Archezoa was a
kingdom
Kingdom commonly refers to:
* A monarchy ruled by a king or queen
* Kingdom (biology), a category in biological taxonomy
Kingdom may also refer to:
Arts and media Television
* ''Kingdom'' (British TV series), a 2007 British television drama s ...
proposed in the 20th century by
Thomas Cavalier-Smith
Thomas (Tom) Cavalier-Smith, FRS, FRSC, NERC Professorial Fellow (21 October 1942 – 19 March 2021), was a professor of evolutionary biology in the Department of Zoology, at the University of Oxford.
His research has led to discov ...
(1942–2021), and was believed to encompass
eukaryotes which did not have
mitochondria (and are therefore ''amitochondriate'') or peroxisomes (e.g. ''
Giardia''). The category was dropped after it was discovered that all the amitochondriates were descendants of eukaryotes with mitochondria that had lost them.
History
Origin
This taxonomic category was proposed upon the discovery that some
protists lacked mitochondria, which suggested to Cavalier-Smith that the initial ancestor of eukaryotes emerged prior to the
endosymbiosis
An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship.
(The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...
of mitochondria (cf. ''
Giardia'').
Eukaryotes that eventually acquired a bacterial endosymbiont that became the mitochondria were placed in a taxonomic group which Cavalier-Smith called the Metakaryota, whereas the Archezoa represented a
paraphyletic group involving four phyla representing primitive eukaryotes which evolved prior to the acquisition of mitochondria.
Organisms making up the Archezoa are species of amitochondriate
protists and some fungi, whereas Cavalier-Smith categorized mitochondriate protists within a group he termed the Mitozoa.
[ Included in the Archezoa are ]Microsporidia
Microsporidia are a group of spore-forming unicellular parasites. These spores contain an extrusion apparatus that has a coiled polar tube ending in an anchoring disc at the apical part of the spore. They were once considered protozoans or pr ...
, Metamonada, and Parabasalids.
Original mitochondria lost
In light of discoveries which took place not long after this taxonomic category was proposed, the validity of this category could not be maintained: Eukaryotic protists lacking mitochondria were discovered to have experienced secondary mitochondrial loss, meaning that their ancestors once possessed mitochondria but that these mitochondria had, over time, been lost or reduced. In some of these organisms, mitochondria had degraded into simpler double-membrane bound organelles known as mitosomes
A mitosome is an organelle found in some unicellular eukaryotic organisms, like in members of the supergroup Excavata. The mitosome was found and named in 1999, and its function has not yet been well characterized. It was termed a ''crypton'' by o ...
and hydrogenosomes
A hydrogenosome is a membrane-enclosed organelle found in some anaerobic ciliates, flagellates, and fungi. Hydrogenosomes are highly variable organelles that have presumably evolved from protomitochondria to produce molecular hydrogen and ATP i ...
. Some of both types of organelles are known to have fully lost their genome.
Initial discoveries found that amitochondriate organisms appeared to express mitochondrial Hsp60 and Hsp70 proteins from the nuclear DNA of the organism. This indicated that the ancestors of these organisms once possessed mitochondria which expressed these proteins, but that these genes had migrated to their nuclear DNA over time as a result of endosymbiotic gene transfer.
As a result, it could not be said that there are any eukaryotes lacking mitochondria which had emerged from an earlier part of the eukaryotic lineage that preceded the acquisition of mitochondria.
Long branch attraction
Another supporting evidence for the Archezoa group was that amitochondriate protists appeared to branch off early on from the eukaryotic lineage in phylogenetic analyses, supporting the supposition that Archezoa were more closely linked to primitive eukaryotes that evolved prior to the endosymbiosis event that generated the mitochondria.[ However, this early divergence later turned out to be a type of systematic error that is possible in phylogenetic analysis called "]long branch attraction
In phylogenetics, long branch attraction (LBA) is a form of systematic error whereby distantly related lineages are incorrectly inferred to be closely related. LBA arises when the amount of molecular or morphological change accumulated within a lin ...
".
References
Obsolete eukaryote taxa
Biological hypotheses
Mitochondria
Kingdoms (biology)
{{Eukaryote-stub