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The metamonads are a large group of flagellate amitochondriate microscopic . They include the , , , , 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 symbiosis or parasitism of animals, as is the case with Giardia lamblia which causes diarrhea in .
These flagellates are unusual in lacking aerobic mitochondrion. Originally they were considered among the most primitive , 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 , which produce hydrogen (and make ATP), and small structures called .
It now appears the Metamonada are, together with Malawimonas, sister clades of the Unikont.
All of these groups have flagellum or basal bodies in characteristic groups of four (or more, in parabasalids), which are often associated with the cell 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.
Metamonada were once again proposed to be basal eukaryotes in 2018.
A view of the metamonad taxonomy is:
A 2023 study found it likely that Metamonada is a Paraphyly 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.
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