Do viruses form lineages across different domains of life?

The scarce characterisation of the viral world has hampered our efforts to appreciate the magnitude and diversity of the viral domain. It appears that almost every species can be infected by a number of viruses. As our knowledge of viruses increases, it appears that this myriad of viruses may be organised into a reasonably low number of viral lineages including members infecting hosts belonging to different domains of life. Viruses belonging to a lineage share a common innate "self" that refers to structural and assembly principles of the virion. This hypothesis has a few consequences. All viruses are old, maybe preceding cellular life, and virus origins are polyphyletic, as opposed to the idea of a monophyletic origin of cellular life. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.