A common characteristic of opisthokonts is thatflagellate cells, such as thesperm of most animals and thespores of thechytridfungi, propel themselves with a singleposterior flagellum. It is this feature that gives the group its name. In contrast, flagellate cells in other eukaryote groups propel themselves withone or moreanterior flagella. Flagellate cells however have been secondarily lost in some opisthokont groups, including most of the fungi.[7]
Opisthokont characteristics include synthesis of extracellularchitin in exoskeleton, cyst/spore wall, or cell wall of filamentous growth and hyphae; the extracellular digestion of substrates with osmotrophic absorption of nutrients; and other cell biosynthetic and metabolic pathways. Genera at the base of each clade are amoeboid andphagotrophic.[12]
The close relationship between animals and fungi was suggested byThomas Cavalier-Smith in 1987,[3] who used the informal name opisthokonta (the formal name has been used for thechytrids byCopeland in 1956), and was supported by later genetic studies.[13]
Early phylogenies placed fungi near theplants and other groups that havemitochondria with flatcristae, 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 ofcarbamoyl phosphate synthetase,dihydroorotase, andaspartate carbamoyltransferase that is not present in plants, and plants have a fusion ofthymidylate synthase anddihydrofolate reductase not present in the opisthokonts. Animals and fungi are also more closely related to amoebas than to plants, and plants are more closely related to theSAR supergroup of protists than to animals or fungi.[citation needed] Animals and fungi are bothheterotrophs, unlike plants, and while most fungi aresessile like plants, there are also sessile animals.
Cavalier-Smith and Stechmann argue that the uniciliate eukaryotes such as opisthokonts andAmoebozoa, collectively calledunikonts, split off from the otherbiciliate eukaryotes, calledbikonts, shortly after theyevolved.[14]
Opisthokonts are divided intoHolomycota or Nucletmycea (fungi and all organisms more closely related to fungi than to animals) andHolozoa (animals and all organisms more closely related to animals than to fungi); no opisthokonts basal to the Holomycota/Holozoa split have yet been identified.[citation needed] The Opisthokonts was largely resolved by Torriella et al.[15] Holomycota and Holozoa are composed of the following groups.[citation needed]
The followingphylogenetic tree indicates the evolutionary relationships between the different opisthokont lineages, and the time divergence of the clades in millions of years ago (Mya).[17][18][19]
^abRayner, Alan D. M., ed. (1987). "The origin of fungi and pseudofungi".Evolutionary biology of Fungi. Cambridge: Cambridge University Press. pp. 339–353.ISBN0-521-33050-5.OL2936766W.
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