Evidence supporting a viral origin of the eukaryotic nucleus.

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).