The metamonads are a large group of

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 the ...

amitochondriate microscopic

eukaryote The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

s. They include the retortamonads,

diplomonad 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 ...

parabasalid The parabasalids are a group of flagellated protists within the supergroup Excavata. Most of these eukaryotic organisms form a symbiosis, symbiotic relationship in animals. These include a variety of forms found in the intestines of termites and ...

oxymonad The Oxymonads (or Oxymonadida) are a group of flagellated protists found exclusively in the intestines of animals, mostly termites and other Xylophagy, wood-eating insects. Along with the similar parabasalid flagellates, they harbor the Symbiosi ...

s, and a range of more poorly studied taxa, most of which are free-living flagellates. All metamonads are

anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: *Adhesive#Anaerobic, Anaerobic ad ...

(many being aerotolerant anaerobes), and most members of the four groups listed above are symbiotes or

parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...

s of animals, as is the case with ''Giardia lamblia'' which causes diarrhea in

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

Characteristics

A number of parabasalids and oxymonads are found in

termite Termites are a group of detritivore, detritophagous Eusociality, eusocial cockroaches which consume a variety of Detritus, decaying plant material, generally in the form of wood, Plant litter, leaf litter, and Humus, soil humus. They are dist ...

guts, and play an important role in breaking down the

cellulose Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...

found in

wood Wood is a structural tissue/material found as xylem in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulosic fibers that are strong in tension and embedded in a matrix of lignin t ...

. Some other metamonads are

parasites Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The en ...

. These flagellates are unusual in lacking aerobic

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 us ...

. Originally they were considered among the most primitive

s, diverging from the others before mitochondria appeared. However, they are now known to have lost aerobic mitochondria secondarily, and retain both organelles and nuclear genes derived ultimately from the mitochondrial endosymbiont genome. Mitochondrial relics include

hydrogenosome A hydrogenosome is a membrane-enclosed organelle found in some Anaerobic organism, anaerobic Ciliate, ciliates, Flagellate, flagellates, Fungus, fungi, and three species of Loricifera, loriciferans. Hydrogenosomes are highly variable organelles t ...

s, which produce

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

(and make ATP), and small structures called

mitosome A mitosome (also called a ''crypton'' in early literature) is a mitochondrion-related organelle (MRO) found in a variety of parasitic unicellular eukaryotes, such as members of the supergroup Excavata. The mitosome was first discovered in 1999 in ...

s. It now appears the Metamonada are, together with ''

Malawimonas ''Malawimonas'' is genus of unicellular, heterotrophic flagellates with uncertain phylogenetic affinities. They have variably being assigned to Excavata and Loukozoa. Recent studies suggest they may be closely related to the Podiata. Discovery ...

'', sister clades of the Podiata. All of these groups have

flagella A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...

or basal bodies in characteristic groups of four (or more, in parabasalids), which are often associated with the nucleus, forming a structure called a karyomastigont. In addition, genera such as '' Carpediemonas'' and '' Trimastix'' are now known to be close relatives of the retortamonad-diplomonad lineage and the oxymonads, respectively. Most of the closer relatives of the retortamonad-diplomonad lineage actually have two flagella and basal bodies.

Classification

The metamonads were thought to make up part of the

Excavata Excavata is an obsolete, extensive and diverse paraphyletic group of unicellular Eukaryota. The group was first suggested by Simpson and Patterson in 1999 and the name latinized and assigned a rank by Thomas Cavalier-Smith in 2002. It contains ...

, a proposed eukaryotic supergroup including flagellates with feeding grooves and their close relatives. Their relationships are uncertain, and they do not always appear together on molecular trees. Current opinion is that Excavata is not a

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

group, but it might be paraphyletic. The following higher level treatment from 2013 is based on works of

Cavalier-Smith Thomas (Tom) Cavalier-Smith, Royal Society, FRS, Royal Society of Canada, FRSC, Natural Environment Research Council, NERC Professorial Fellow (21 October 1942 – 19 March 2021), was a professor of evolutionary biology in the Departme ...

with amendments within Fornicata from Yubuki, Simpson & Leander. Metamonada were once again proposed to be basal eukaryotes in 2018. * Phylum Metamonada (Grassé 1952) Cavalier-Smith 1987 emend. Cavalier-Smith 2003 ** Family Anaeramoebidae Táborský, Pánek & Čepička 2017 ** Subphylum Anaeromonada Cavalier-Smith 1997 emend. 2003 *** Class Anaeromonadea Cavalier-Smith 1997 emend. 1999 **** Family Paratrimastigidae Zhang et al. 2015 **** Order Trimastigida Cavalier-Smith 2003 ***** Family Trimastigidae Saville Kent 1880 **** Order

Oxymonadida The Oxymonads (or Oxymonadida) are a group of flagellated protists found exclusively in the intestines of animals, mostly termites and other wood-eating insects. Along with the similar parabasalid flagellates, they harbor the symbiotic bacteria ...

