![Excavata cell schemes](https://upload.wikimedia.org/wikipedia/commons/5/5c/Excavata_cell_schemes.svg)
Excavata is a major supergroup of
unicellular organism
A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms a ...
s belonging to the
domain Eukaryota.
It was first suggested by Simpson and Patterson in 1999
and introduced by
Thomas Cavalier-Smith
Thomas (Tom) Cavalier-Smith, FRS, FRSC, NERC Professorial Fellow (21 October 1942 – 19 March 2021), was a professor of evolutionary biology in the Department of Zoology, at the University of Oxford.
His research has led to discov ...
in 2002 as a formal taxon. It contains a variety of free-living and symbiotic forms, and also includes some important parasites of humans, including ''
Giardia'' and ''
Trichomonas
''Trichomonas'' is a genus of anaerobic excavate parasites of vertebrates. It was first discovered by Alfred François Donné in 1836 when he found these parasites in the pus of a patient suffering from vaginitis, an inflammation of the vagina. ...
''.
Excavates were formerly considered to be included in the now obsolete
Protista
A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...
kingdom.
They are classified based on their flagellar structures,
and they are considered to be the most basal
flagellate lineage. Phylogenomic analyses split the members of Excavata into three different and not all closely related groups:
Discobids
Excavata is a major supergroup of unicellular organisms belonging to the Domain (biology), domain Eukaryota. It was first suggested by Simpson and Patterson in 1999 and introduced by Thomas Cavalier-Smith in 2002 as a formal taxon. It contains ...
,
Metamonads and
Malawimonads. 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.
[ Some excavates lack "classical" mitochondria, and are called "amitochondriate", although most retain a mitochondrial organelle in greatly modified form (e.g. a ]hydrogenosome
A hydrogenosome is a membrane-enclosed organelle found in some anaerobic ciliates, flagellates, and fungi. Hydrogenosomes are highly variable organelles that have presumably evolved from protomitochondria to produce molecular hydrogen and ATP i ...
or mitosome
A mitosome is an organelle found in some unicellular eukaryotic organisms, like in members of the supergroup Excavata. The mitosome was found and named in 1999, and its function has not yet been well characterized. It was termed a ''crypton'' by ...
). Among those with mitochondria, the mitochondrial cristae may be tubular, discoidal, or in some cases, laminar. Most excavates have two, four, or more flagella. Many have a conspicuous ventral feeding groove with a characteristic ultrastructure
Ultrastructure (or ultra-structure) is the architecture of cells and biomaterials that is visible at higher magnifications than found on a standard optical light microscope. This traditionally meant the resolution and magnification range of a co ...
, supported by microtubules—the "excavated" appearance of this groove giving the organisms their name. However, various groups that lack these traits may be considered excavates based on genetic evidence (primarily phylogenetic trees of molecular sequences).
The Acrasidae
The family Acrasidae ( ICZN, or Acrasiomycota, ICBN) is a family of slime molds which belongs to the excavate group Percolozoa. The name element - comes from the Greek ''akrasia'', meaning "acting against one's judgement". This group consists ...
slime molds are the only excavates to exhibit limited multicellularity. Like other cellular slime molds
Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic organisms with a life cycle that includes a free-living single-celled stage and the formation of spores. Spores are often produced in macroscopic mu ...
, they live most of their life as single cells, but will sometimes assemble into larger clusters.
Classification
Excavates are classified into six major subdivisions at the phylum/class level. These are shown in the table below. An additional group, Malawimonadida (e.g. ''Malawimonas
''Malawimonas'' is a Loukozoa genus, possible sister of the Podiata.
History of the discovery of Malawimonads
In 1993, Charles J O’ Kelly studied the jakobid groups flagellates and implications for the early diversification of eukaryotes and ...
''), may also be included amongst excavates, though phylogenetic evidence is equivocal.
Discoba or JEH clade
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
Discicristata is a proposed eukaryotic clade. It consists of Euglenozoa plus Percolozoa.
It was proposed that Discicristata plus Cercozoa yielded Cabozoa. Another proposal is to group Discicristata with Jakobida into Discoba superphylum.
See ...
). A close relationship has been shown between Discicristata and Jakobida, 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.
Metamonads
Metamonads are unusual in having lost classical mitochondria—instead they have hydrogenosome
A hydrogenosome is a membrane-enclosed organelle found in some anaerobic ciliates, flagellates, and fungi. Hydrogenosomes are highly variable organelles that have presumably evolved from protomitochondria to produce molecular hydrogen and ATP i ...
s, mitosome
A mitosome is an organelle found in some unicellular eukaryotic organisms, like in members of the supergroup Excavata. The mitosome was found and named in 1999, and its function has not yet been well characterized. It was termed a ''crypton'' by ...
s or uncharacterised organelles. The oxymonad ''Monocercomonoides
''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 trailin ...
'' is reported to have completely lost homologous organelles.
Monophyly
Excavate relationships are still uncertain; it is possible that they are not a monophyletic group. The monophyly of the excavates is far from clear, although there seem to be several clades within the excavates that are monophyletic.
Certain excavates are often considered among the most primitive eukaryotes, based partly on their placement in many evolutionary trees. This could encourage proposals that excavates are a paraphyletic grade that includes the ancestors of other living eukaryotes. However, the placement of certain excavates as 'early branches' may be an analysis artifact caused by long branch attraction
In phylogenetics, long branch attraction (LBA) is a form of systematic error whereby distantly related lineages are incorrectly inferred to be closely related. LBA arises when the amount of molecular or morphological change accumulated within a lin ...
