Guest contribution by John Tillman
I previously reported on recent research into the formation of mitochondria, one of the two characteristics that distinguish eukaryotes from prokaryotes (domains bacteria and archaea). The other defining characteristic, the cell nucleus, gives the eukaryotes their name. Mitochondria almost certainly originate from the endosymbiosis of an alphaproteobacterium by an Asgard Archaeon. Now there is further evidence to support the hypothesis that the eukaryotic nucleus was caused by a viral infection of an archaeon.
This post reports on an unpaid paper in Virus Research last November that provides support for this viral eukaryogenesis hypothesis.
Evidence of a viral origin of the eukaryotic nucleus
The defining characteristic of the eukaryotic cell is the possession of a nucleus that decouples transcription from translation. According to the updated hypothesis of viral eukaryogenesis (VE) presented here, the eukaryotic nucleus is derived from the viral factory of a DNA virus that infected the archaic ancestor of the eukaryotes. The VE hypothesis implies that many unique features of the nucleus, including the mechanisms by which the eukaryotic nucleus decouples transcription from translation, should be of viral rather than cellular origin. The modern eukaryotic nucleus decouples transcription from translation using a complex process in which hundreds of eukaryotic specific genes work together. This complicated process is triggered by the eukaryote-specific 7-methylguanylate (m7G) cap on eukaryotic mRNA, which 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, but apparently absent in archaea in general and specifically in Lokiarchaeota, a suggested archaic relative of the eukaryotes. The phylogenetic analysis of the m7G capping apparatus shows that eukaryotic nuclei and Mimiviridae received this common path from a common ancestral source that predates the origin of the last eukaryotic common ancestor (LECA). These results are consistent with the hypothesis that the eukaryotic nucleus and 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 that is found in the nucleus of eukaryotes. Translation means using this mRNA to build the protein it codes for, whereby transfer RNA (tRNA) carries the required amino acids to the polypeptide assembly site on a ribosome in the cellular cytoplasm.
Ribosomes are made up of larger and smaller bundles of ribosomal RNA (rRNA) that are decorated with proteins. They are similar in size and composition to RNA viruses. Ribosomal RNA is tRNA.
If this is further confirmed, this hypothesis means that the eukaryota domain is nested in the archaic phylum Lokiarchaeota and that we eukaryotes are descended from the symbiogenesis of an archaeon, bacterium and virus. There is growing support for Family Mimiviridae, also known as "giant viruses", the status of a living organism rather than just a "replicant" among the many underlying "mobile genetic elements". They are large double-stranded DNA viruses.
The first family member, genus Mimivirus, was identified in 2003. It was discovered in 1992 when it infected an amoeba but was mistaken for a bacterium. Typical viruses are about 100 times smaller than normal bacteria, but giant viruses are similar in size.
In related news about friendly viruses, a study from a year ago:
How the placenta evolved from an ancient virus
Of the approximately 180 recognized virus families, 26 contain human pathogens, 18 of which are RNA.
Classification of human viruses