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Wikipedia / Metamonad

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 symbiotess or parasitess of animals, as is the case with Giardia lamblia which causes diarrhea in mammals.

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.

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, 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.

A view of the metamonad taxonomy is:

Clade Metamonada

Phylum Fornicata

Order Diplomonadida

Family Hexamitidae

Order Retortamonadida (paraphyletic)

Informal group Carpediemonas-like organisms (polyphyletic)

Order Dysnectida

Phylum Parabasalia

Class Hypotrichomonadea

Order Hypotrichomonadida

Class Pimpavickea

Order Pimpavickida

Class Trichomonadea

Order Honigbergiellida

Order Trichomonadida

Family Incertae sedis

Clade Tla

Class Lophomonadea

Order Lophomonadida

Class Trichonymphea

Order Trichonymphida

Clade Cadamassta

Class Cristamonadea

Order Calonymphida

Order Devescovinida

Order Gigantomonadida

Order Incertae sedis

Class Dientamoebea

Order Dientamoebida

Class Monocercomonadea

Order Monocercomonadida

Class Simplicimonadea

Order Simplicimonadida

Class Tritrichomonadea

Order Tritrichomonadida

Class Spirotrichonymphea

Order Cononymphida

Order Holomastigotoidida

Order Spirotrichonymphida

Class Incertae sedis

Phylum Preaxostyla

Order Oxymonadida

Order Trimastigida

Order Paratrimastigida

Phylum Anaeramoebae

Phylum Incertae sedis

Class Notopharyngea

Order Barthelonida

Order Skoliomonadida

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 and the recently isolated Skoliomonas; and the Preaxostyla.

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.

References