{{Short description|Clade of excavate protists}} {{For|an explanation of related terms|Excavata}} {{Automatic taxobox | image = Giardia lamblia.jpg | image_alt = "Giardia lamblia", a parasitic diplomonad | image_caption = ''Giardia lamblia'', a parasitic diplomonad | display_parents = 2 | taxon = Metamonada | authority = Grassé 1952 emend. Cavalier-Smith 2003 | subdivision_ranks = Subgroups | subdivision = * Fornicata * Parabasalia * Preaxostyla (=Anaeromonadea) * Anaeramoebae (''Anaeramoeba'') * Notopharyngea (=BaSk) | synonyms = * Archezoa <small>Cavalier-Smith 1983 stat. n. emend. Cavalier-Smith 2022</small> * Centrosomea <small>Chatton Villeneuve 1937</small> * Metamonadina <small>Grassé 1952</small> * Polymastigota <small>Butschli 1884</small> * Tetramastigota <small>Hulsmann & Hausmann 1994</small> }}
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.<ref name="Al Jewari Baldauf 2023">{{Cite journal |last1=Al Jewari |first1=Caesar |last2=Baldauf |first2=Sandra L. |date=2023-04-28 |title=An excavate root for the eukaryote tree of life |journal=Science Advances |volume=9 |issue=17 |article-number=eade4973 |doi=10.1126/sciadv.ade4973 |issn=2375-2548 |pmc=10146883 |pmid=37115919|bibcode=2023SciA....9E4973A }}</ref>
==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.<ref>{{Cite journal|last1=Cavalier-Smith|first1=Thomas|last2=Chao|first2=Ema E.|last3=Lewis|first3=Rhodri|date=2016-06-01|title=187-gene phylogeny of protozoan phylum Amoebozoa reveals a new class (Cutosea) of deep-branching, ultrastructurally unique, enveloped marine Lobosa and clarifies amoeba evolution|journal=Molecular Phylogenetics and Evolution|volume=99|pages=275–296|doi=10.1016/j.ympev.2016.03.023|pmid=27001604|doi-access=free}}</ref>
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,<ref name="pmid14657102">{{cite journal |author=Cavalier-Smith T |title=The excavate protozoan phyla Metamonada Grassé emend. (Anaeromonadea, Parabasalia, Carpediemonas, Eopharyngia) and Loukozoa emend. (Jakobea, Malawimonas): their evolutionary affinities and new higher taxa |journal=Int. J. Syst. Evol. Microbiol. |volume=53 |issue=Pt 6 |pages=1741–58 |date=November 2003 |pmid=14657102 |doi= 10.1099/ijs.0.02548-0|doi-access=free }}</ref> 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.
Metamonada were once again proposed to be basal eukaryotes in 2018.<ref>{{Cite journal|last1=Krishnan|first1=Arunkumar|last2=Burroughs|first2=A. Max |last3=Iyer |first3=Lakshminarayan|last4=Aravind|first4=L.|year=2018 |title=Unexpected Evolution of Lesion-Recognition Modules in Eukaryotic NER and Kinetoplast DNA Dynamics Proteins from Bacterial Mobile Elements |biorxiv=10.1101/361121 |journal=iScience |volume=23 |issue=9 |pages=192–208 |doi=10.1016/j.isci.2018.10.017 |doi-access=free|pmc=6222260 }}</ref>
A view of the metamonad taxonomy is:<ref>{{Cite journal |last=Adl |first=Sina M. |last2=Bass |first2=David |last3=Lane |first3=Christopher E. |last4=Lukeš |first4=Julius |last5=Schoch |first5=Conrad L. |last6=Smirnov |first6=Alexey |last7=Agatha |first7=Sabine |last8=Berney |first8=Cedric |last9=Brown |first9=Matthew W. |last10=Burki |first10=Fabien |last11=Cárdenas |first11=Paco |last12=Čepička |first12=Ivan |last13=Chistyakova |first13=Lyudmila |last14=del Campo |first14=Javier |last15=Dunthorn |first15=Micah |date=2019 |title=Revisions to the Classification, Nomenclature, and Diversity of Eukaryotes |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/jeu.12691 |journal=Journal of Eukaryotic Microbiology |language=en |volume=66 |issue=1 |pages=4–119 |doi=10.1111/jeu.12691 |issn=1550-7408 |pmc=6492006 |pmid=30257078 |doi-access=free}}</ref><ref>{{Cite journal |last=Boscaro |first=Vittorio |last2=James |first2=Erick R. |last3=Fiorito |first3=Rebecca |last4=del Campo |first4=Javier |last5=Scheffrahn |first5=Rudolf