DESCRIPTION OF THE SUPERGROUP UNIKONTA (CAVALIER-SMITH 2002)
| EUKARYA> UNIKONTA |
SUPERGROUP UNIKONTA LINKS
| Unikonta (yu-nah-KON-tah) is derived from the Latin root for one (unus) and the Greek kinein or kino (κινώ), which means to move, it is the same root from which kinetic is derived. The reference is to motile cells having a single flagellum. |
| INTRODUCTION TO THE UNIKONTA Baldauf and Palmer (1993) demonstrated the phylogenetic relationship between the animals and fungi by using sequences of 25 proteins. This followed a suggestion by Cavalier-Smith (1987) that animals and fungi evolved from a choanoflagellate-like line. Since then, other work (Cavalier-Smith and Chao, 1995; and Cavalier-Smith, 1998b) strengthened the link and called the group, the Opisthokonts in reference to the posteriorly-directed flagellum in motile cells. Other work (e.g. van de Peer et al., 2000; and Keeling, 2004) has confirmed a sister group relationship between the Amoebozoae and the Opisthokonts. Tentatively, this group is called the Unikonta, in reference to the occurrence of a single flagellum in their motile cells. The Unikont question hinges on the relationships of the Apusozoa with other taxa, especially the Amoebozoa. Is Apusozoa a member of the Bikonts like the Excavata or the Unikonts? Literally, because the Apusomonads have two flagella that are heterodynamic, they are Bikonts. If that is where they really belong, then the Unikonta collapses, and the Opisthokonts and Amoebozoans would have to be considered separate supergroups. Cavalier-Smith and Chao (2003; see Figure 1) present a scenario in which the unikonts, those organisms with single flagella, are paraphyletic. They root the eukaryotes in a line from a group of Gram positive eubacteria, which gave rise to the eukaryotes and the archaea [see an explanation of the Archaezoa Hypothesis in the description of the Domain Eukarya]. The stages in the origin of the eukaryote line were the organization of microtubules to form a flagellum (not the endosymbiotic origin as Margulis claims) and the later acquisition of a mitochondrial endosymbiont. The single flagellar structure gave rise to the Opisthokonts (taxa with a single trailing or pushing flagellum and flat mitochondrial cristae) and the Anterokonts (taxa with a single anterior or pulling flagellum and tubular mitochondrial cristae). The anterokonts (e.g. Amoebozoae) gave rise to the Bikont line, which they presumed to have arisen through an apusozoan-like organism. In a curious turnabout from Cavalier-Smith and Chao (2003), Cavalier-Smith (2003; Figure 2) presents a larger view of the Eukaryotes and suggests that the Amoebozoa is in the clade with the Opisthokonts. In this scenario, all Eukaryotes can be segregated into Unikonta and Bikonta. The Apusomonads are at the base of the Bikont line. Kim et al. (2006; Figure 3) suggest a third alternative. They conclude that the Apusozoans are sisters to the Opisthokonta and that the Amoebozoa are sisters to all other Bikont Eukaryotes. In this scenario, the Unikonts collapse as monophyletic taxon. The Unikonts also suffer from a lack of defining synapomorphies. Table 1 explores the distribution of states of flagella, cell covering, mitochondria, and meiosis through 12 groups of Unikonts. The flagella range from being absent to one posterior, one anterior, and both posterior and anterior. Similarly, mitochondria can be absent, reduced, or present with flat cristae or present with tubular cristae. Character states for cell covering and meiosis are equally dispersed. |
FIGURE 1. This is an illustration that is modified from Cavalier-Smith and Chao (2003) in which the major Unikont groups (Opisthokonts + Amoebozoans) are paraphyletic.
FIGURE 2. Cavalier-Smith (2003) presents a scenario in which the bikonts and unikonts arise from an amoeboid uniflagellated ancestor. In this scenario based on molecular, ultrastructural, and biochemical characters, the Apusozoans are basal to the Bikont line. Compare this scenario to Cavalier-Smith and Chao (2003).
FIGURE 3. Kim et al. (2006) examined the position of Apusomonas and concluded that it was a sister to the Opisthokonts. If their scenario is correct, the Unikonts are not monophyletic.
