A number of parabasalids and oxymonads are found intermiteguts, and play an important role in breaking down thecellulose found inwood. Some other metamonads areparasites.
These flagellates are unusual in lacking aerobicmitochondria. Originally they were considered among the most primitiveeukaryotes, 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 includehydrogenosomes, which producehydrogen (and make ATP), and small structures calledmitosomes.
It now appears the Metamonada are, together withMalawimonas, sister clades of thePodiata.[5]
All of these groups haveflagella or basal bodies in characteristic groups of four (or more, in parabasalids), which are often associated with thenucleus, forming a structure called a karyomastigont. In addition, genera such asCarpediemonas andTrimastix 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.
The metamonads were thought to make up part of theExcavata, a proposed eukaryotic supergroup including flagellates with feeding grooves and their close relatives. Their relationships are uncertain,[6] and they do not always appear together on molecular trees. Current opinion is that Excavata is not amonophyletic group, but it might be paraphyletic.
The following higher level treatment from 2013 is based on works ofCavalier-Smith[7] with amendments withinFornicata from Yubuki, Simpson & Leander.[8]
Metamonada were once again proposed to be basal eukaryotes in 2018.[9]
A 2023 study found it likely that Metamonada is aparaphyletic group at the base of Eukaryota, meaning their anaerobic metabolism possibly represents the ancestral condition in eukaryotes (similar to what theArchezoa-Metakaryota hypothesis proposed) and that aerobic mitochondria might not have the same origin as hydrogenosomes.[4]
^Cavalier-Smith T (2013). "Early evolution of eukaryote feeding modes, cell structural diversity, and classification of the protozoan phyla Loukozoa, Sulcozoa, and Choanozoa".Eur. J. Protistol.49 (2):115–178.doi:10.1016/j.ejop.2012.06.001.PMID23085100.
^Yubuki; Simpson; Leander (2013). "Comprehensive Ultrastructure of Kipferlia bialata Provides Evidence for Character Evolution within the Fornicata (Excavata)".Protist.164 (3):423–439.doi:10.1016/j.protis.2013.02.002.PMID23517666.
^Cepicka, Ivan; Hampl, Vladimír; Kulda, Jaroslav (July 2010). "Critical Taxonomic Revision of Parabasalids with Description of one New Genus and three New Species".Protist.161 (3):400–433.doi:10.1016/j.protis.2009.11.005.PMID20093080.