H. |last6=Keeling |first6=Patrick J. |date=2024 |title=Updated classification of the phylum Parabasalia |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/jeu.13035 |journal=Journal of Eukaryotic Microbiology |language=en |volume=71 |issue=4 |article-number=e13035 |doi=10.1111/jeu.13035 |issn=1550-7408 |doi-access=free|hdl=10261/374507 |hdl-access=free }}</ref><ref>{{Cite journal |last=Cavalier-Smith |first=Thomas |date=2013-05-01 |title=Early evolution of eukaryote feeding modes, cell structural diversity, and classification of the protozoan phyla Loukozoa, Sulcozoa, and Choanozoa |url=https://www.sciencedirect.com/science/article/pii/S0932473912000508 |journal=European Journal of Protistology |volume=49 |issue=2 |pages=115–178 |doi=10.1016/j.ejop.2012.06.001 |issn=0932-4739|url-access=subscription }}</ref><ref>{{Cite journal |last=Vargová |first=Romana |last2=Hanousková |first2=Pavla |last3=Salamonová |first3=Jana |last4=Žihala |first4=David |last5=Silberman |first5=Jeffrey D. |last6=Eliáš |first6=Marek |last7=Čepička |first7=Ivan |date=2022-05-19 |title=Evidence for an Independent Hydrogenosome-to-Mitosome Transition in the CL3 Lineage of Fornicates |url=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2022.866459/full |journal=Frontiers in Microbiology |language=English |volume=13 |doi=10.3389/fmicb.2022.866459 |issn=1664-302X |pmc=9161772 |pmid=35663895 |doi-access=free}}</ref><ref>{{Cite journal |last=Jørgensen |first=Anders |last2=Sterud |first2=Erik |date=2007-04-18 |title=Phylogeny of Spironucleus (Eopharyngia: Diplomonadida: Hexamitinae) |url=https://www.sciencedirect.com/science/article/pii/S1434461007000028 |journal=Protist |volume=158 |issue=2 |pages=247–254 |doi=10.1016/j.protis.2006.12.003 |issn=1434-4610|url-access=subscription }}</ref><ref>{{Cite journal |last=Kolisko |first=Martin |last2=Cepicka |first2=Ivan |last3=Hampl |first3=Vladimir |last4=Leigh |first4=Jessica |last5=Roger |first5=Andrew J. |last6=Kulda |first6=Jaroslav |last7=Simpson |first7=Alastair GB |last8=Flegr |first8=Jaroslav |date=2008-07-15 |title=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 |journal=BMC Evolutionary Biology |language=en |volume=8 |issue=1 |page=205 |doi=10.1186/1471-2148-8-205 |issn=1471-2148 |pmc=2496913 |pmid=18627633 |doi-access=free}}</ref><ref>{{Cite journal |last=Martínez-Díaz |first=Rafael A. |last2=Castro |first2=Ana Teresa |last3=Herrera |first3=Silvia |last4=Ponce |first4=Francisco |date=2001 |title=First report of the genus Retortamonas (Sarcomastigophora: Retortamonadidae) in birds |url=https://www.scielo.br/j/mioc/a/QSdwsZnQtnv4FjqWqWXSFQp/?lang=en |journal=Memórias do Instituto Oswaldo Cruz |language=en |volume=96 |pages=961–963 |doi=10.1590/S0074-02762001000700013 |issn=0074-0276 |doi-access=free}}</ref><ref name="stairs2021">{{Cite journal |last=Stairs |first=Courtney W. |last2=Táborský |first2=Petr |last3=Salomaki |first3=Eric D. |last4=Kolisko |first4=Martin |last5=Pánek |first5=Tomáš |last6=Eme |first6=Laura |last7=Hradilová |first7=Miluše |last8=Vlček |first8=Čestmír |last9=Jerlström-Hultqvist |first9=Jon |last10=Roger |first10=Andrew J. |last11=Čepička |first11=Ivan |date=2021-12-20 |title=Anaeramoebae are a divergent lineage of eukaryotes that shed light on the transition from anaerobic mitochondria to hydrogenosomes |url=https://www.cell.com/current-biology/abstract/S0960-9822(21)01364-6 |journal=Current Biology |language=English |volume=31 |issue=24 |pages=5605–5612.e5 |doi=10.1016/j.cub.2021.10.010 |issn=0960-9822 |pmid=34710348 |doi-access=free}}</ref><ref name="Yazaki2020">{{Cite journal |last=Yazaki |first=Euki |last2=Kume |first2=Keitaro |last3=Shiratori |first3=Takashi |last4=Eglit |first4=Yana |last5=Tanifuji |first5=Goro |last6=Harada |first6=Ryo |last7=Simpson |first7=Alastair G. 