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Excavata is an extensive and diverse but paraphyletic group of unicellular Eukaryota. The group was first suggested by Simpson and Patterson in 1999 and the name latinized and assigned a rank by Thomas Cavalier-Smith in 2002. It contains a variety of free-living and symbiotic protists, and includes some important parasites of humans such as Giardia and Trichomonas. Excavates were formerly considered to be included in the now obsolete Protista kingdom. They were distinguished from other lineages based on electron-microscopic information about how the cells are arranged. They are considered to be a basal flagellate lineage.
The metamonads are a large group of flagellate amitochondriate microscopic eukaryotes. Their composition is not entirely settled, but they include the retortamonads, diplomonads, and possibly the parabasalids and oxymonads as well. These four groups are all anaerobic, occurring mostly as symbiotes or parasites of animals, as is the case with Giardia lamblia which causes diarrhea in mammals.
Pelomyxa is a genus of giant flagellar amoebae, usually 500–800 μm but occasionally up to 5 mm in length, found in anaerobic or microaerobic bottom sediments of stagnant freshwater ponds or slow-moving streams.
The retortamonads are a small group of flagellates, most commonly found in the intestines of animals as commensals, although a free-living species called the Chilomastix cuspidata exists. They are grouped under the taxon, Archezoa. They are usually around 5-20 μm in length, and all of their small subunit ribosomal RNA gene sequences are very similar to each other. There are two genera: Retortamonas with two flagella, and Chilomastix with four. In both cases there are four basal bodies anterior to a prominent feeding groove, and one flagellum is directed back through the cell, emerging from the groove.
The Oxymonads are a group of flagellated protists found exclusively in the intestines of animals, mostly termites and other wood-eating insects. Along with the similar parabasalid flagellates, they harbor the symbiotic bacteria that are responsible for breaking down cellulose. There is no evidence for presence of mitochondria in oxymonads and 3 species have been shown to completely lack any molecular markers of mitochondria.
Carpediemonas is genus of Metamonada, and belongs to the group Excavata. This organism is a unicellular flagellated eukaryote that was first discovered in substrate samples from the Great Barrier Reef. Carpediemonas can be found in anaerobic intertidal sediment, where it feeds on bacteria. A feature of this species is the presence of a feeding groove, a characteristic of the excavates. Like most other metamonads, Carpediemonas does not rely on an aerobic mitochondrion to produce energy. Instead, it contains hydrogenosomes that are used to produce ATP. This organism has two flagella: a posterior one used for feeding on the substrate, and an anterior one that moves in a slower sweeping motion. Carpediemonas is assigned to the fornicates, where similar Carpediemonas-like organisms are used in researching the evolution within excavates. Although Carpediemonas is a member of the metamonads, it is unusual in the sense that it is free-living and has three basal bodies.
Anaeromonadea, also known as Preaxostyla, is a class of excavate protists, comprising the oxymonads, Trimastix, and Paratrimastix. This group is studied as a model system for reductive evolution of mitochondria, because it includes both organisms with anaerobic mitochondrion-like organelles, and those that have completely lost their mitochondria.
Jakobids are an order of free-living, heterotrophic, flagellar eukaryotes in the supergroup Excavata. They are small, and can be found in aerobic and anaerobic environments. The order Jakobida, believed to be monophyletic, consists of only twenty species at present, and was classified as a group in 1993. There is ongoing research into the mitochondrial genomes of jakobids, which are unusually large and bacteria-like, evidence that jakobids may be important to the evolutionary history of eukaryotes.
Colpodella is a genus of alveolates comprising 5 species, and two further possible species: They share all the synapomorphies of apicomplexans, but are free-living, rather than parasitic. Many members of this genus were previously assigned to a different genus - Spiromonas.
Proteromonas is a genus of single-celled biflagellated microbial eukaryotes belonging to the Superphylum Stramenopiles which are characterized by the presence of tripartite, hair-like structures on the anteriorly-directed larger of the two flagella. Proteromonas on the other hand are notable by having tripartite hairs called somatonemes not on the flagella but on the posterior of the cell. Proteromonas are closely related to Karotomorpha and Blastocystis, which belong to the Opalines group.
Mastigamoeba is a genus of pelobionts, and treated by some as members of the Archamoebae group of protists. Mastigamoeba are characterized as anaerobic, amitochondriate organisms that are polymorphic. Their dominant life cycle stage is as an amoeboid flagellate. Species are typically free living, though endobiotic species have been described.
Psalteriomonas is a genus of excavates in the group of Heterolobosea. The genus was first discovered and named in 1990. It contains amoeboflagellate cells that live in freshwater anaerobic sediments all over the world. The microtubule-organizing ribbon and the associated microfibrillar bundles of the mastigote system is the predominant feature in Psalteriomonas. This harp-shaped complex gives rise to the name of this genus. Psalteriomonasforms an endosymbiotic relationship with methanogenic bacteria, especially with Methanobacterium formicicum There are currently three species in this genus: P. lanterna, P. vulgaris, and P. magna.
Oxymonas is a genus of Excavata.
Breviatea, commonly known as breviate amoebae, are a group of free-living, amitochondriate protists with uncertain phylogenetic position. They are biflagellate, and can live in anaerobic (oxygen-free) environments. They are currently placed in the Obazoa clade. They likely do not possess vinculin proteins. Their metabolism relies on fermentative production of ATP as an adaptation to their low-oxygen environment.
