Excavate
Encyclopedia
The excavates are a major kingdom
of unicellular eukaryote
s, often known as Excavata. The phylogenetic category Excavata, proposed by Cavalier-Smith in 2002, contains a variety of free-living and symbiotic forms, and also includes some important parasites of humans.
e. Most excavates have two, four, or more flagella
and many have a conspicuous ventral feeding groove with a characteristic ultrastructure
, supported by microtubule
s. However, various groups that lack these traits may be considered excavates based on genetic evidence (primarily phylogenetic trees of molecular sequences).
The closest that the excavates come to multicellularity are the Acrasidae slime molds. Like other cellular slime molds, they live most of their life as single cells, but will sometimes assemble into a larger cluster.
Heterolobosea (Percolozoa) and Euglenozoa appear to be particularly close relatives, and are united by the presence of discoid crista
e within the mitochondria
(Superphylum Discicristata
). More recently a close relationship has been shown between Discicristata and Jakobida. Most jakobids have tubular cristae, like most other protists, while the metamonads are unusual in having lost classical mitochondria—instead they have 'hydrogenosomes', 'mitosome
s' or uncharacterised organelles. In addition to the groups mentioned in the table above, the genus Malawimonas
is generally considered to be a member of Excavata owing to its typical excavate morphology, and phylogenetic affinity to excavate groups in some molecular phylogenies. However, its position among excavates remains elusive.
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 it seems like there are several clades within the excavates which are monophyletic.
Certain excavates are often considered among the most primitive eukaryote
s, 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
, as has been seen with some other groups, for example, microsporidia.
Kingdom (biology)
In biology, kingdom is a taxonomic rank, which is either the highest rank or in the more recent three-domain system, the rank below domain. Kingdoms are divided into smaller groups called phyla or divisions in botany...
of unicellular eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s, often known as Excavata. The phylogenetic category Excavata, proposed by Cavalier-Smith in 2002, contains a variety of free-living and symbiotic forms, and also includes some important parasites of humans.
Characteristics
Many excavates lack 'classical' mitochondria - these organisms are often referred to as 'amitochondriate', although most, perhaps all, retain a mitochondrial organelle in greatly modified form. Others have mitochondria with tubular, discoidal, or in some cases, laminar cristaCrista
Cristae are the internal compartments formed by the inner membrane of a mitochondrion. They are studded with proteins, including ATP synthase and a variety of cytochromes. The maximum surface for chemical reactions to occur is within the mitochondria...
e. Most excavates have two, four, or more flagella
Flagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
and many have a conspicuous ventral feeding groove with a characteristic ultrastructure
Ultrastructure
Ultrastructure is the detailed structure of a biological specimen, such as a cell, tissue, or organ, that can be observed by electron microscopy...
, supported by microtubule
Microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
s. However, various groups that lack these traits may be considered excavates based on genetic evidence (primarily phylogenetic trees of molecular sequences).
The closest that the excavates come to multicellularity are the Acrasidae slime molds. Like other cellular slime molds, they live most of their life as single cells, but will sometimes assemble into a larger cluster.
