Abstract
The majority of characterized plant viruses have RNA genomes. Genetic variability is a fundamental feature of RNA viruses. High mutation rates, recombination and reassortment are the three basic mechanisms that are responsible for the enormous genetic polymorphism and rapid evolution of RNA viruses. Mutations are most frequently introduced into the viral genome during the replication process due to the low fidelity of RNA-dependent RNA polymerases. Recombination is a widespread phenomenon described in many plant viruses with both RNA and DNA genomes and is responsible for more profound changes within the viral genome (sequence deletion or insertion or strand exchange). Reassortment is also an important mechanism responsible for swapping or introducing a whole genomic segment of the viral genome, but is limited only to the segmented viruses. However, these three mechanisms are counterbalanced by selection and genetic bottlenecks which reduce the genetic variation of plant viruses in nature. Recently, genetic bottlenecks have been identified experimentally in plant virus populations during the systemic movement within the plant and horizontal transmission from plant to plant by aphid vectors.