|Entamoeba histolytica cyst|
The Archamoebae are a group of Amoebozoa distinguished by the absence of mitochondria. They include genera that are internal parasites or commensals of animals (Entamoeba and Endolimax). A few species are human pathogens, causing diseases such as amoebic dysentery. The other genera of archamoebae live in freshwater habitats, and are unusual among amoebae in possessing flagella. Most have a single nucleus and flagellum, but the giant amoeba Pelomyxa has many of each.
Early molecular trees based on rRNA placed the parasitic or commensal genera (entamoebids) and the flagellate genera (pelobionts) as separate groups that diverged from other eukaryotes very early on, suggesting that the absence of mitochondria was a primitive condition. The absence of dictyosomes in Pelomyxa was also considered primitive, which would separate it from Entamoeba, which has dictyosomes.
The name Archamoebae, from Greek αρχη first, refers to this presumed antiquity. However, studies based on other genes have shown that this placement is an artifact of long-branch attraction. Instead, the Archamoebae are part of the Amoebozoa that have lost their mitochondria, and are particularly close relatives of the slime moulds. It also appears the pelobionts and entamoebids are not separate groups, i.e. Entamoeba and Endolimax developed separately from free-living ancestors.
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