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Description of OpisthokontsCircumscription: This group contains the true fungi and their protist relatives (the chytrids) and the animals and their protist relatives (the choanoflagellates). The close relationship of the multicellular taxa was initially indicated by molecular means. The group contains uniflagellated solitary and colonial protists with one flagellum at some stage in their life history, phagotrophs, saprophytes, mycelial organisms with spore-forming bodies but no flagella, multicellular heterotrophs formed from layers of cells (epithelia), and the sponges with a less structured arrangement of cells. Collagen, one of the components of the extracellular matrix of the animals has also been reported from some fungi. All of the flagellated taxa and flagellated cells swim with a single flagellum beating behind the cell. The term opisthokonta has been applied to this grouping (Cavalier-Smith and Chao 1995; Cavalier-Smith 1996). However, Cavalier-Smith and Chao do not use the term as a formal taxon because it would require that more important taxa are subordinated to less important taxa. Here, the opisthokonts is a taxon with the composition as indicated. The name may need to be revisited as the term Opisthokonta has previously been used by Copeland (1956) for the chytrids. Some other permanently or temporarily opisthokont protists-such as several nominal pelobionts or the unassigned Phalansterium or Pseudaphelidium-are not included, and it is not yet known if they form part of this group. Ultrastructural identity: The diversity of organization within this group is great, extending from uniflagellated protists with or without the ability to make siliceous products to multicellular myeelial or epitheliate organisms. Apart from having platycristate mitochondria and being dictyosomate, this group has few discriminating characters that extend throughout this group. Nuclear division is variable within the fungi but in the animals, the envelope breaks down during mitosis. Synapomorphy: Unspecified but probably may relate to the radiating and arcing anchorage structures associated with the single flagellum. Very few studies of the protistan (ancestral) members have been conducted, and until more detailed ultrastructural studies are carried Out, such a determination would probably be premature. Most included taxa have secondarily lost this character. Composition: The largest of the major eukaryote lineages with probably in excess of 1,000,000 species, in two major clusters: (chytrids + true fungi) + (choanoflagellates + Metazoa). These two clusters themselves require appropriately defined names. This taxon includes the Myxospora (previously thought of as a group of protozoa) as a subset of the Cnidaria; and the Microspora are placed with the fungi in line with recent molecular evidence.