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 or a bacterium. The virus later evolved into the eukaryotic nucleus by acquiring genes from the host genome and eventually usurping its role. The hypothesis was first proposed by Philip Bell in 2001[1] and was further popularized with the discovery of large, complex DNA viruses (such as Mimivirus) that are capable of protein biosynthesis.
Viral eukaryogenesis has been controversial for several reasons. For one, it is sometimes argued that the posited evidence for the viral origins of the nucleus can be conversely used to suggest the nuclear origins of some viruses.[2] Secondly, this hypothesis has further inflamed the longstanding debate over whether viruses are living organisms.[2]
The viral eukaryogenesis hypothesis posits that eukaryotes are composed of three ancestral elements: a viral component that became the modern nucleus; a prokaryotic cell (an archaeon according to the eocyte hypothesis) which donated the cytoplasm and cell membrane of modern cells; and another prokaryotic cell (here bacterium) that, by endocytosis, became the modern mitochondrion or chloroplast.
In 2006, researchers suggested that the transition from RNA to DNA genomes first occurred in the viral world.[3] 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).[2] 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.[3] In the original paper it was also an RNA cell at the origin of eukaryotes, but eventually more complex, featuring RNA processing. Although this is in contrast to nowadays's more probable eocyte hypothesis, viruses seem to have contributed to the origin of all three domains of life ('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.[4]
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.[5] 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.[6][7]
The viral eukaryogenesis hypothesis points to the cell cycle of eukaryotes, particularly sex and meiosis, as evidence.[6] Little is known about the origins of DNA or reproduction in prokaryotic or eukaryotic cells. It is thus possible that viruses were involved in the creation of Earth's first cells.[8] The eukaryotic nucleus contains linear DNA with specialized end sequences, like that of viruses (and 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.[2] Transfers of "infectious" nuclei have been documented in many parasitic red algae.[9]
Recent supporting evidence includes the discovery that upon the infection of a bacterial cell, the giant bacteriophage 201 Φ2-1 (of the genus Phikzvirus) assembles a nucleus-like structure around the region of genome replication and uncouples transcription and translation, and synthesized mRNA is then transported into the cytoplasm where it undergoes translation.[10] The same researchers also found that this same phage encodes a eukaryotic homologue to tubulin (PhuZ) that plays the role of positioning the viral factory in the center of the cell during genome replication.[11] The PhuZ spindle shares several unique properties with eukaryotic spindles: dynamic instability, bipolar filament arrays, and centrally positioning DNA.[7] Further, many classes of nucleocytoplasmic large DNA viruses (NCLDVs) such as mimiviruses have the apparatus to produce m7G capped mRNA and contain homologues of the eukaryotic cap-binding protein eIF4E. Those supporting viral eukaryogenesis also point to the lack of these features in archaea, and so believe that a sizable gap separates the archaeal groups most related to the eukaryotes and the eukaryotes themselves in terms of the nucleus. In light of these and other discoveries, Bell modified his original thesis to suggest that the viral ancestor of the nucleus was an NCLDV-like archaeal virus rather than a pox-like virus.[7] Another piece of supporting evidence is that the m7G capping apparatus (involved in uncoupling of transcription from translation) is present in both Eukarya and Mimiviridae but not in Lokiarchaeota that are considered the nearest archaeal relatives of Eukarya according to the Eocyte hypothesis (also supported by the phylogenetic analysis of the m7G capping pathway).[7]
Several 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.[12]