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^[1] and gained support as large, complex DNA viruses (such as Mimivirus) capable of protein biosynthesis were discovered. Most recently, it was revealed that upon the infection of a bacterial cell, the giant bacteriophage 201Phi2-1 assembles a nucleus-like structure^[2]. This also strengthens the hypothesis of Viral eukaryogenesis.

While viruses remain one of the more poorly understood aspects of biology, recent genomic research and the discovery of complex DNA viruses indicate that they may have played a role in the development of eukaryotic nuclei. This research further opens a hotly debated question: Are viruses living organisms? Most virologists took the stand that viruses were not alive; currently, viruses are hypothesized by some researchers to have been ancestors of modern eukaryotic cells—most importantly with respect to DNA, the shared genetic code of all eukaryotes and prokaryotes alive today.^[3]

Hypothesis

Like viruses, a eukaryotic nucleus contains linear chromosomes with specialized end sequences (in contrast to bacterial genomes, which have a circular topology); it 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.^[3] Transfers of "infectious" nuclei have been documented in many parasitic red algae.^[4] These traits suggest that complex eukaryotic DNA viruses could also have originated through nuclear viriogenesis.^[3]

The viral eukaryogenesis hypothesis posits that eukaryotes are composed of three ancestral elements: a viral component that became the modern nucleus; a prokryotic cell (originally thought to be very primitive but more probably an archaeon according to Eocyte hypothesis) that donated the cytoplasm and cell membrane of modern cells; and another prokaryotic (bacterial) cell that, by endocytosis, became the modern mitochondrion or chloroplast. Molecular homology suggests that all life shares a single common ancestor, and evidence, alongside the fossil record, can be found in the evolution of prokaryotes, and Archaea prior to the eukaryotes. The viral eukaryogenesis hypothesis points to the cell cycle of eukaryotes, particularly sex and meiosis, as evidence.^[5] Little is known about DNA or the origins of reproduction in prokaryotic or eukaryotic cells. It is thus possible that viruses were involved in the creation of Earth's first cells.^[6]

In 2006, researchers suggested that the transition from RNA to DNA genomes first occurred in the viral world.^[7] A DNA-based virus may have provided storage for an ancient host that had previously used RNA to store its genetic information (such host is called ribocell or ribocyte).^[3] Viruses may initially have adopted 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 contribution from chloroplasts or mitochondria. Following this hypothesis, archaea, Bacteria, and eukaryotes each obtained their DNA informational system from a different virus.^[7] In the original paper it was also an RNA cell at the origin of eukaryotes, but eventually more complex, featuring RNA processing. Althuogh this is in contrast to nowadays more probable Eocyte hypothesis, viruses seem to have contributed to the origin of all three domains of live ('out of virus hypothesis'). It has also been suggested that telomerase and telomeres, key aspects of eukaryotic cell replication, have viral origins. Further, the viral origins of the modern eukaryotic nucleus may have relied on multiple infections of archaeal cells carrying bacterial mitochondrial precursors with lysogenic viruses.^[8]

The viral eukaryogenesis 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 existing bacterial and archaeal species. The lysogenic virus then became the information storage center 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 because of the balances struck by viruses, which characteristically follow a pattern of tradeoff between infecting as many hosts as possible and killing an individual host through viral proliferation. Hypothetically, viral replication cycles may mirror those of plasmids and viral lysogens. However, this theory is controversial, and additional experimentation involving archaeal viruses is necessary, as they are probably the most evolutionarily similar to modern eukaryotic nuclei.^[5]

Implications

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). In theory, a large DNA virus could 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 dilemma typically faced by viruses. With the virus in control of the host cell's molecular machinery, it would effectively become a functional nucleus. Through the processes of mitosis and cytokinesis, the virus would thus recruit the entire cell as a symbiont — a new way to survive and proliferate.

See also

• Endogenous retrovirus
• Endogenous viral element
• Paleovirology

References

Further reading

• Livingstone Bell, Philip John (2001). "Viral Eukaryogenesis: was the ancestor of the nucleus a complex DNA virus?". Journal of Molecular Evolution. 53 (3): 251–6. PMID 11523012. doi:10.1007/s002390010215. 
• Trevors, Jack Thomas (2003). "Genetic material in the early evolution of bacteria". Microbiological Research. 158 (11): 1–6. doi:10.1078/0944-5013-00171.