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From Wikipedia, the free encyclopedia
The Eocyte hypothesis is a biological classification that indicates eukaryotes emerged within the prokaryotic Crenarchaeota (formerly known as eocytes), a phylum within the archaea. This hypothesis was originally proposed by James A. Lake and colleagues in 1984 based on the discovery that the shapes of ribosomes in the Crenarchaeota and eukaryotes are more similar to each other than to either bacteria or the second major kingdom of archaea, the Euryarchaeota.[2][3]
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Karyota | |
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Ignicoccus hospitalis (and its symbiote Nanoarchaeum equitans) | |
Scientific classification | |
Domain: | Archaea or Arkarya |
(unranked): | Karyota Lake et al. 1988[1] |
Domain & Regnum | |
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Synonyms | |
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The eocyte hypothesis gained considerable attention after its introduction due to the interest in determining the origin of the eukaryotic cell. This hypothesis has primarily been in contrast with the three-domain system introduced by Carl Woese in 1977. Additional evidence supporting the eocyte hypothesis was published in the 1980s, but despite fairly unequivocal evidence, support waned in favor of the three-domain system.[1][2]
With advancements in genomics, the eocyte hypothesis experienced a revival beginning in the mid-2000s. As more archaeal genomes were sequenced, numerous genes coding for eukaryotic traits have been discovered in various archaean phyla, seemingly providing support for the eocyte hypothesis. Proteomics based research has also found supporting data with the use of elongation factor 1-α (eEF-1), a common housekeeping protein, to compare structural homology between eukaryotic and archaean lineages.[4] Furthermore, other proteins have been sequenced through proteomics with homologous structures in heat shock proteins found in both eukaryotes and archaea. The structure of these heat shock proteins were identified though X-ray crystallography to find the three dimensional structure of the proteins. [5]These proteins however have differing purposes as the eukaryote heat shock protein is a part of the T-complex while the archaeal heat shock protein is a molecular chaperone.[6] This creates an issue with the sequence homology that has been seen between 70 kilodalton heat shock proteins in eukaryotes and gram negative bacteria.[7]
In addition to a Crenarchaeal origin of eukaryotes, some studies have suggested that eukaryotes may also have originated in the Thaumarchaeota.[2][8][9][10][11] A superphylum - TACK - has been proposed that includes the Thaumarchaeota, Crenarchaeota, and other groups of archaea[12], so that this superphylum may be related to the origin of eukaryotes. It is seen that eukaryotes share a large number of proteins with members of the TACK superphylum and that these complex archaea may have had rudimentary phagocytosis abilities to engulf bacteria.[13]
As a result of metagenomic analysis of material found nearby hydrothermal vents, another superphylum -- Asgard[14] -- has been named and proposed to be more closely related to the original eukaryote and a sister group to TACK more recently.[15][16] The eocyte tree root may be located in the RNA World, that is the root organism may have been a Ribocyte (aka Ribocell). For cellular DNA and DNA handling an "out of virus" scenario has been proposed, i. e. string genetic information in DNA may have been an invention performed by viruses later handed over to Ribocytes twice, once transforming them into bacteria and once transforming them into archaea.[17][18] Although archaeal viruses aren't as studied as bacterial phages, it is thought that dsDNA viruses lead to the incorporation of the viral genome into archaeal genomes. [19] The transduction of genetic material through a viral vector lead to an increase in complexity in the pre-eukaryotic cells.[20] All these findings do not change the eocyte tree as given here in principle, but zoom into a higher resolution of it.