Archezoa was a kingdom proposed in the 20th century by Thomas Cavalier-Smith (1942–2021), and was believed to encompass

eukaryotes Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

which did not have

mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...

(and are therefore ''amitochondriate'') or peroxisomes (e.g. ''

Giardia ''Giardia'' ( or ) is a genus of anaerobic flagellated protozoan parasites of the phylum Metamonada that colonise and reproduce in the small intestines of several vertebrates, causing the disease giardiasis. Their life cycle alternates between ...

''). 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 A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the excl ...

lacked mitochondria, which suggested to Cavalier-Smith that the initial ancestor of eukaryotes emerged prior to the endosymbiosis of mitochondria (cf. ''

''). 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 In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

group involving four phyla representing primitive eukaryotes which evolved prior to the acquisition of mitochondria. Organisms making up the Archezoa are species of amitochondriate

and some fungi, whereas Cavalier-Smith categorized mitochondriate protists within a group he termed the Mitozoa. Included in the Archezoa are Microsporidia, Metamonada, and

Parabasalids The parabasalids are a group of flagellated protists within the supergroup Excavata. Most of these eukaryotic organisms form a symbiotic relationship in animals. These include a variety of forms found in the intestines of termites and cockroaches ...

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 and hydrogenosomes. 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".

References

Obsolete eukaryote taxa Biological hypotheses Mitochondria Kingdoms (biology) {{Eukaryote-stub