Archezoa

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Archezoa was a kingdom proposed by Thomas Cavalier-Smith (1942–2021 AD) in the 20th century, and was believed to encompass eukaryotes which did not have mitochondria (and are therefore "amitochondriate") or peroxisomes. 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 of mitochondria. Eukaryotes that eventually inducted a mitochondria were placed in a taxonomic group which Cavalier-Smith called the Metakaryota, whereas the Archezoa represented a paraphyletic group representing primitive eukaryotes which evolved prior to the acquisition of mitochondria. Furthermore, the Archezoa kingdom was divided into four phyla.[1][2]

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 reduced and lost. Some mitochondria had degraded into simpler double-membrane bound organelles known as mitosomes and hydrogenosomes in some of these organisms, which lack any genetic material. Initial discoveries demonstrating that this category was incorrect were that all categories placed in this group 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 migrated to the nuclear DNA over time as a result of endosymbiotic gene transfer.[3][4] 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.[5]

An early, incomplete classification system[]

Cavalier-Smith's system was developed before the presence and function of mitochondria in life forms was well-codified into taxonomic systems. As the understanding of the functions of mitochondria became more advanced, phylogeny of these life forms became more nuanced.

At various times, a few life forms were included in Archezoa. Two groupings of protists- Pelobionts and entamoebids (now Archamoebae)- were included. Metamonads, and the Microsporidia (both currently unranked within the Eukaryota domain) were included. These groups appear near the base of eukaryotic evolution on rRNA trees[clarify]. Today, all these groups are known to have developed from mitochondriate ancestors, and trees based on other genes do not support their basal placement.

References[]

  1. ^ Tom Cavalier-Smith (May 1989). "Archaebacteria and Archezoa". Nature. 339 (6220): 100–101. Bibcode:1989Natur.339..100C. doi:10.1038/339100a0. PMID 2497352. S2CID 30405691.
  2. ^ Tom Cavalier-Smith (December 1993). "Kingdom protozoa and its 18 phyla". Microbiological Reviews. 57 (4): 953–994. doi:10.1128/mmbr.57.4.953-994.1993. PMC 372943. PMID 8302218.
  3. ^ Clark, C. G. & Roger, A. J. Direct evidence for secondary loss of mitochondria in Entamoeba histolytica. Proc. Natl Acad. Sci. USA 92, 6518–6521 (1995)
  4. ^ Roger, A. J. Reconstructing early events in eukaryotic evolution. Am. Nat. 154, S146–S163 (1999)
  5. ^ Embley TM, Martin W. 2006. Eukaryotic evolution, changes and challenges. Nature 440(7084):623–630.
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