Giant Viruses and the Creation of Eukaryotes

Trying to understand how the complex eukaryotic cell evolved is not fully understood. How did a simple prokaryote cell with no membrane-enclosed organelles give rise to cells that contained mitochondria, chloroplasts and a nucleus? One possible answer for this is the viral eukaryogenesis (VE) hypothesis.

According to the VC hypothesis the mitochondria was believed to have originally been a free-living marine alpha-proteobacterium. Possibly 2 billion years ago, this alpha-proteobacterium found its way into a larger archaeon. Whether this was an infection of the archaeon or the archaeon cell consuming the alpha-proteobacterium via phagocytosis remains unclear. In either case, the cells then developed an evolutionary symbiosis. The same hypothesis explains the rise of chloroplasts having come from an engulfed cyanobacterium and eventually gave rise to algae and then plants. What remains less understood is the sudden development of a nucleus within a cell. The VC hypothesis tries to explain this as being the result of a virus that infected the cell but instead of killing the cell became an essential part of it.

In 2003, a new virus was described that infects the genus of amoebae called Acanthamoeba. What is special about this virus is its giant size, both physically (750nm) and the size of its double-stranded DNA genome (1.2Mb) (Raoult et al, 2004). This is actually bigger than some marine bacteria. These viruses are now called mimiviruses (Mimiviridae) and have been given the nickname ‘giruses’.

In this study, an ancestor of the mimivirus is believed to be the origin of the eukaryotic nucleus for several reasons. Just like a eukaryotic nucleus the mimivirus genome is double-stranded, there are repetitive base sequences at their telomeres and have DNA polymerases that are homologous to eukaryotes but not prokaryotes. Mimiviruses also contain enzymes that are used in the capping of mRNA to protect it from degradation inside the host’s cytoplasm. This is not seen in prokaryotic cells but is in eukaryotic nuclei. This also means that once the viral capsid is inside the host cell it is able to carry out DNA replication and transcription independently. Although it still relies on the cell for translation and energy production.

Bell (2009) suggests that like some other viruses, the mimivirus causes the cell membrane to pinch off internal vesicles that is then packaged with DNA to produce new virions. Possibly, as mimivirus are so big, the host genetic material started to get packaged into these vesicles too. The virus remained in the cell and took on the role of DNA storage instead.

The VC hypothesis was originally suggested back in 2001 but with the discovery of the mimivirus a couple of years later this hypothesis had even more evidence to back it up. It is also an expansion on the endosymbiotic theory which is an older and better known theory. It is still just a theory of coarse but I think one that has a lot of strong evidence to back it up. It will be very interesting to see if there are more breakthroughs in this field in the near future.

A Review of:
Bell, P. J. L. (2009) The Viral Eukaryogenesis Hypothesis: A Key Role for Viruses in the Emergence of Eukaryotes from a Prokaryotic World Environment. Ann. NY Acad. Sci, 1178, 91-105.

Additional References:
Raoult, D. Audic, S. Robert, C. Abergel, C. Renesto, P. Ogata, H. La Scola, B. Suzan, M and Claverie, J (2004) The 1.2-Megabase Genome Sequence of Mimivirus. Science, 306, 1344-1350.