Grassé 1952 emend. Cavalier-Smith 2003 ***** Family Polymastigidae Bütschli 1884 ***** Family Saccinobaculidae Brugerolle & Lee 2002 ex Cavalier-Smith 2013 ***** Family Pyrsonymphidae Grassé 1892 ***** Family Oxymonadidae Kirby 1928 ** Subphylum Trichozoa Cavalier-Smith 1996 emend. Cavalier-Smith 2003 stat. n. 2013 *** Superclass Fornicata Simpson 2003 stat. n. Cavalier-Smith 2013 **** Family Kipferliidae Cavalier-Smith 2013 **** Class

Carpediemonadea ''Carpediemonas'' is genus of Metamonada, and belongs to the group Excavata. This organism is a unicellular flagellated eukaryote that was first discovered in substrate samples from the Great Barrier Reef. ''Carpediemonas'' can be found in anaero ...

Cavalier-Smith 2013 s.s. ***** Order Carpediemonadida Cavalier-Smith 2003 emend. 2013 s.s. ****** Family Carpediemonadidae Cavalier-Smith 2003 **** Class Eopharyngea Cavalier-Smith 1993 stat. n. Cavalier-Smith 2003 ***** Order Dysnectida Cavalier-Smith 2013 ****** Family Dysnectidae Cavalier-Smith 2013 ***** Order Retortamonadida Grassé 1952 emend. Cavalier-Smith 2013 ****** Family Caviomonadidae Cavalier-Smith 2013 ****** Family Chilomastigidae Cavalier-Smith 2013 ****** Family Retortamonadidae Wenrich 1932 ***** Order Diplomonadida Wenyon 1926 emend. Brugerolle et al. 1975 ****** Family Giardiidae Kulda & Nohy´nkova´ 1978 ****** Family Octomitidae Cavalier-Smith 1996 ****** Family Spironucleidae Cavalier-Smith 1996 ****** Family Hexamitidae Kent 1880 emend. Brugerolle et al. 1975 *** Superclass Parabasalia Honigberg 1973 stat. n. Cavalier-Smith 2003 **** Class Trichonymphea Cavalier-Smith 2003 ***** Order Lophomonadida Light 1927 ****** Family Lophomonadidae Saville Kent 1880 ***** Order

Trichonymphida The parabasalids are a group of flagellated protists within the supergroup Excavata. Most of these eukaryotic organisms form a symbiosis, symbiotic relationship in animals. These include a variety of forms found in the intestines of termites and ...

Poche 1913 ****** Family † Burmanymphidae Poinar 2009 ****** Family Retractinymphidae Radek & Brune 2023 ****** Family Spirotrichosomidae Hollande & Caruette-Valentin 1971 ****** Family Staurojoeninidae Grassé 1917 ****** Family Trichonymphidae Saville Kent 1880 ****** Family Hoplonymphidae Light 1926 ****** Family Teratonymphidae Koidzumi 1921 ucomonymphidae**** Class Trichomonadea Kirby 1947 stat. n. Cavalier-Smith 2003 ***** Order Pimpavickida Céza & Čepička 2022 ****** Family Pimpavickidae Céza & Čepička 2022 ***** Order Trichomonadida Kirby 1947 ****** Family Lacusteriidae Céza & Čepička 2022 ****** Family Trichomonadidae Chalmers & Pekkoloa 1918 sensu Hampl et al. 2006 ***** Order Honigbergiellida Čepička et al. 2010 ****** Family Honigbergiellidae Čepička, Hampl & Kulda 2010 ****** Family Hexamastigidae Čepička, Hampl & Kulda 2010 ****** Family Tricercomitidae Čepička, Hampl & Kulda 2010 ***** Order Hypotrichomonadida Čepička et al. 2010 ****** Family Hypotrichomonadidae (Honigberg 1963) Čepička, Hampl & Kulda 2010 ***** Order Spirotrichonymphida Grassé 1952 ****** Family Spirotrichonymphidae Grassé 1917 ***** Order Tritrichomonadida Čepička et al. 2010 ****** Family Dientamoebidae Grassé 1953 ****** Family Monocercomonadidae Kirby 1944 ****** Family Simplicimonadidae Čepička et al. 2010 ****** Family Tritrichomonadidae Honigberg 1963 ***** Order Cristamonadida Brugerolle & Patterson 2001 emend. Cavalier-Smith 2013 ****** Family Calonymphidae Grassé 1911 ****** Family Devescovinidae Doflein 1911

Evolution

Within Metamonada, two main branches are recovered in recent

phylogenetic analyses In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as Computational phylogenetics, phylogenetic inference. It infers the relationship among organ ...

. One branch contains the Parabasalia and the closely related anaeramoebae. The other branch contains two large groups: the Fornicata, which is closely related to barthelonids and the recently isolated '' Skoliomonas''; and the

Preaxostyla Anaeromonadea, also known as Preaxostyla, is a class of excavate protists, comprising the oxymonads, '' Trimastix'', and '' Paratrimastix''. This group is studied as a model system for reductive evolution of mitochondria, because it includes bo ...

. A 2023 study found it likely that Metamonada is a

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

group at the base of Eukaryota, meaning their anaerobic metabolism possibly represents the ancestral condition in eukaryotes (similar to what the Archezoa- Metakaryota hypothesis proposed) and that aerobic mitochondria might not have the same origin as hydrogenosomes.

References

Citations

Cited literature

* * *

External links

Tree of Life: Fornicata
{{Taxonbar, from=Q499091 * Taxa named by Pierre-Paul Grassé