| "Excavates" Temporal range:Neoproterozoic–present | |
|---|---|
 | |
| Giardia lamblia, a parasitic diplomonad | |
Scientific classification (obsolete as paraphyletic) | |
| Domain: | Eukaryota |
| Informal group: | Excavata (Cavalier-Smith), 2002 |
| Phyla andclasses | |
See text | |
| Cladistically included but traditionally excluded taxa | |
Excavata is an extensive and diverse butparaphyletic group of unicellularEukaryota.[1][2] The group was first suggested by Simpson and Patterson in 1999[3][4] and the name latinized and assigned a rank byThomas Cavalier-Smith in 2002. It contains a variety of free-living and symbiotic protists, and includes some important parasites of humans such asGiardia andTrichomonas.[5] Excavates were formerly considered to be included in the now obsoleteProtista kingdom.[6] They were distinguished from other lineages based on electron-microscopic information about how the cells are arranged (they have a distinctiveultrastructural identity).[4] They are considered to be abasalflagellate lineage.[7]
On the basis of phylogenomic analyses, the group was shown to contain three widely separated eukaryote groups, thediscobids,metamonads, andmalawimonads.[8][9][10][11] A current view of the composition of the excavates is given below, indicating that the group is paraphyletic. Except for someEuglenozoa, all are non-photosynthetic.
Most excavates are unicellular, heterotrophic flagellates. Only someEuglenozoa are photosynthetic. In some (particularly anaerobic intestinal parasites), the mitochondria have been greatly reduced.[5] Some excavates lack "classical"mitochondria, and are called "amitochondriate", although most retain a mitochondrial organelle in greatly modified form (e.g. ahydrogenosome ormitosome). Among those with mitochondria, the mitochondrialcristae may be tubular,discoidal, or in some cases, laminar. Most excavates have two, four, or moreflagella.[4] Many have a conspicuous ventral feeding groove with a characteristicultrastructure, supported bymicrotubules—the "excavated" appearance of this groove giving the organisms their name.[3][6] However, various groups that lack these traits are considered to be derived excavates based on genetic evidence (primarily phylogenetic trees of molecular sequences).[6]
TheAcrasidae slime molds are the only excavates to exhibit limited multicellularity. Like other cellularslime molds, they live most of their life as single cells, but will sometimes assemble into larger clusters.
Excavate relationships were always uncertain, suggesting that they are not amonophyletic group.[12] Phylogenetic analyses often do not place malawimonads on the same branch as the other Excavata.[13]
Excavates were thought to include multiple groups:
| Kingdom/Superphylum | Included taxa | Representative genera (examples) | Description |
|---|---|---|---|
| Discoba or JEH or Eozoa | Tsukubea | Tsukubamonas | |
| Euglenozoa | Euglena,Trypanosoma | Many important parasites, one large group with plastids (chloroplasts) | |
| Heterolobosea (Percolozoa) | Naegleria,Acrasis | Most alternate between flagellate andamoeboid forms | |
| Jakobea | Jakoba,Reclinomonas | Free-living, sometimes loricate flagellates, with very gene-rich mitochondrial genomes | |
| Metamonada or POD | Preaxostyla | Oxymonads,Trimastix | Amitochondriate flagellates, either free-living (Trimastix,Paratrimastix) or living in the hindguts of insects |
| Fornicata | Giardia,Carpediemonas | Amitochondriate, mostly symbiotes and parasites of animals. | |
| Parabasalia | Trichomonas | Amitochondriate flagellates, generally intestinalcommensals of insects. Some human pathogens. | |
| Anaeramoeba | Anaeramoeba | Anaerobic protists withhydrogenosomes instead of mitochondria. | |
| Malawimonada | Malawimonadida | Malawimonas |
Euglenozoa and Heterolobosea (Percolozoa) or Eozoa (as named by Cavalier-Smith[14]) appear to be particularly close relatives, and are united by the presence of discoidcristae within themitochondria (SuperphylumDiscicristata). A close relationship has been shown between Discicristata andJakobida,[15] the latter having tubular cristae like most other protists, and hence were united under the taxon nameDiscoba, which was proposed for this supposedly monophyletic group.[1] Thisclade was defined as anode-based clade, receiving the definition "The least inclusive clade containingJakoba libera (Ruinen, 1938) Patterson, 1990;Andalucia godoyi, Laraet al., 2006;Euglena gracilis Klebs 1883; andNaegleria gruberi (Schardinger, 1899) Alexeieff, 1912." Alternatively, the clade has been termed the jakobid, euglenazoan and heterolobosean groupJEH.[16]
Metamonads are unusual in not having classical mitochondria—instead they havehydrogenosomes,mitosomes or uncharacterised organelles. The oxymonadMonocercomonoides is reported to have completely lost homologous organelles. There are competing explanations.[17][18]
Themalawimonads have been proposed to be members of Excavata owing to their typical excavate morphology, and phylogenetic affinity to other excavate groups in some molecular phylogenies. However, their position among eukaryotes remains elusive.[2]
Ancyromonads are small free-living cells with a narrow longitudinal groove down one side of the cell. The ancyromonad groove is not used for "suspension feeding", unlike in "typical excavates" (e.g. malawimonads, jakobids,Trimastix,Carpediemonas,Kiperferlia, etc). Ancyromonads instead capture prokaryotes attached to surfaces. The phylogenetic placement of ancyromonads is poorly understood (in 2020), however some phylogenetic analyses place them as close relatives of malawimonads.[9]
The conventional explanation for the origin of the Eukaryotes is that aheimdallarchaeian or another Archaea acquired analphaproteobacterium[19] as anendosymbiont, and that this became themitochondrion, theorganelle providingoxidative respiration to the eukaryotic cell.[20]
Caesar al Jewari and Sandra Baldauf argue instead that the Eukaryotes possibly started with an endosymbiosis event of aDeltaproteobacterium orGammaproteobacterium, accounting for the otherwise unexplained presence of anaerobic bacterial enzymes in Metamonada. The sister of the Preaxostyla within Metamonada represents the rest of the Eukaryotes which acquired an Alphaproteobacterium. In their scenario, thehydrogenosome andmitosome, both conventionally considered "mitochondrion-derived organelles", would predate the mitochondrion, and instead be derived from the earlier symbiotic bacterium.[18]
In 2023, using molecular phylogenetic analysis of 186 taxa, Al Jewari and Baldauf proposed a phylogenetic tree with the metamonad Parabasalia as basal Eukaryotes. Discoba and the rest of the Eukaryota appear to have emerged assister taxon to the Preaxostyla, incorporating a single alphaproteobacterium as mitochondria by endosymbiosis. Thus the Fornicata are more closely related to e.g. animals than to Parabasalia. The rest of the Eukaryotes emerged within the Excavata as sister of the Discoba; as they are within the same clade but are notcladistically considered part of the Excavata yet, the Excavata are in this analysis highly paraphyletic.[18]
| "Excavata" | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
TheAnaeramoeba are associated with Parabasalia, but could turn out to be more basal as the root of a tree is often difficult to pinpoint.[21]
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