Metamonad

For an explanation of related terms, see Excavata.

Metamonad
"Giardia lamblia", a parasitic diplomonad
Giardia lamblia, a parasitic diplomonad
Scientific classification
Domain:	Eukaryota
Clade:	Metamonada Grassé 1952 emend. Cavalier-Smith 2003
Subdivisions
Anaeramoebidae Preaxostyla Barthelona Skoliomonas Trichozoa
Synonyms
Archezoa? Centrosomea Chatton Villeneuve 1937 Metamonadina Grassé 1952 Polymastigota Butschli 1884 Tetramastigota Hulsmann & Hausmann 1994

The metamonads are a large group of flagellate amitochondriate microscopic eukaryotes. They include the retortamonads, diplomonads, parabasalids, oxymonads, and a range of more poorly studied taxa, most of which are free-living flagellates. All metamonads are anaerobic (many being aerotolerant anaerobes), and most members of the four groups listed above are symbiotes or parasites of animals, as is the case with Giardia lamblia which causes diarrhea in mammals.

Characteristics

A number of parabasalids and oxymonads are found in termite guts, and play an important role in breaking down the cellulose found in wood. Some other metamonads are parasites.

These flagellates are unusual in lacking aerobic mitochondria. Originally they were considered among the most primitive eukaryotes, 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 hydrogenosomes, which produce hydrogen (and make ATP), and small structures called mitosomes.

It now appears the Metamonada are, together with Malawimonas, sister clades of the Podiata.

All of these groups have flagella 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, 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 group, but it might be paraphyletic.

The following higher level treatment from 2013 is based on works of Cavalier-Smith 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 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 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 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 [Eucomonymphidae]
      • 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. 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.

A 2023 study found it likely that Metamonada is a paraphyletic 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.

External links

• Tree of Life: Fornicata