Thursday, March 06, 2008

Opisthokonts

Unikont
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Unikont is a eukaryotic cell with a single flagellum, at least ancestrally. Current research suggests that a unikont was the ancestor of opisthokonts (animals, fungi and related forms) and Amoebozoa, and a bikont (a eukaryotic cell with two flagella) was the ancestor of Archaeplastida (plants and relatives), Excavata, Rhizaria, and Chromalveolata. The unikonts also have a triple-gene fusion that is lacking in the
bikonts, and a single centriole (Cavalier-Smith, 2002, 2006). (Some unikonts have two centrioles but their origins are developmentally different than in the bikonts, indicating convergent evolution (Cavalier-Smith 2006). The three genes that are fused together in the unikonts but not bacteria or bikonts encode enzymes for synthesis of the pyrimidine nucleotides: carbamoyl phosphate synthase, dihydroorotase, aspartate carbamoyltransferase (Cavalier-Smith 2006). This must have involved a double fusion, a rare pair of events, further supporting the shared ancestry of Opisthokonta and Amoebozoa.

References

Cavalier-Smith, Thomas (2002). "The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa". International Journal of Systematic and Evolutionary Microbiology 52 (2): 297–354. Retrieved on 2007-06-08.

Cavalier-Smith, Thomas (2006). "Protist phylogeny and the high-level classification of Protozoa". European Journal of Protistology 39 (4): 338-348.

Stechmann, Alexandra; Cavalier-Smith, Thomas (2003). "The root of the eukaryote tree pinpointed". Current Biology 13 (17): R665-R666.

Opisthokont
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The opisthokonts (Greek: οπίσθω- (opisthō-) = "rear, posterior" + κοντός (kontos) = "pole" i.e. flagellum) are a broad group of eukaryotes, including both the animal and fungus kingdoms, together with the phylum Choanozoa of the protist kingdom. Both genetic and ultrastructural studies strongly support that opisthokonts form a monophyletic group. One common characteristic is that flagellate cells, such as most animal sperm and chytrid spores, propel themselves with a single posterior flagellum. This gives the groups 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 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.

See also

Unikonts and bikonts
Choanozoa

References

Cavalier-Smith, T. (1987). Evolutionary biology of Fungi. Cambridge: Cambridge Univ. Press.
Wainwright, P.O.; et al. (1993). "Monophyletic origins of the metazoa: an evolutionary link with fungi". Science 260 340-342.
Stechmann, A.; Cavalier-Smith, T. (2002). "Rooting the eukaryote tree by using a derived gene fusion". Science 297 89–91.

Bikont
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A Bikont is a eukaryotic cell with two flagella. Another shared trait of bikonts is the fusion of two genes into a single unit: the genes for thymidylate synthase (TS) and dihydrofolate reductase (DHFR) encode a single protein with two functions (Cavalier-Smith, 2006). The genes are separately translated in unikonts. Some research suggests that a unikont (a eukaryotic cell with a single flagellum) was the ancestor of opisthokonts (Animals, Fungi and related forms) and Amoebozoa, and a bikont was the ancestor of Archaeplastida (Plants and relatives), Excavata, Rhizaria, and Chromalveolata. Cavalier-Smith has suggested that Apusozoa, which are typically considered incertae sedis, are in fact bikonts.

References

Thomas Cavalier-Smith (2006). "Protist phylogeny and the high-level classification of Protozoa". European Journal of Protistology 39 (4): 338-348.

Alexandra Stechmann and Thomas Cavalier-Smith (2003). "The root of the eukaryote tree pinpointed". Current Biology 13 (17):

Choanozoa
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Choanozoa (Greek: χόανος (choanos) = "funnel" + ζῶον (zōon) = "animal") is the name of a phylum of protists that belongs to the line of opisthokonts. Most appear closer to the animals than to the fungi, and they are of great interest to biologists studying animal origins. The chytrids have also been included among the Protista, but are now more often placed among the Fungi. It has been suggested that the nucleariids are in fact a sister group to the fungi, and that the rest of the classes form a monophyletic or paraphyletic sister group to the animals.

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