We read with interest the recent article in Nature Reviews Microbiology by Moreira and López-García (Ten reasons to exclude viruses from the tree of life. Nature Rev. Microbiol. 7, 306–311 (2009))1 who courageously attempted to exclude viruses from the tree of life. The inability of viruses to self-sustain and self-replicate, their phylogenetic diversity, the origin of their cell-like genes and the instability of their genomes over time were emphasized to strengthen the argument. However, as discussed in this Correspondence, we emphasize that although we might be able to call viruses 'pseudo living entities' or 'molecular parasites' we cannot deprive them of their status as living entities.
We cannot compare primitive organisms such as viruses with complex living entities such as plants and animals. In fact, because viruses are 'mono-unitary' organisms, the most plausible comparison would be with animal spermatozoa or ova. Can we expect either spermatozoa or ova to survive and replicate in their natural environment without any support? Definitely not. In fact, many of the cell types isolated from an organism cannot be sustained even when we provide the best in vitro environment for them to grow in. Therefore, the argument that viruses should be excluded from the tree of life because they cannot sustain themselves under natural conditions does not hold. Also, 'nature' does not mean mere soil, light and water. It also includes all living entities, including plants and animals. Although viruses need host cells for survival and replication, we cannot argue that viruses will not survive in nature if we leave them alone, as hosts themselves are part of the continuum of nature. In fact, even animals and birds are not sustained in nature unless they obtain food from other sources of life, that is, plants or other animals. Thus, similarly to viruses, animals and birds are dependent on other species to be sustained in nature. So, virus replication in the host means that viruses replicate in nature. An interesting case is that of Sputnik, a recently described virus that exists inside another virus, acanthamoeba polyphaga mimivirus2. If a virus can live inside another virus, is the bigger virus similar to a host cell or is it simply a parasite within a parasite?
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