Guest post by John Tillman

I previously posted on recent research into the origin of mitochondria, one of the two traits distinguishing eukaryotes from prokaryotes (Domains Bacteria and Archaea). The other defining characteristic, the cell nucleus, gives eukaryotes their name. Mitochondria almost certainly derive from endosymbiosis of an alphaproteobacterium by an Asgard archaeon. Now more evidence has emerged favoring the hypothesis that the eukaryotic nucleus arose from viral infection of an archaeon.

This post reports on an unpaywalled paper in the journal Virus Research from November last year, offering support for this Viral Eukaryogenesis hypothesis.

Evidence supporting a viral origin of the eukaryotic nucleus

Abstract
The defining feature of the eukaryotic cell is the possession of a nucleus that uncouples transcription from translation. According to the updated Viral Eukaryogenesis (VE) hypothesis presented here, the eukaryotic nucleus descends from the viral factory of a DNA virus that infected the archaeal ancestor of the eukaryotes. The VE hypothesis implies that many unique features of the nucleus, including the mechanisms by which the eukaryotic nucleus uncouples transcription from translation, should be viral rather than cellular in origin. The modern eukaryotic nucleus uncouples transcription from translation using a complex process employing hundreds of eukaryotic specific genes acting in concert. This intricate process is primed by the eukaryote specific 7-methylguanylate (m7G) cap on eukaryotic mRNA that targets mRNA for splicing, nuclear export, and cytoplasmic translation. It is shown here that homologues of the eukaryotic m7G capping apparatus are present in viruses of the Mimiviridae yet are apparently absent from archaea generally, and specifically from Lokiarchaeota, a proposed archaeal relative of the eukaryotes. Phylogenetic analysis of the m7G capping apparatus shows that eukaryotic nuclei and Mimiviridae obtained this shared pathway from a common ancestral source that predated the origin of the Last Eukaryotic Common Ancestor (LECA). These results are consistent with the hypothesis that the eukaryotic nucleus and the Mimiviridae obtained these abilities from an ancient virus that could be considered the ‘First Eukaryotic Nuclear Ancestor’ (FENA).

Transcription means the formation of Messenger RNA (mRNA) from a DNA template, which in eukaryotes occurs in the nucleus. Translation means using this mRNA to build the protein for which it codes, with Transfer RNA (tRNA) carrying the needed amino acids to the polypeptide assembly site on a ribosome in the cellular cytoplasm.

Ribosomes consist of a larger and smaller wad of Ribosomal RNA (rRNA), decorated with proteins. They’re similar in size and composition to RNA viruses. Ribosomal RNA is tRNA.

If further confirmed, this hypothesis means that Domain Eukaryota is nested within archaeal Phylum Lokiarchaeota, and that we eukaryotes descend from the symbiogenesis of an archaeon, bacterium and a virus. There is growing support for according Family Mimiviridae, aka “Giant viruses”, the status of living organism, rather than mere “replicant” among the many sub-alive “Mobile Genetic Elements”. They are large, double-stranded DNA viruses.

The first member of the family, Genus Mimivirus, was identified in 2003. It had been discovered in 1992 infecting an amoeba, but was mistaken for a bacterium. Typical viruses are about 100 times smaller than normal bacteria, but giant viruses are similar to them in size.

In related news about friendly viruses, a study from a year ago: 

How the placenta evolved from an ancient virus

Of the about 180 recognized viral families, 26 contain human pathogens, of which 18 are RNA.

Classification of Human Viruses

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