyaloraphidium (previously thought to be a green algaChlorophytaChlorophyta is a division of green algae, informally called chlorophytes. The name is used in two very different senses so that care is needed to determine the use by a particular author...
, now thought to be a chytrid) - excluding oomycetes (water molds) (now thought to be stramenopiles)
- excluding labyrinthulomycetesLabyrinthulomycetesThe Labyrinthulomycetes also known as Labyrinthulomycota or Slime nets are a group of protists that produce a network of filaments or tubes, which serve as tracks for the cells to glide along and absorb nutrients for them. There are two main groups, the labyrinthulids and thraustochytrids...
(slime nets) (now thought to be stramenopiles) - excluding myxomycetes (now thought to be amoebozoaAmoebozoaThe Amoebozoa are a major group of amoeboid protozoa, including the majority that move by means ofinternal cytoplasmic flow. Their pseudopodia are characteristically blunt and finger-like,...
ns)
- including chytrids (previously thought to be protist
- RozellidaRozellidaCryptomycota , or Rozellida are a clade of micro-organisms which are either fungi or a sister group to fungi. They differ from classical fungi in that they lack chitinous cell walls....
- FonticulidaFonticulaFonticula is a cellular slime mold which forms a fruiting body in a volcano shape. As long ago as 1979 it has been known to not have a close relationship with either the Dictyosteliida or the Acrasidae, the two well-established groups of cellular slime molds...
(fonticulid slime molds) - Nucleariida or Cristidiscoidea (nucleariid amoebae)
- Fungi
- Holozoa
- MesomycetozoeaMesomycetozoeaThe Mesomycetozoea are a small group of protists, mostly parasites of fish and other animals.-Significance:...
- DermocystidaDermocystidaDermocystida is a order of parasitic protists.-Genera:* Amphibiocystidium* Amphibiothecum * Dermocystidium* Rhinosporidium* Sphaerothecum...
- IchthyophonidaIchthyophonidaIchthyophonida is an order of parasitic protists.-Taxonomy:Members of Ichthyophonida include* Abeoforma whisleri* Amoebidiidae** Amoebidium** Paramoebidium* Anurofeca richardsii* Caullerya mesnili* Creolimax fragantissima...
- EccrinalesEccrinalesEccrinales are an order of protists, previously thought to be zycomycete fungi belonging to the class Trichomycetes, but now considered to be members of the opisthokont protist group Mesomycetozoea.- Taxonomy :* Order: Eccrinales** Family: Eccrinaceae...
- Amoebidiales
- CorallochytriumCorallochytriumCorallochytrium is a single-celled eukaryote which was found in coral reef lagoons of the Lakshadweep islands of the Arabian sea. Although it is an opisthokont, its closest relatives in that group are not clear. For example, it is not clear whether it is more closely related to animals or...
- Dermocystida
- FilozoaFilozoaThe Filozoa are a monophyletic grouping within the Opisthokonta. They include animals along with their nearest unicellular relatives ....
- FilastereaFilastereaFilasterea is a grouping that includes Ministeria and Capsaspora....
- CapsasporidaCapsasporaCapsaspora is a single-celled eukaryote which is a symbiont in the haemolymph of the tropical freshwater snail Biomphalaria glabrata.- Taxonomy :...
(capsasporid amoebae) - MinisteriidaMinisteriaMinisteria is a small single-celled eukaryote which feeds on bacteria. Although it is an opisthokont, it seems not to fit in any of the established opisthokont lineages such as choanoflagellates or animals. It might form a group with Capsaspora, another opisthokont of uncertain placement....
(ministeriid amoebae)
- Capsasporida
- Choanoflagellata
- Animalia
- including MyxozoaMyxozoaThe Myxozoa are a group of parasitic animals of aquatic environments. Over 1300 species have been described and many have a two-host lifecycle, involving a fish and an annelid worm or bryozoan. The average size of a Myxosporea spore usually ranges from 10 μm to 20 μm and Malacosporea up...
(previously thought to be protists)
- including Myxozoa
- Filasterea
- Mesomycetozoea
The paraphyletic taxon Choanozoa
Choanozoa
Choanozoa is the name of a phylum of protists that belongs to the line of opisthokonts....
includes either non-animal holozoans, or non-animal, non-fungi opisthokonts.
The choanoflagellates have a circular mitochondrial DNA genome with long intergenic regions. This is four times as large and contains two times as many protein genes as do animal mitochondrial mitochondria.
Corallochytrium
Corallochytrium
Corallochytrium is a single-celled eukaryote which was found in coral reef lagoons of the Lakshadweep islands of the Arabian sea. Although it is an opisthokont, its closest relatives in that group are not clear. For example, it is not clear whether it is more closely related to animals or...
seem likely to be more closely related to the fungi than to the animals on the basis of the presence of ergosterol
Ergosterol
Ergosterol is a sterol found in fungi, and named for ergot, a common name for the members of the fungal genus Claviceps from which ergosterol was first isolated. Ergosterol does not occur in plant or animal cells...
in their membranes and being capable of synthesis of lysine
Lysine
Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....
via the AAA pathway.
The ichthyosporeans have a two amino acid deletion in their elongation factor
Elongation factor
Elongation factors are a set of proteins that facilitate the events of translational elongation, the steps in protein synthesis from the formation of the first peptide bond to the formation of the last one.Elongation is the most rapid step in translation:...
1 α gene that is considered characteristic of fungi.
The ichthyosporean mitochondrial genome is >200 kilobase pairs in length and consists of several hundred linear chromosomes that share elaborate terminal-specific sequence patterns.