Abstract
The nucleocytoplasmic large DNA viruses (NCLDVs) are a monophyletic group of diverse eukaryotic viruses that reproduce primarily in the cytoplasm of the infected cells and include the largest viruses currently known: the giant mimiviruses, pandoraviruses, and pithoviruses. With virions measuring up to 1.5 μm and genomes of up to 2.5 Mb, the giant viruses break the now-outdated definition of a virus and extend deep into the genome size range typical of bacteria and archaea. Additionally, giant viruses encode multiple proteins that are universal among cellular life forms, particularly components of the translation system, the signature cellular molecular machinery. These findings triggered hypotheses on the origin of giant viruses from cells, likely of an extinct fourth domain of cellular life, via reductive evolution. However, phylogenomic analyses reveal a different picture, namely multiple origins of giant viruses from smaller NCLDVs via acquisition of multiple genes from the eukaryotic hosts and bacteria, along with gene duplication. Thus, with regard to their origin, the giant viruses do not appear to qualitatively differ from the rest of the virosphere. However, the evolutionary forces that led to the emergence of virus gigantism remain enigmatic.
Highlights
Viruses are obligate intracellular parasites that are generally considered to be tiny compared with cellular life forms
We present the results of the phylogenomic analysis of the nucleo-cytoplasmic large DNA viruses (NCLDVs), including the reconstruction of gene gain and loss events
Apart from the branches containing giant viruses, there is a consistent trend of gene loss among those NCLDVs that apparently have switched from protist to animal hosts, namely irido-ascoviruses, asfarviruses, and poxviruses (Figure 2). These findings reveal a highly dynamic evolution of the NCLDVs and strongly suggest that giant viruses evolved from simpler viruses with smaller genomes on many independent occasions[16]
Summary
Faculty Reviews are review articles written by the prestigious Members of Faculty Opinions. The articles are commissioned and peer reviewed before publication to ensure that the final, published version is comprehensive and accessible. The reviewers who approved the final version are listed with their names and affiliations. Virus evolution, virus-host interaction, gene gain; gene loss, phagocytosis. 2. Purificacion Lopez-Garcia, CNRS, Université Paris-Sud/Paris-Saclay, Paris, France. Any comments on the article can be found at the end of the article
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