Excavata is a former supergroup of unicellular organisms belonging to the domainEukaryota.[1][2][3] It was first suggested by Simpson and Patterson in 1999[4][5] and introduced by Thomas Cavalier-Smith in 2002 as a formal taxon. It contains a variety of free-living and symbiotic forms, and includes some important parasites of humans, such as Giardia and Trichomonas.[6] Excavates were formerly considered to be included in the now obsolete Protista kingdom.[7] They were classified based on their flagellar structures,[5] and were considered to be the most basal flagellate lineage.[8]
Phylogenomic analyses indicate that the Excavata are polyphyletic, not forming a single natural group.[9] Excavata has therefore been split into three groups, not all closely related: Discobids, Metamonads and Malawimonads.[10][11][12] Except for Euglenozoa, they are all non-photosynthetic.
Characteristics
Most excavates are unicellular, heterotrophic flagellates. Only the Euglenozoa are photosynthetic. In some (particularly anaerobic intestinal parasites), the mitochondria have been greatly reduced.[6] Some excavates lack "classical" mitochondria, and are called "amitochondriate", although most retain a mitochondrial organelle in greatly modified form (e.g. a hydrogenosome or mitosome). Among those with mitochondria, the mitochondrial cristae may be tubular, discoidal, or in some cases, laminar. Most excavates have two, four, or more flagella.[5] Many have a conspicuous ventral feeding groove with a characteristic ultrastructure, supported by microtubules—the "excavated" appearance of this groove giving the organisms their name.[4][7] However, various groups that lack these traits may be considered excavates based on genetic evidence (primarily phylogenetic trees of molecular sequences).[7]
The Acrasidae slime molds are the only excavates to exhibit limited multicellularity. Like other cellular slime 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 a monophyletic group.[13] The malawimonads often do not branch with the other Excavata.[14]
Excavates were thought to include multiple groups:
Euglenozoa and Heterolobosea (Percolozoa) or Eozoa (Cavalier-Smith) appear to be particularly close relatives, and are united by the presence of discoid cristae within the mitochondria (Superphylum Discicristata). A close relationship has been shown between Discicristata and Jakobida,[15] the latter having tubular cristae like most other protists, and hence were united under the taxon name Discoba, which was proposed for this apparently monophyletic group.[2]
Metamonads
Metamonads are unusual in having lost classical mitochondria—instead they have hydrogenosomes, mitosomes or uncharacterised organelles. The oxymonad Monocercomonoides is reported to have completely lost homologous organelles.[16]
Malawimonads
The malawimonads 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.[3]
Ancyromonads
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.[17]
The conventional explanation for the origin of the Eukaryotes is that a Heimdallarchaeia[18] or another Archaea acquired an alphaproteobacterium as an endosymbiont, and that this became the mitochondrion, the organelle providing oxidative respiration to the eukaryotic cell. Caesar al Jewari and Sandra Baldauf argue instead that the Eukaryotes possibly started with an endosymbiosis event of a deltaproteobacterium or gammaproteobacterium, accounting for the otherwise unexplained presence of anaerobic bacterial enzymes in metamonada in this scenario. The sister of the Preaxostyla within Metamonada represents the rest of the Eukaryotes which acquired a alphaproteobacterium.[19]
Phylogeny
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 Eukaryote appear to have emerged within the traditional metamonada group as sister 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. By cladistically granting these groups to metamonada, the following tree is reached. The rest of the Eukaryotes emerged within the Excavata as sister of the Discoba.[19]
^ abSimpson, Alastair G.B.; Patterson, David J. (Dec 1999). "The ultrastructure of Carpediemonas membranifera (Eukaryota) with reference to the 'excavate hypothesis'". European Journal of Protistology. 35 (4): 353–370. doi:10.1016/S0932-4739(99)80044-3.
^ abcCavalier-Smith, T (2002). "The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa". International Journal of Systematic and Evolutionary Microbiology. 52 (2): 297–354. doi:10.1099/00207713-52-2-297. PMID11931142.
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