The metamonads are microscopic

eukaryotic 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. Bacte ...

organisms, 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 thei ...

amitochondriate 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 use ...

Loukozoa. Their composition is not entirely settled, but they include the

retortamonad 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 ...

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 ...

s, and possibly the

parabasalid 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 cockroach ...

s and

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

s as well. These four groups are all

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: * Anaerobic adhesive, a bonding a ...

(many being aerotolerant anerobes), occurring mostly as symbiotes or

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 is adapted structurally to this way of life. The entomologist E. O. Wilson h ...

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

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...

Characteristics

A number of parabasalids and oxymonads are found in

termite Termites are small insects that live in colonies and have distinct castes ( eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blat ...

guts The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and ...

, and play an important role in breaking down the

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...

found in

wood Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin ...

. Some other metamonads are

parasites 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 is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...

. These flagellates are unusual in lacking

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 ...

. Originally they were 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, diverging from the others before mitochondria appeared. However, they are now known to have lost mitochondria secondarily, and retain both organelles and nuclear genes derived from them. Mitochondrial relics include

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, which produce

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

, and small structures called

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. It now appears the Metamonada are, together with '' Malawimonas'', sister clades of the Podiata. All of these groups are united by having

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 ...

or basal bodies in characteristic groups of four, which are often associated with the

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: * Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...

, forming a structure called a ''karyomastigont''. In addition, the genera ''Carpediemonas'' and ''Trimastix'' are now known to be close relatives of the retortamonad-diplomonad line and the oxymonads, respectively. Both are free-living and amitochondriate.

Classification

The metamonads make up part of the

Excavata Excavata is a major supergroup of unicellular organisms belonging to the 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 a variety of free- ...

, a 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. It is possible that the metamonads as defined here do not form 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 ...

subgroup. The following higher level treatment is based on works of Cavalier-Smith with amendments within Fornicata from Yubukia, Simpson & Leander Metamonada were once again proposed to being basal eukaryotes in 2018. * Phylum Metamonada (Grassé 1952) Cavalier-Smith 1987 emend. Cavalier-Smith 2003 ** Subphylum Anaeromonada Cavalier-Smith 1997 emend. 2003 *** Class

Anaeromonadea Anaeromonadea is a class of excavates, comprising the oxymonads and ''Trimastix ''Trimastix'' is a genus of excavates, the sole occupant of the order Trimastigida. ''Trimastix'' are bacterivorous, free living and anaerobic. When first observ ...

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 protozoa found exclusively in the intestines of termites and other wood-eating insects. Along with the similar parabasalid flagellates, they harbor the symbiotic bacteria that are respons ...

Grassé 1952 emend. Cavalier-Smith 2003 ***** Family Polymastigidae Bütschli 1884 ***** Family Saccinobaculidae Brugerolle & Lee 2002 ex Cavalier-Smith 2013 ***** Family Pyrsonymphidae Grassé 1892 ***** Family Streblomastigidae Kofoid & Swezy 1919 ***** Family Oxymonadidae Kirby 1928 ** Subphylum

Trichozoa Trichozoa is a group of excavates. "Fornicata" is a similar grouping, but it excludes Parabasalia.Tree
a ...

Cavalier-Smith 1996 emend. Cavalier-Smith 2003 stat. n. 2013 *** Superclass Fornicata Simpson 2003 stat. n. Cavalier-Smith 2013 **** Genus '' Ergobibamus'' **** Genus '' Hicanonectes'' **** Family Kipferliidae Cavalier-Smith 2013 **** Order Chilomastigida Cavalier-Smith 2013 ***** Family Chilomastigidae Cavalier-Smith 2013 **** Order Dysnectida Cavalier-Smith 2013 ***** Family Dysnectidae Cavalier-Smith 2013 **** Class Carpediemonadea 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 Retortamonadida Grassé 1952 emend. 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 Chilomitea ***** Order Chilomitida ****** Family Chilomitidae **** Class Trichonymphea Cavalier-Smith 2003 ***** Order Lophomonadida Light 1927 ****** Family Lophomonadidae Saville Kent 1880 ***** Order Trichonymphida Poche 1913 ****** Family Spirotrichosomidae Hollande & Caruette-Valentin 1971 ****** Family Staurojoeninidae Grassé 1917 ****** Family Trichonymphidae Saville Kent 1880 ****** Family Hoplonymphidae Light 1926 ****** Family Teranymphidae Koidzumi 1921 ucomonymphidae**** Class Trichomonadea Kirby 1947 stat. n. Cavalier-Smith 2003 ***** Order Trichocovinida ****** Family Trichocovinidae ***** Order Trichomonadida Kirby 1947 ****** Suborder Hypotrichomonadina Čepička et al. 2010 stat. n. ******* Family Hypotrichomonadidae (Honigberg 1963) Čepička, Hampl & Kulda 2010 ****** Suborder Trichomonadina (Kirby 1947) Cavalier-Smith 2013 ******* Family Trichomonadidae Chalmers & Pekkoloa 1918 sensu Hampl et al. 2006 ****** Suborder Honigbergiellina Čepička et al. 2010 ******* Family Honigbergiellidae Čepička, Hampl & Kulda 2010 ******* Family Hexamastigidae Čepička, Hampl & Kulda 2010 ******* Family Tricercomitidae Čepička, Hampl & Kulda 2010 ***** Order Tritrichomonadida Čepička et al. 2010 ****** Family Dientamoebidae Grassé 1953 ****** Family Monocercomonadidae Kirby 1944 ****** Family Simplicimonadidae Čepička et al. 2010 ****** Family Tritrichomonadidae Honigberg 1963 ***** Subclass Cristomonadia Cavalier-Smith, 2013 ****** Order Spirotrichonymphida Grassé 1952 ******* Family Spirotrichonymphidae Grassé 1917 ****** Order Cristamonadida Brugerolle & Patterson 2001 emend. Cavalier-Smith 2013 ******* Family Calonymphidae Grassé 1911 ******* Family Devescovinidae Doflein 1911 oeniidae; Rhizonymphidae; Kofoidiidae; Deltotrichonymphidae

References

External links

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