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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 Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined ** Domain of definition of a partial function ** Natural domain of a partial function **Domain of holomorphy of a function * ...
Eukaryota Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...
. 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 ''Giardia'' ( or ) is a genus of anaerobic flagellated protozoan parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and ...
'' 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 e ...
kingdom. They are classified based on their flagellar structures, and they are considered to be the most basal
flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and thei ...
lineage. Phylogenomic analyses split the members of Excavata into three different and not all closely related groups: Discobids, Metamonads and Malawimonads. Except for
Euglenozoa Euglenozoa are a large group of flagellate Discoba. They include a variety of common free-living species, as well as a few important parasites, some of which infect humans. Euglenozoa are represented by three major clades, i.e., Kinetoplastea, D ...
, they are all non-photosynthetic.


Characteristics

Most excavates are unicellular, heterotrophic flagellates. Only the
Euglenozoa Euglenozoa are a large group of flagellate Discoba. They include a variety of common free-living species, as well as a few important parasites, some of which infect humans. Euglenozoa are represented by three major clades, i.e., Kinetoplastea, D ...
are photosynthetic. In some (particularly anaerobic intestinal parasites), the mitochondria have been greatly reduced. Some excavates lack "classical"
mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...
, and are called "amitochondriate", although most retain a mitochondrial organelle in greatly modified form (e.g. a hydrogenosome 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
crista A crista (; plural cristae) is a fold in the inner membrane of a mitochondrion. The name is from the Latin for ''crest'' or ''plume'', and it gives the inner membrane its characteristic wrinkled shape, providing a large amount of surface area fo ...
e may be tubular, discoidal, or in some cases, laminar. Most excavates have two, four, or more
flagella A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have fro ...
. Many have a conspicuous ventral feeding groove with a characteristic ultrastructure, supported by
microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27  nm and have an inner diameter between 1 ...
s—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 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''), 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
crista A crista (; plural cristae) is a fold in the inner membrane of a mitochondrion. The name is from the Latin for ''crest'' or ''plume'', and it gives the inner membrane its characteristic wrinkled shape, providing a large amount of surface area fo ...
e within the
mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...
(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 hydrogenosomes,
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 trailing ...
'' is reported to have completely lost homologous organelles.


Monophyly

Excavate relationships are still uncertain; it is possible that they are not a
monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gr ...
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
eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...
s, based partly on their placement in many evolutionary trees. This could encourage proposals that excavates are a
paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...
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 Ancyromonadida or Planomonadida is a small group of biflagellated protists found in the soil and in aquatic habitats, where they feed on bacteria.Cavalier-Smith, T. (2013)Early evolution of eukaryote feeding modes, cell structural diversity, and ...
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 Euglenozoa are a large group of flagellate Discoba. They include a variety of common free-living species, as well as a few important parasites, some of which infect humans. Euglenozoa are represented by three major clades, i.e., Kinetoplastea, D ...
: Euglenoida) 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'' 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'' (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:
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 ''Giardia'' ( or ) is a genus of anaerobic flagellated protozoan parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and ...
'' 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