Clade of excavate protists
For an explanation of related terms, see
Excavata .
Themetamonads are a large group offlagellate amitochondriate microscopiceukaryotes . They include theretortamonads ,diplomonads ,parabasalids ,oxymonads , and a range of more poorly studied taxa, most of which are free-living flagellates. All metamonads areanaerobic (many being aerotolerant anaerobes), and most members of the four groups listed above aresymbiotes orparasites of animals, as is the case withGiardia lamblia mammals .[ 1]
A number of parabasalids and oxymonads are found intermite guts , 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 clades of thePodiata .[ 2]
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 Trimastix
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,[ 3] monophyletic group, but it might beparaphyletic .
Metamonada were once again proposed to be basal eukaryotes in 2018.[ 4]
A view of the metamonad taxonomy is:[ 5] [ 6] [ 7] [ 8] [ 9] [ 10] [ 11] [ 12] [ 13] [ 14] [ 15] [ 16] [ 17] [ 18]
Within Metamonada, two main branches are recovered in recentphylogenetic analyses . One branch contains theParabasalia and the closely relatedanaeramoebae . The other branch contains two large groups: theFornicata , which is closely related tobarthelonids [ 13] Skoliomonas [ 14] Preaxostyla .[ 12]
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.[ 1]
Sister group to barthelonids areskoliomonads . These two groups of free-living anaerobic metamonads form together a monophyletic group known asBaSk clade. It was identified by phylogenomic analysis as a deeply branching sister group to theFornicata . The extreme mitochondrial reduction observed within this free-living anaerobic protistan clade demonstrates that mitochondrial functions may be completely lost even in free-living organisms, not onlyparasites .[ 18]
^a b Al Jewari, Caesar; Baldauf, Sandra L. (2023-04-28)."An excavate root for the eukaryote tree of life" .Science Advances .9 (17) eade4973.Bibcode :2023SciA....9E4973A .doi :10.1126/sciadv.ade4973 .ISSN 2375-2548 .PMC 10146883 PMID 37115919 . ^ Cavalier-Smith, Thomas; Chao, Ema E.; Lewis, Rhodri (2016-06-01)."187-gene phylogeny of protozoan phylum Amoebozoa reveals a new class (Cutosea) of deep-branching, ultrastructurally unique, enveloped marine Lobosa and clarifies amoeba evolution" .Molecular Phylogenetics and Evolution .99 :275– 296.doi :10.1016/j.ympev.2016.03.023 PMID 27001604 . ^ Cavalier-Smith T (November 2003)."The excavate protozoan phyla Metamonada Grassé emend. (Anaeromonadea, Parabasalia, Carpediemonas, Eopharyngia) and Loukozoa emend. (Jakobea, Malawimonas): their evolutionary affinities and new higher taxa" .Int. J. Syst. Evol. Microbiol .53 (Pt 6):1741– 58.doi :10.1099/ijs.0.02548-0 PMID 14657102 . ^ Krishnan, Arunkumar; Burroughs, A. Max; Iyer, Lakshminarayan; Aravind, L. (2018)."Unexpected Evolution of Lesion-Recognition Modules in Eukaryotic NER and Kinetoplast DNA Dynamics Proteins from Bacterial Mobile Elements" .iScience .23 (9):192– 208.bioRxiv 10.1101/361121 doi :10.1016/j.isci.2018.10.017 PMC 6222260 ^ Adl, Sina M.; Bass, David; Lane, Christopher E.; Lukeš, Julius; Schoch, Conrad L.; Smirnov, Alexey; Agatha, Sabine; Berney, Cedric; Brown, Matthew W.; Burki, Fabien; Cárdenas, Paco; Čepička, Ivan; Chistyakova, Lyudmila; del Campo, Javier; Dunthorn, Micah (2019)."Revisions to the Classification, Nomenclature, and Diversity of Eukaryotes" .Journal of Eukaryotic Microbiology .66 (1):4– 119.doi :10.1111/jeu.12691 ISSN 1550-7408 .PMC 6492006 PMID 30257078 . ^ Boscaro, Vittorio; James, Erick R.; Fiorito, Rebecca; del Campo, Javier; Scheffrahn, Rudolf H.; Keeling, Patrick J. (2024)."Updated classification of the phylum Parabasalia" .Journal of Eukaryotic Microbiology .71 (4) e13035.doi :10.1111/jeu.13035 hdl :10261/374507 ISSN 1550-7408 . ^ Cavalier-Smith, Thomas (2013-05-01)."Early evolution of eukaryote feeding modes, cell structural diversity, and classification of the protozoan phyla Loukozoa, Sulcozoa, and Choanozoa" European Journal of Protistology .49 (2):115– 178.doi :10.1016/j.ejop.2012.06.001 .ISSN 0932-4739 . ^ Vargová, Romana; Hanousková, Pavla; Salamonová, Jana; Žihala, David; Silberman, Jeffrey D.; Eliáš, Marek; Čepička, Ivan (2022-05-19)."Evidence for an Independent Hydrogenosome-to-Mitosome Transition in the CL3 Lineage of Fornicates" .Frontiers in Microbiology .13 .doi :10.3389/fmicb.2022.866459 ISSN 1664-302X .PMC 9161772 PMID 35663895 . ^ Jørgensen, Anders; Sterud, Erik (2007-04-18)."Phylogeny of Spironucleus (Eopharyngia: Diplomonadida: Hexamitinae)" Protist .158 (2):247– 254.doi :10.1016/j.protis.2006.12.003 .ISSN 1434-4610 . ^ Kolisko, Martin; Cepicka, Ivan; Hampl, Vladimir; Leigh, Jessica; Roger, Andrew J.; Kulda, Jaroslav; Simpson, Alastair GB; Flegr, Jaroslav (2008-07-15)."Molecular phylogeny of diplomonads and enteromonads based on SSU rRNA, alpha-tubulin and HSP90 genes: Implications for the evolutionary history of the double karyomastigont of diplomonads" .BMC Evolutionary Biology .8 (1): 205.doi :10.1186/1471-2148-8-205 ISSN 1471-2148 .PMC 2496913 PMID 18627633 . ^ Martínez-Díaz, Rafael A.; Castro, Ana Teresa; Herrera, Silvia; Ponce, Francisco (2001)."First report of the genus Retortamonas (Sarcomastigophora: Retortamonadidae) in birds" .Memórias do Instituto Oswaldo Cruz .96 :961– 963.doi :10.1590/S0074-02762001000700013 ISSN 0074-0276 . ^a b Stairs, Courtney W.; Táborský, Petr; Salomaki, Eric D.; Kolisko, Martin; Pánek, Tomáš; Eme, Laura; Hradilová, Miluše; Vlček, Čestmír; Jerlström-Hultqvist, Jon; Roger, Andrew J.; Čepička, Ivan (2021-12-20)."Anaeramoebae are a divergent lineage of eukaryotes that shed light on the transition from anaerobic mitochondria to hydrogenosomes" .Current Biology .31 (24): 5605–5612.e5.doi :10.1016/j.cub.2021.10.010 ISSN 0960-9822 .PMID 34710348 . ^a b Yazaki, Euki; Kume, Keitaro; Shiratori, Takashi; Eglit, Yana; Tanifuji, Goro; Harada, Ryo; Simpson, Alastair G. B.; Ishida, Ken-ichiro; Hashimoto, Tetsuo; Inagaki, Yuji (2020-09-09)."Barthelonids represent a deep-branching metamonad clade with mitochondrion-related organelles predicted to generate no ATP" .Proceedings of the Royal Society B: Biological Sciences .287 (1934) 20201538.doi :10.1098/rspb.2020.1538 ISSN 0962-8452 .PMC 7542792 PMID 32873198 . ^a b Eglit, Yana; Williams, Shelby K.; Roger, Andrew J.; Simpson, Alastair G. B. (2024)."Characterization of Skoliomonas gen. nov., a haloalkaliphilic anaerobe related to barthelonids (Metamonada)" .Journal of Eukaryotic Microbiology .71 (6) e13048.doi :10.1111/jeu.13048 ISSN 1550-7408 .PMC 11603281 PMID 39225178 . ^ Zhang, Qianqian; Táborský, Petr; Silberman, Jeffrey D.; Pánek, Tomáš; Čepička, Ivan; Simpson, Alastair G. B. (2015-09-01)."Marine Isolates of Trimastix marina Form a Plesiomorphic Deep-branching Lineage within Preaxostyla, Separate from Other Known Trimastigids (Paratrimastix n. gen.)" Protist .166 (4):468– 491.doi :10.1016/j.protis.2015.07.003 .ISSN 1434-4610 . ^ Cavalier-Smith, Thomas (2022-05-01)."Ciliary transition zone evolution and the root of the eukaryote tree: implications for opisthokont origin and classification of kingdoms Protozoa, Plantae, and Fungi" .Protoplasma .259 (3):487– 593.doi :10.1007/s00709-021-01665-7 ISSN 1615-6102 .PMC 9010356 PMID 34940909 . ^ Poinar, George (2009-10-01)."Early Cretaceous protist flagellates (Parabasalia: Hypermastigia: Oxymonada) of cockroaches (Insecta: Blattaria) in Burmese amber" Cretaceous Research .30 (5):1066– 1072.doi :10.1016/j.cretres.2009.03.008 .ISSN 0195-6671 . ^a b Williams, Shelby K.; Jerlström Hultqvist, Jon; Eglit, Yana; Salas-Leiva, Dayana E.; Curtis, Bruce; Orr, Russell J. S.; Stairs, Courtney W.; Atalay, Tuğba N.; MacMillan, Naomi; Simpson, Alastair G. B.; Roger, Andrew J. (2024-08-09)."Extreme mitochondrial reduction in a novel group of free-living metamonads" .Nature Communications .15 (1): 6805.doi :10.1038/s41467-024-50991-w ISSN 2041-1723 .PMC 11316075 PMID 39122691 .
Pharyngomonada
Tetramitia
Lunosea Neovahlkampfiea Eutetramitia
Anaeromonada
Fornicata
Parabasalia
Pimpavickea Hypotrichomonadea Trichomonadea Spirotrichonymphea Cononymphida Cristamonadida Holomastigotoidida Monocercomonadida Simplicimonadida Spirotrichonymphida Tritrichomonadida
