Metamonad
Retortamonadida
Diplomonadida
Carpediemonas
Parabasalia
Anaeromonada
Oxymonadida
Trimastix
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The
metamonads are a group of
flagellate protozoa, including the retortamonads,
diplomonads, and possibly the
parabasalids and oxymonads as well. These four groups are all
anaerobic, lacking
mitochondria, and are mostly found as
symbiotes of of animals. 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 important parasites.
Originally these flagellates were considered to have diverged from other
eukaryotes before mitochondria appeared, along with a few other lines (
pelobionts,
entamoebae, and microsporidia).
Cavalier-Smith proposed a separate kingdom Archezoa for such forms. However, all are now known to have lost mitochondria secondarily, and metamonads retain both organelles and nuclear genes derived from them. Mitochondrial relics include hydrogenosomes, which produce hydrogen, and small structures called mitosomes.
In addition to these groups, the genera
Carpediemonas and
Trimastix are now known to be close relatives of the retortamonad-diplomonad line and oxymonads, respectively. All of these belong among the
excavates, a group of flagellates that have feeding grooves or are closely related to such forms. Their relationships are still being worked out, and it is possible the organisms listed here do not make up a monophyletic subgroup. However, they are linked by having
flagella or at least basal bodies in characteristic groups of four, often associated with the
nucleus in a structure called a karyomastigont.
References
- T. Cavalier-Smith (2003). The excavate protozoan phyla Metamonada Grassé emend. (Anaeromoadea, Parabasalia, Carpediemonas, Eopharyngia) and Loukozoa emend. (Jakobea, Malawimonas): their evolutionary affinities and new higher taxa. International Journal of Systematic and Evoluionary Microbiology 53: 1741-1758
