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 95% confidence interval for our estimate of W is 1.016-1.040 d À1 , suggesting that 1) there is a cost to depend on in trans NIb expression, but 2) this cost is partially compensated for by faster replication of TEV-DNIb, resulting in a higher W than the wild-type virus. Our data therefore support the idea of a relationship between genome size and fitness, and suggest that viruses harboring deletions in NIb will outcompete the wild-type virus in NIbexpressing plants. Therefore, NIRV expression might also lead to conditions favoring the replication of virus variants, with deletions in the gene(s) encoded for by the NIRV. However, the accumulation of viruses with large genomic deletions due to in trans expression of virus' genes will also depend on mutational supply, specifically how often recombination generates viruses with deletions in NIb that conserve the reading frame in downstream genes.