Division of Plant Industry, Commonwealth Scientific and Industrial Research Organisation, Canberra, Australia.
Viruses represent the epitome of genetic diversity, encompassing both DNA and RNA genomes. Their variability probably results from high mutation rates, extensive genetic recombination, gene duplication, large populations and ancient origins. In addition, at least a third of all viral genes may have de novo origins. There is also recent evidence that additional novelty may be generated by genetic recombination between viruses and their plant hosts. Viruses are indeed master explorers of evolutionary space.