Viral particles are much more abundant than cells and viral genes outnumber cellular ones in the biosphere. Cellular genomes also harbour many integrated viruses whereas cellular genes are rare in viral genomes. The gene flux from virus to cell is thus overwhelming if compared with the opposite event. Novel viral genes continuously arose during replication/recombination of viral genomes in the virocell. These genes can become 'cellular genes' when viral genomes integrate into cellular ones. Together with the arm race between viruses and cells, this explains why viruses have played a major role in shaping cellular gene contents. Several documented cases show that viruses have been involved in the emergence of evolutionary innovations. This gives credit to hypotheses suggesting that viruses have played an important role in the formation of modern cells.
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