Shrinkage of Genome Size in a Plant RNA Virus upon Transfer of an Essential Viral Gene into the Host Genome

Tromas, Nicolas; Zwart, Mark P.; Forment, Javier; Elena, Santiago F.

The genome size of RNA viruses is highly variable, ranging from less than 4 kb for members of the Narnaviridae to more than 30 kb for some Coronaviridae. However, most RNA viruses have genome sizes in the 5-15-kb range (Holmes 2009 ). What could explain this wide range of genome sizes in RNA viruses? Small genomes probably replicate faster than large ones, while genome size limits enzymatic complexity. This limited complexity results in lower-fidelity replication enzymes, which in turn leads to higher variability (Smith and Szathmá ry 1995) . On the other hand, larger genomes will replicate more slowly, while being less constrained in the complexity of the proteins they code for. Therefore, larger genomes allow for evolutionary innovations such as polymerases with greater replication fidelity (Denison et al. 2011) or the modulation of host immune responses (Shackelton and Holmes 2004) . However, the evolution of larger genomes and greater complexity may be impeded by Eigen's paradox: to obtain proofreading activity, an RNA virus must already have a large genome (Eigen 1971; Smith and Szathmá ry 1995) . It has therefore been suggested that RNA virus genome size evolves to the upper limit imposed by mutation rate (Swetina and Schuster 1982) .