Subgroups
Excavates are classified into four major subgroups at the phylum/superphylum level. These are shown in the table below.| Superphylum | Phylum | Representative genera | Description |
|---|---|---|---|
| Discoba or JEH | Euglenozoa Euglenozoa The Euglenozoa are a large group of flagellate protozoa. They include a variety of common free-living species, as well as a few important parasites, some of which infect humans. There are two main subgroups, the euglenids and kinetoplastids... |
e.g. Euglena Euglena Euglena is a genus of unicellular flagellate protists. It is the best known and most widely studied member of the phylum Euglenozoa , a diverse group containing some 44 genera and at least 800 species. Species of Euglena are found in fresh and salt waters... , Trypanosoma Trypanosoma Trypanosoma is a genus of kinetoplastids , a monophyletic group of unicellular parasitic flagellate protozoa. The name is derived from the Greek trypano and soma because of their corkscrew-like motion. All trypanosomes are heteroxenous and are transmitted via a vector... |
Many important parasites, one large group with plastids (chloroplasts) |
| Percolozoa Percolozoa The Percolozoa are a group of colourless protozoa, including many that can transform between amoeboid, flagellate, and encysted stages.-Terminology and classification:... (Heterolobosea) |
e.g. Naegleria Naegleria Naegleria is a eukaryotic genus. The genus was named after French zoologist Mathieu Naegler.Species include:* Naegleria fowleri, which causes a rare and generally fatal disease in humans* Naegleria gruberi-External links:... , Acrasis |
Most alternate between flagellate and amoeboid Amoeboid Amoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amœba" are often used interchangeably even by biologists, and especially refer to a creature moving by using pseudopodia. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to... forms |
|
| Jakobida (see also Loukozoa Loukozoa Loukozoa is a paraphyletic taxon used in some classifications of excavate protists.The largest group within Loukozoa is the "jakobids". Loukozoa, however, also includes Malawimonas... ) |
e.g. Jakoba Jakoba Jakoba is a genus of Excavata.It includes the species:* Jakoba libera.* Jakoba bahamensis... , Reclinomonas Reclinomonas Reclinomonas a genus of small protozoans that feed on bacteria. Reclinomonas americana has 98 genes encoded by mitochondrial DNA, the highest number among organisms analyzed up to date.It has been described as an Excavate.-External links:*... |
Free-living, sometimes loricate flagellates, with very gene-rich mitochondrial genomes | |
| Metamonada or POD | Preaxostyla | e.g. Oxymonads, Trimastix Trimastix Trimastix is an Excavate grouping.One species is Trimastix pyriformis.... |
Amitochontriate flagellates, either free-living (Trimastix Trimastix Trimastix is an Excavate grouping.One species is Trimastix pyriformis.... ) or living in the hindguts of insects |
| Fornicata | e.g. Giardia Giardia Giardia is a genus of anaerobic flagellated protozoan parasites of the phylum Metamonada in the supergroup "Excavata" that colonise and reproduce in the small intestines of several vertebrates, causing giardiasis, commonly known as Beaver fever... , Carpediemonas Carpediemonas Carpediemonas is a genus of protozoa. The known species, Carpediemonas membranifera, is a small flagellate that was originally isolated from anaerobic intertidal sediment. The group is related, distantly, to diplomonads... |
Amitochondriate, mostly symbiotes and parasites of animals. | |
| Parabasalia | e.g. Trichomonas Trichomonas Trichomonas is a genus of anaerobic protists that are parasites of vertebrates. They are included with the parabasalids.Species of Trichomonas include:*Trichomonas vaginalis, an organism generally living inside the vagina of humans... |
Amitochondriate flagellates, generally intestinal commensals Commensalism In ecology, commensalism is a class of relationship between two organisms where one organism benefits but the other is neutral... of insects. Some human pathogens. |
Heterolobosea (Percolozoa) and Euglenozoa appear to be particularly close relatives, and are united by the presence of discoid crista
Crista
Cristae are the internal compartments formed by the inner membrane of a mitochondrion. They are studded with proteins, including ATP synthase and a variety of cytochromes. The maximum surface for chemical reactions to occur is within the mitochondria...
e within the mitochondria
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
(Superphylum Discicristata
Discicristata
Discicristata is a proposed eukaryotic clade.It consists of Euglenozoa plus Percolozoa.It was proposed that Discicristata plus Cercozoa yielded Cabozoa....
). More recently a close relationship has been shown between Discicristata and Jakobida. Most jakobids have tubular cristae, like most other protists, while the metamonads are unusual in having lost classical mitochondria—instead they have 'hydrogenosomes', 'mitosome
Mitosome
A mitosome is an organelle found in some unicellular eukaryotic organisms. The mitosome has only recently been found and named, and its function has not yet been well characterized. It was termed a 'crypton' by one group, but that name is no longer in use....
s' or uncharacterised organelles. In addition to the groups mentioned in the table above, the genus Malawimonas
Malawimonas
Malawimonas is an excavate genus.Species include:* Malawimonas californiana* Malawimonas jakobiformis-External links:*...
is generally considered to be a member of Excavata owing to its typical excavate morphology, and phylogenetic affinity to excavate groups in some molecular phylogenies. However, its position among excavates remains elusive.
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 it seems like there are several clades within the excavates which are monophyletic.
Certain excavates are often considered among the most primitive eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s, 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
Long branch attraction
Long branch attraction is a phenomenon in phylogenetic analyses when rapidly evolving lineages are inferred to be closely related, regardless of their true evolutionary relationships. For example, in DNA sequence-based analyses, the problem arises when sequences from two lineages evolve rapidly...
, as has been seen with some other groups, for example, microsporidia.