Monocercomonoides is a genus of flagellate Excavata belonging to the order Oxymonadida. It was established by Bernard V. Travis and was first described as those with "polymastiginid flagellates having three anterior flagella and a trailing one originating at a single basal granule located in front of the anteriorly positioned nucleus, and a more or less well-defined axostyle". It is the first eukaryotic genus to be found to completely lack mitochondria, and all hallmark proteins responsible for mitochondrial function. The genus also lacks any other mitochondria related organelles (MROs) such as hydrogenosomes or mitosomes. Data suggests that the absence of mitochondria is not an ancestral feature, but rather due to secondary loss. Monocercomonoides sp. was found to obtain energy through an enzymatic action of nutrients absorbed from the environment. The genus has replaced the iron-sulfur cluster assembly pathway with a cytosolic sulfur mobilization system, likely acquired by horizontal gene transfer from a eubacterium of a common ancestor of oxymonads. These organisms are significant because they undermine assumptions that eukaryotes must have mitochondria to properly function. The genome of Monocercomonoides exilis has approximately 82 million base pairs, with 18 152 predicted protein-coding genes.
Stygiella /ˌstɪ.d͡ʒiˈɛ.lə/ is a genus of free-living marine flagellates belonging to the family Stygiellidae in the jakobids (excavata).
Anaeramoeba is a genus of anaerobic protists of uncertain phylogenetic position, first described in 2016.
Stygiellidae is a family of free-living marine flagellates belonging to the order Jakobida, a deep-branching lineage within the eukaryotic supergroup Discoba. They are unicellular organisms that commonly inhabit anoxic, sulfide-rich and ammonium-rich marine habitats worldwide.
Paratrimastix is a genus of free-living freshwater anaerobic excavate protists from the group Metamonada, that was segregated from the genus Trimastix in 2015. The best studied species is Paratrimastix pyriformis.
Paratrimastix pyriformis is a species of free-living anaerobic freshwater bacteriovorous flagellated protists formerly known as Trimastix pyriformis and Tetramitus pyriformis.
References
- ↑ Cavalier-Smith T (November 2003). "The excavate protozoan phyla Metamonada Grassé emend. (Anaeromonadea, Parabasalia, Carpediemonas, Eopharyngia) and Loukozoa emend. (Jakobea, Malawimonas): their evolutionary affinities and new higher taxa". Int. J. Syst. Evol. Microbiol. 53 (Pt 6): 1741–58. doi: 10.1099/ijs.0.02548-0 . PMID 14657102.
- 1 2 3 4 Kent, W. S. (1881). A manual of the infusoria: including a description of all known flagellate, ciliate, and tentaculiferous protozoa, British and foreign, and an account of the organization and affinities of the sponges (Vol. 1). David Bogue, London, England.
- 1 2 3 4 5 6 7 Brugerolle, G., & Patterson, D. (1997). Ultrastructure of Trimastix convexa Hollande, an amitochondriate anaerobic flagellate with a previously undescribed organization. European Journal of Protistology, 33(2), 121-130.
- ↑ Alastair Simpson & Vladimir Hampl. "Tree of Life: Trimastix". Tree of Life. Retrieved July 24, 2012.
- ↑ Hampl V, Silberman JD, Stechmann A, Diaz-Triviño S, Johnson PJ, Roger AJ (2008). Redfield R (ed.). "Genetic evidence for a mitochondriate ancestry in the 'amitochondriate' flagellate Trimastix pyriformis". PLOS ONE. 3 (1): e1383. Bibcode:2008PLoSO...3.1383H. doi: 10.1371/journal.pone.0001383 . PMC 2148110 . PMID 18167542.
- ↑ Zhang, Qianqian; Táborský, Petr; Silberman, Jeffrey D.; Pánek, Tomáš; Čepička, Ivan; Simpson, Alastair G.B. (September 2015). "Marine Isolates of Trimastix marina Form a Plesiomorphic Deep-branching Lineage within Preaxostyla, Separate from Other Known Trimastigids (Paratrimastix n. gen.)". Protist. 166 (4): 468–491. doi:10.1016/j.protis.2015.07.003. ISSN 1434-4610. PMID 26312987.
- 1 2 Simpson, Alastair G.B.; Bernard, Catherine; Patterson, David J. (September 2000). "The ultrastructure of Trimastix marina Kent 1880 (Eukaryota), an excavate flagellate". European Journal of Protistology. 36 (3): 229–251. doi:10.1016/s0932-4739(00)80001-2. ISSN 0932-4739.
- 1 2 3 Novák, Lukáš V. F.; Treitli, Sebastian C.; Pyrih, Jan; Hałakuc, Paweł; Pipaliya, Shweta V.; Vacek, Vojtěch; Brzoň, Ondřej; Soukal, Petr; Eme, Laura; Dacks, Joel B.; Karnkowska, Anna; Eliáš, Marek; Hampl, Vladimír (2023-12-07). "Genomics of Preaxostyla Flagellates Illuminates the Path Towards the Loss of Mitochondria". PLOS Genetics. 19 (12): e1011050. doi: 10.1371/journal.pgen.1011050 . ISSN 1553-7404. PMC 10703272 . PMID 38060519.