Viral eukaryogenesis

Viral eukaryogenesis is the hypothesis that the cell nucleus of eukaryotic life forms evolved from a large DNA virus in a form of endosymbiosis within a methanogenic archaeon. The virus later evolved into the eukaryotic nucleus by acquiring genes from the host genome and eventually usurping its role. The hypothesis was proposed by Philip Bell in 2001, and gained support as large complex DNA viruses capable of protein biosynthesis (such as Mimivirus) have been discovered.

While viruses remain one of the more poorly understood aspects of biology, recent genomic research coupled with the discovery of the existence of large, complex DNA viruses indicate that viruses may have played a role in the development of eukaryotic nuclei. This research further opens a hotly debated topic: are viruses living organisms? Viruses have long been considered to not be alive, but now are hypothesized to have been an ancestor of modern eukaryotic cells, most importantly, for life on earth, with respect to DNA, the shared genetic code of all eukaryotes and prokaryotes alive today.

Like viruses, a eukaryotic nucleus contains linear chromosomes with specialized end sequences (bacterial genomes have a circular topology), uses mRNA capping and separates transcription from translation. Eukaryotic nuclei are also capable of cytoplasmic replication. Some large viruses have their own DNA-directed RNA polymerase. A transfer of an 'infectious' nucleus is documented in many parasitic red algae Existing complex eukaryotic DNA viruses could also have originated through nuclear viriogenesis.

The viral eukaryogenesis (VE) hypothesis posits that eukaryotes are composed of three ancestral elements: a viral component that preceded the modern nucleus, another, eukaryotic, cell that forms what is now the cytoplasm of modern cells, and a bacterium that, by endocytosis, became the modern mitochondrion. Molecular homology suggests that all life shares one single common ancestor, and further evidences, alongside the fossil record, the evolution of Prokaryotes and Archaea before Eukaryotes. The viral eukaryogenesis hypothesis points to the cell cycle of eukaryotes, with particular focus on sex and meiosis, as evidence for viral eukaryogenesis. Little is known about either DNA or the origins of reproduction in both prokaryotic and eukaryotic cells. It is thus entirely possibly that viruses were involved in the creation of Earth's first cells.

Recently, it was suggested that the transition from RNA to DNA genomes first occurred in the viral world. A DNA-based virus may have provided a DNA-based storage for the ancient host that was previously using RNA to store all its genetic information. Viruses could initially adopt DNA as a way to resist RNA degrading enzymes in the host cells. Hence the "contribution" from such a new component may have been as significant as the one of chloroplast or mitochondria. Following this hypothesis, Archaea, Bacteria, and Eukarya each obtained it's DNA informational system from a different virus. The RNA cell at the origin of Eukarya was probably more complex, featuring RNA processing. It has further been suggested that telomerase and telomeres, key aspects of eukaryotic cell replication, have viral origins. Further, it has been suggested that viral origins of the modern eukaryotic nucleus may have relied on multiple infections of archaeal cells carrying bacterial mitochondrial precursors with lysogenic viruses in order to achieve the event of viral ancestry for all modern eukaryotes.

The VE hypothesis depicts a model of eukaryotic evolution in which a virus, similar to a modern pox virus, evolved into a nucleus via gene acquisition from previously existing bacterial and archaeal species. Upon this occurrence, the lysogenic virus became the storage center of information for the cell, while the cell retained its capacities for gene translation and general function despite the viral genome’s entry. Similarly, the bacterial species involved in this eukaryogenesis retained its capacity to produce energy in the form of ATP while also passing much of its genetic information into this new virus-nucleus organelle. It is hypothesized that the modern cell cycle, whereby mitosis, meiosis, and sex occur in all eukaryotes, evolved due to the balances struck by viruses: they characteristically follow a pattern of tradeoff between infecting as many hosts as possible and killing an individual host due to viral proliferation. Hypothetically, viral replication cycles may mirror those of plasmids and viral lysogens. However, this theory is very much controversial and it is clear that additional experimentation involving viruses within Archaea is necessary, as they are likely the most evolutionary similar to modern eukaryotic nuclei.

A number of precepts in the theory are possible. For instance, a helical virus with a bilipid envelope bears a distinct resemblance to a highly simplified cellular nucleus (i.e.: a DNA chromosome encapsulated within a lipid membrane). To consider the concept logically, a large DNA virus would take control of a bacterial or archaeal cell. Instead of replicating and destroying the host cell, it would remain within the cell, thus overcoming the tradeoff paradox typically faced by viruses. With the virus in control of the host cell's molecular machinery it would effectively become a "nucleus" of sorts. Through the processes of mitosis and cytokinesis, the virus would thus hijack the entire cell—an extremely favorable way to ensure its survival.

Although this hypothesis has been evaluated by scientists in the field of microbiology in the last decade, more research is necessary to support the theory of viral eukaryogenesis. There are significant gaps in what is currently known about origins of life on Earth, and many implications for further research on both viruses and the ancestral state of our genetic code. As such, the VE theory brings up interesting questions about what it means for organisms to be alive, about organelles, and about the ancestral state of all life on Earth.

See also

• Endogenous retrovirus
• Endogenous viral element
• Paleovirology