, as has been seen with some other groups, for example, microsporidia. The malawimonads often do not branch with the other Excavata.
Malawimonads
The malawimonads are generally considered 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.[
]
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. Consequently, it is possible that ancyromonads are relevant for understand the evolution of 'true' excavates.
Cladogram
Here is a proposed cladogram for the positioning of the Excavata, with the Eukaryote root in the excavates, mainly following Cavalier-Smith.
In this view, excavata is highly polyphyletic, and is proposed to be abandoned. In alternative view, the Discoba are sister to the rest of the Diphoda.
Gallery
File:Two Euglena.jpg, ''Euglena
''Euglena'' is a genus of single cell flagellate eukaryotes. It is the best known and most widely studied member of the class Euglenoidea, a diverse group containing some 54 genera and at least 200 species. Species of ''Euglena'' are found in f ...
'' ( Euglenozoa: Euglenoid
Euglenids (euglenoids, or euglenophytes, formally Euglenida/Euglenoida, ICZN, or Euglenophyceae, ICBN) are one of the best-known groups of flagellates, which are excavate eukaryotes of the phylum Euglenophyta and their cell structure is typical o ...
a)
File:TrypanosomaBrucei ProcyclicTrypomastigote SEM.jpg, ''Trypanosoma brucei
''Trypanosoma brucei'' is a species of parasitic kinetoplastid belonging to the genus '' Trypanosoma'' that is present in sub-Saharan Africa. Unlike other protozoan parasites that normally infect blood and tissue cells, it is exclusively extrace ...
'' (Euglenozoa: Kinetoplastida
Kinetoplastida (or Kinetoplastea, as a class) is a group of flagellated protists belonging to the phylum Euglenozoa, and characterised by the presence of an organelle with a large massed DNA called kinetoplast (hence the name). The organisms are ...
)
File:Bodo_saltans_-_400x_(13895749563).jpg, ''Bodo Bodo may refer to:
Ethnicity
* Boro people, an ethno-linguistic group mainly from Northwest Assam, India
* Bodo-Kachari people, an umbrella group from Nepal, India and Bangladesh that includes the Bodo people
Culture and language
* Boro cu ...
'' sp. (Euglenozoa: Kinetoplastida)
File:Percolomonas_sp.jpg, ''Percolomonas
''Percolomonas'' is a genus of free-living flagellate Heteroloboseans, forming a clade with ''Stephanopogon''.
The genus includes six described species (see infobox). However, ''P. cosmopolitus'' is likely a species complex containing multiple c ...
'' sp. (Percolozoa)
File:Stephanopogon_sp.jpg, '' Stephanopogon'' sp. (Percolozoa)
File:Naegleria_(formes).png, Stages of ''Naegleria
''Naegleria'' is a free living amoebae protist genus consisting of 47 described species often found in warm aquatic environments as well as soil habitats worldwide. It has three life cycle forms: the amoeboid stage, the cyst stage, and the flage ...
'' sp. (Percolozoa
The Percolozoa are a group of colourless, non-photosynthetic Excavata, including many that can transform between amoeboid, flagellate, and cyst stages.
Characteristics
Most Percolozoa are found as bacterivores in soil, fresh water and occasio ...
: Heterolobosea
The Percolozoa are a group of colourless, non-photosynthetic Excavata, including many that can transform between amoeboid, flagellate, and cyst stages.
Characteristics
Most Percolozoa are found as bacterivores in soil, fresh water and occasionall ...
)
File:Acrasis_rosea_31095.jpg, ''Acrasis rosea
Amoebae emerging from spores
''Acrasis rosea'' is a species of heterolobosean, previously described as a slime mold
Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic organisms with a life cycle th ...
'' (Percolozoa: Heterolobosea)
File:Jakobida.svg, Jakobids ( Jakobida)
File:Trichomonas Giemsa DPDx.JPG, ''Trichomonas vaginalis
''Trichomonas vaginalis'' is an anaerobic, flagellated protozoan parasite and the causative agent of a sexually transmitted disease called trichomoniasis. It is the most common pathogenic protozoan that infects humans in industrialized countri ...
'' ( Metamonada: Parabasalia
The parabasalids are a group of flagellated protists within the supergroup Excavata. Most of these eukaryotic organisms form a symbiotic relationship in animals. These include a variety of forms found in the intestines of termites and cockroaches ...
)
File:Metamonada_retortamonas_hexamita_giardia.svg, ''Retortamonas
''Retortamonas'' is a genus of flagellated excavates.Geiman, Q. M. 1932: Retortamonas caudacus (n. sp.), an Intestinal Flagellate from a Beetle Larva, Gyrinidae sp. Transactions of the American Microscopical Society 51(4), 219-224. Doi: DOI: 10 ...
'' sp., left ( Metamonada: Fornicata
Trichozoa is a group of excavates.
"Fornicata" is a similar grouping, but it excludes Parabasalia.[Tree](_blank)
a ...
: Retortamonadida
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 ...
)
File:Giardia_1.jpg, '' Giardia'' sp. (Metamonada: Fornicata: Diplomonadida
The diplomonads (Greek for "two units") are a group of flagellates, most of which are parasitic. They include ''Giardia duodenalis'', which causes giardiasis in humans. They are placed among the metamonads, and appear to be particularly close ...
)
References
External links
Open Tree of Life
Taxonomicon
Tree of Life Eukaryotes
Tree of Life: Jakobida
Tree of Life: Fornicata
{{Taxonbar, from=Q691551
Infrakingdoms
Taxa named by Thomas Cavalier-Smith
2002 introductions