|Orange elephant ear sponge, Agelas clathrodes, in foreground. Two corals in the background: a sea fan, Iciligorgia schrammi, and a sea rod, Plexaurella nutans.|
Cavalier-Smith 1987, emend. Adl et al. 2005
The opisthokonts (Greek: ὀπίσθιος (opísthios) = "rear, posterior" + κοντός (kontós) = "pole" i.e. "flagellum") 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 (conventionally assigned to the protist 'kingdom'). The opisthokonts, sometimes referred to as the 'Fungi/Metazoa group', are generally recognized as a monophyletic clade.
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.
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 supported by later genetic studies.
Early phylogenies placed them near the plants and other groups that have mitochondria with flat cristae, but this character varies. More recently, it has been said that holozoa (animals) and holomycota (fungi) are much more closely related to each other than either is to plants, because opisthokonts have a triple fusion of carbamoyl phosphate synthase, dihydroorotase, and aspartate carbamoyltransferase that is not present in plants, and plants have a fusion of thymidylate synthase and dihydrofolate reductase not present in the opisthokonts. Animals and fungi are also more closely related to amoebas than they are to plants, and plants are more closely related to the SAR supergroup of protists than they are to animals or fungi.
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.
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.
- 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)
- Fonticulida (fonticulid slime molds)
- Nucleariida or Cristidiscoidea (nucleariid amoebae)
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 genomes.
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 ichthyosl genome is >200 kilobase pairs in length and consists of several hundred linear chromosomes that share elaborate terminal-specific sequence patterns.
One view of the great kingdoms and their stem groups.
- Shalchian-Tabrizi K, Minge MA, Espelund M. et al. (2008). Aramayo, Rodolfo, ed. "Multigene phylogeny of choanozoa and the origin of animals". PLoS ONE 3 (5): e2098. Bibcode:2008PLoSO...3.2098S. doi:10.1371/journal.pone.0002098. PMC 2346548. PMID 18461162.
- Steenkamp ET, Wright J, Baldauf SL (2006). "The protistan origins of animals and fungi". Mol. Biol. Evol. 23 (1): 93–106. doi:10.1093/molbev/msj011. PMID 16151185.
- "Fungi/Metazoa group". Retrieved 2009-03-08.
- Huang, Jinling; Xu, Ying; Gogarten, Johann Peter (2005). "The presence of a haloarchaeal type tyrosyl-tRNA synthetase marks the opisthokonts as monophyletic". Molecular biology and evolution 22 (11): 2142–2146.
- Steenkamp, Emma T; Wright, Jane; Baldauf, Sandra L. (2006). "The protistan origins of animals and fungi". Molecular biology and Evolution 23 (1): 93–106.
- Parfrey, Laura Wegener et al. (2006). "Evaluating support for the current classification of eukaryotic diversity". PLoS genetics 2 (12): e220.
- Torruella, Guifré, et al. (2012). "Phylogenetic relationships within the Opisthokonta based on phylogenomic analyses of conserved single-copy protein domains". Molecular Biology and Evolution 29 (2): 531–544.
- Cavalier-Smith, T. (1987). "The origin of fungi and pseudofungi". In Rayner, Alan D. M. (ed.). Evolutionary biology of Fungi. Cambridge: Cambridge Univ. Press. pp. 339–353. ISBN 0-521-33050-5.
- Wainright PO, Hinkle G, Sogin ML, Stickel SK (April 1993). "Monophyletic origins of the metazoa: an evolutionary link with fungi". Science 260 (5106): 340–2. Bibcode:1993Sci...260..340W. doi:10.1126/science.8469985. PMID 8469985.
- Stechmann, A.; Cavalier-Smith, T. (2002). "Rooting the eukaryote tree by using a derived gene fusion". Science 297 (5578): 89–91. Bibcode:2002Sci...297...89S. doi:10.1126/science.1071196. PMID 12098695.
- Del Campo, J.; Ruiz-Trillo, I. (2013). "Environmental survey meta-analysis reveals hidden diversity among unicellular opisthokonts". Molecular Biology and Evolution. doi:10.1093/molbev/mst006.
- Phylogeny based on:
- Eichinger, L.; Pachebat, J. A.; Glöckner, G.; Rajandream, M. A.; Sucgang, R.; Berriman, M.; Song, J.; Olsen, R. et al. (2005). "The genome of the social amoeba Dictyostelium discoideum". Nature 435 (7038): 43–57. doi:10.1038/nature03481. PMC 1352341. PMID 15875012.
- Steenkamp, E. T.; Wright, J.; Baldauf, S. L. (2005). "The Protistan Origins of Animals and Fungi". Molecular Biology and Evolution 23 (1): 93–106. doi:10.1093/molbev/msj011. PMID 16151185.
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