| TABLE 1. A table of four distinguishing features (flagella, covering, mitochondria, and meiosis) among 12 groups of unikonts (from Cavalier-Smith 2003 and Adl et al. 2005). | ||||
| GROUP | FLAGELLA | COVERING | MITOCHONDRIA | MEIOSIS |
| PELOBIONTS | ANTERIOR OR INTERNAL | NAKED | HYDROGENOSOMES | ABSENT |
| RHIZOPODA | ANTERIOR (1) | NAKED/ ORGANIC COVERING TESTS & SPORES | TUBULAR CRISTAE | ABSENT |
| MYXOMYCOTA | ANTERIOR (2) | NAKED/CHITIN | TUBULAR CRISTAE | PRESENT |
| DICTYOSTELA | ABSENT | NAKED/ ORGANIC COVERING SPORES | TUBULAR CRISTAE | ABSENT |
| APUSOZOA | ANTERIOR + POSTERIOR | ORGANIC THECA | FLAT & TUBULAR CRISTAE | ABSENT |
| CHYTRIDS | POSTERIOR (1) | CHITIN | FLAT CRISTAE | PRESENT |
| MICROSPORIDIA | ABSENT | SPORE WALL OF 3 LAYERS | ABSENT | SUSPECTED |
| AMASTIGOTE FUNGI | ABSENT | CHITIN/ CHITOSAN/GLUCAN | FLAT CRISTAE | PRESENT/ ABSENT IN FUNGI IMPERFECTI |
| CHOANOFLAGELLATES | POSTERIOR (1) | NAKED WITH SILICACEOUS SPICULES | FLAT CRISTAE | NOT KNOWN |
| NUCLEARIIDA | ABSENT | NAKED | FLAT CRISTAE | NOT KNOWN |
| ICHTHYOSPOREA | POSTERIOR (1) | NAKED | FLAT CRISTAE (ONE WITH TUBULAR CRISTAE) | NOT KNOWN |
| METAZOA | POSTERIOR (1) | VARIABLE; GENERALLY NAKED/ ORGANIC OF CHITIN CUTIN | MOSTLY WITH FLAT CRISTAE | PRESENT |
| KINGDOMS OF THE UNIKONTA |
- ANIMALIA (LINNAEUS 1758)
AMOEBOZOAE (LÜHE 1913, em. CORLISS 1984)
FUNGI (LINNAEUS 1753)
| LITERATURE CITED Adl, S. M., A. G. B. Simpson, M. A. Farmer, R. A. Andersen, O. R. Anderson, J. R. Barta, S. S. Bowser, G. Brugerolle, R. A. Fensome, S. Fredericq, T. Y. James, S. Karpov, P. Kugrens, J. Krug, C. E. Lane, L. A. Lewis, J. Lodge, D. H. Lynn, D. G. Mann, R. M. McCourt, L. Mendoza, O. Moestrup, S. E. Mozley-Standridge, T. A. Nerad, C. A. Shearer, A. V. Smirnov, F. W. Spiegel, and M. F. J. R. Taylor. 2005. The new higher level classification of eukaryotes with emphasis on the taxonomy of protists. Journal of Eukaryotic Microbiology. 52(5):399-451. Baldauf, S. L. 2003a. The deep roots of eukaryotes. Science. 300 (5626): 1701-1703. Baldauf, S. L. and J.D. Palmer. 1993. Animals and fungi are each other’s closest relatives: Congruent evidence from multiple proteins. Proceedings of the National Academy of Science USA. 90:11558-11562. Cavalier-Smith, T. 1983. A six-kingdom classification and a unified phylogeny. In: Schenk, H.E.A. and W.S. Schwemmler, eds. Endocytobiology II. de Gruyter , Berlin . pp. 1027-1034. Cavalier-Smith, T. 1987b. The origin of eukaryote and archaebacterial cells. Annals of the New York Academy of Sciences. 503: 17-54. Cavalier-Smith, T. 1998b, Neomonada and the origin of animals and fungi: In: Coombs, G. H., Vickerman, K., Sleigh, M. A. & Warren, A. (ed.), Evolutionary Relationships Among Protozoa. Kluwer Academic Publishers. Dordrecht , The Netherlands , pp. 375–407. Cavalier-Smith, T. 2002a. The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa: International Journal of Systematic Evolutionary Microbiology. 52:297–354. Cavalier-Smith, T. 2003a. Protist phylogeny and the high-level classification of Protozoa. European Journal of Protistology. 39:338-348. Cavalier-Smith, T. and E. E. Chao. 1995. The opalozoan Apusomonas is related to the common ancestor of animals, fungi, and choanoflagellates. Proceedings of the Royal Society of London B. 261:1-6. Cavalier-Smith, T. and E. E. Chao. 2003b. Phylogeny of Choanozoa, Apusozoa, and other Protozoa and early eukaryote megaevolution. Journal of Molecular Evolution. 56:540-563. Corliss, J. 1984. The kingdom Protista and its 45 phyla. Biosystems. 17(2): 87-126. Keeling P. J. 2004. The diversity and evolutionary history of plastids and their hosts. American Journal of Botany. 91(10): 1481-1493. Linnaeus, C. 1753. Species Plantarum. Laurentii Salvii. Stockholm. Linnaeus, C. 1758. Systema Naturae per Regna Tria Naturae. Regnum Animale. Laurentii Salvii. Stockholm. Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company. New York. Patterson, D. J. 1999. The diversity of eukaryotes. American Naturalist. 154 (Suppl.): S96–S124. Van de Peer, Y., A. Ben Ali, and A. Meyer. 2000. Microsporidia: accumulating molecular evidence that a group of amitochondriate and suspectedly primitive eukaryotes are just curious fungi. Gene. 246: 1-8. |
| By Jack R. Holt. Last revised: 11/18/2013 |
