The Origin of Multicomponent Small Ribonucleoprotein Viruses

Abstract

One of the striking features of plant virology is the abundance of small ribonucleoprotein plant viruses with a divided genome distributed over two or more particles and not contained within a viral envelope. This type of virus is generally called a multicomponent virus. A study of the origin and evolution of multicomponent small ribonucleoprotein viruses is beset by two problems. The development of the evolutionary theory traditionally rests on two approaches: the study of fossils and the comparative biology of the current species. The first problem is that there are no fossils available, from which to extract meaningful information about the origin and evolution of multicomponent small ribonucleoprotein viruses. Therefore, it seems that the only useful approach to the formulation and testing of a theory of the origin and evolution of these viruses is the comparative study of current viruses, their transmission, and their hosts. The second problem is that so far the origin of small ribonucleoprotein viruses per se has remained obscure. There are many, so far unrefuted, possibilities. First, small ribonucleoprotein viruses could have developed from degenerate bacteria or from now-extinct forms of life based on RNA genomes. Second, they might have originated directly from cellular components, such as nuclear RNA or mRNA, intermediates in the amplification of cellular (DNA) information, or regulatory cellular RNAs. Also, small ribonucleoprotein viruses might have originated from virus-related RNAs, like messengers from double-stranded RNA viruses or via a stage resembling current vertebrate RNA tumor viruses. Alternatively, their ancestors may have resembled current enveloped (+) RNA viruses, such as arbo-or toga viruses, or (-) RNA viruses, such as the myxoviruses. Moreover, it is not clear whether all the small RNA viruses have a common ancestor or whether there were several independent ancestors.