Opisthokont

The opisthokonts (Greek: ὀπίσθιος (opísthios) = "rear, posterior" + κοντός (kontós) = "pole" i.e. "flagellum") or "Fungi/Metazoa group" are a broad group of eukaryotes, including both the animal and fungus kingdoms, together with the eukaryotic microorganisms 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.

Flagella
One common characteristic of opisthokonts is that flagellate 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.

History
The close relationship between animals and fungi was suggested by Cavalier-Smith 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 plants 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 unikonts, split off from the other biciliate eukaryotes, called bikonts, shortly after they evolved.

Taxonomy
Opisthokonts are divided into Holomycota 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. However, a 2013 study suggests that there may be many more unicellular opisthokonts than previously suspected. Holomycota and Holozoa are composed of the following groups.


 * Holomycota
 * Fungi
 * including chytrids (previously thought to be protists)
 * including microsporidia (previously thought to be protists)
 * including Hyaloraphidium (previously thought to be a green alga, now thought to be a chytrid)
 * excluding oomycetes (water molds) (now thought to be stramenopiles)
 * excluding labyrinthulomycetes (slime nets) (now thought to be stramenopiles)
 * excluding myxomycetes (now thought to be amoebozoans)
 * Rozellida
 * Fonticulida (fonticulid slime molds)
 * Nucleariida or Cristidiscoidea (nucleariid amoebae)
 * Holozoa
 * Mesomycetozoea
 * Dermocystida
 * Ichthyophonida
 * Eccrinales
 * Amoebidiales
 * Corallochytrium
 * Filozoa
 * Filasterea
 * Capsasporida (capsasporid amoebae)
 * Ministeriida (ministeriid amoebae)
 * Choanoflagellata
 * Animalia
 * including Myxozoa (previously thought to be protists)

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.



