Virus populations, mutation rates and frequencies

Populations of plant viruses are genetically heterogeneous. This heterogeneity is often linked to mutation, the ultimate source of genetic variation and an uncontested player in plant virus evolution. This review gives basic key information indispensable to understanding mutation in plant viruses, from mutation sources, mutation detection means, to the role of mutation in shaping plant virus evolution in combination with various other evolutionary factors. From information drawn from the recent literature, we confirm or refute some generally held views and we reinstate several unanswered questions. It is clear that low genetic diversity characterizes some plant virus populations, irrespective of their life cycle or their nature (DNA or RNA). Mutation frequencies of plant DNA viruses can be as high as those of RNA viruses. This casts some doubt on a positive correlation between high mutation rates and adaptive evolution, and on the lack of proofreading for RNAdependent RNA polymerases. However, the lack of information on viral mutation rates still precludes a complete understanding of the link between mutation rates and population heterogeneity. Information about plant virus replication mode, generation time and generation size also is still crucially needed before a complete picture of virus evolution will emerge.