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* '''Clade Metamonada''' ** Phylum Fornicata *** Order Diplomonadida **** Family Hexamitidae ***** Subfamily Hexamitinae ***** Subfamily Giardiinae *** Order Retortamonadida (paraphyletic) **** Family Retortamonadidae (paraphyletic) *** Family Caviomonadidae *** Informal group ''Carpediemonas''-like organisms (polyphyletic) **** Order Dysnectida ***** Family Dysnectidae **** Family Kipferliidae **** Genus ''Aduncisulcus'' **** Genus ''Carpediemonas'' **** Genus ''Ergobibamus'' **** Genus ''Hicanonectes'' ** Phylum Parabasalia *** Class Hypotrichomonadea **** Order Hypotrichomonadida ***** Family Hypotrichomonadidae *** Class Pimpavickea **** Order Pimpavickida ***** Family Pimpavickidae *** Class Trichomonadea **** Order Honigbergiellida ***** Family Hexamastigidae ***** Family Honigbergiellidae ***** Family Cthulhuidae ***** Family Tetratrichomastigidae **** Order Trichomonadida ***** Family Lacusteriidae ***** Family Trichomonadidae ***** Family ''Incertae sedis'' ****** Genus ''Pseudotrichomonas'' ****** Genus †''Paleotrichomones'' *** Clade Tla **** Class Lophomonadea ***** Order Lophomonadida ****** Family Lophomonadidae **** Class Trichonymphea ***** Order Trichonymphida ****** Family Barbulanymphidae ****** Family Hoplonymphidae ****** Family Retractinymphidae ****** Family Spirotrichosomidae ****** Family Staurojoeninidae ****** Family Teranymphidae ****** Family Trichonymphidae ****** Family †Burmanymphidae *** Clade Cadamassta **** Class Cristamonadea ***** Order Calonymphida ****** Family Calonymphidae ****** Family Deltotrichonymphidae ****** Family Mixotrichidae ***** Order Devescovinida ****** Family Devescovinidae ***** Order Gigantomonadida ****** Family Gigantomonadidae ***** Order ''Incertae sedis'' ****** Genus ''Cyclojoenia'' ****** Genus ''Joenia'' ****** Genus ''Joenina'' ****** Genus ''Joenoides'' ****** Genus ''Joenopsis'' ****** Genus ''Pachyjoenia'' ****** Genus ''Parajoenopsis'' ****** Genus ''Placojoenia'' ****** Genus ''Projoenia'' **** Class Dientamoebea ***** Order Dientamoebida ****** Family Dientamoebidae **** Class Monocercomonadea ***** Order Monocercomonadida ****** Family Monocercomonadidae **** Class Simplicimonadea ***** Order Simplicimonadida ****** Family Simplicimonadidae **** Class Tritrichomonadea ***** Order Tritrichomonadida ****** Family Tritrichomonadidae **** Class Spirotrichonymphea ***** Order Cononymphida ****** Family Cononymphidae ***** Order Holomastigotoidida ****** Family Holomastigotoididae ***** Order Spirotrichonymphida ****** Family Brugerollinidae ****** Family Fraterculidae ****** Family Holomastigotidae ****** Family Spirotrichonymphidae *** Class ''Incertae sedis'' **** Genus ''Chilomitus'' **** Genus ''Rhizonympha'' **** Genus ''Tricercomitus'' **** Genus ''Trichocovina'' ** Phylum Preaxostyla *** Order Oxymonadida **** Family Oxymonadidae **** Family Pyrsonymphidae **** Family Streblomastigidae **** Family Saccinobaculidae **** Family Polymastigidae *** Order Trimastigida **** Family Trimastigidae *** Order Paratrimastigida **** Family Paratrimastigidae ** Phylum Anaeramoebae *** Family Anaeramoebidae ** Class Notopharyngea *** Order Barthelonida **** Family Barthelonidae *** Order Skoliomonadida **** Family Skoliomonadidae
== 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<ref name="Yazaki2020"/> and the recently isolated ''Skoliomonas'';<ref name="Eglit2024" /> and the Preaxostyla.<ref name="stairs2021"/>
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.<ref name="Al Jewari Baldauf 2023" />
== References == {{Reflist}}
== External links == * [http://tolweb.org/Fornicata/121182 Tree of Life: Fornicata] {{Eukaryota}} {{Excavata}} {{Taxonbar|from=Q499091}}
<!-- Category:Eukaryote phyla moved to Metamonada redirect --> Category:Metamonads Category:Taxa named by Pierre-Paul Grassé