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.

Because the reading frame is conserved in many NIRVs (Taylor and Bruenn 2009) they can in principle be expressed. In practice, this will depend on their location in the genome and the state of the host cell. Some NIRVs are, however, clearly expressed by the host (Taylor and Bruenn 2009) . Host NIRV expression has been proposed to mediate host immunity against the cognate virus (Bertsch et al. 2009; Flegel 2009; Taylor and Bruenn 2009) . However, host NIRV expression may also have important implications for the evolution of the cognate virus. The absence of an effective NIRV-induced host immune response and sufficiently high NIRV expression levels could lead to complementation of host proteins, making the NIRV-expressed genes in the cognate virus functionally redundant. What would be the impact of NIRV expression on viral replication? Is there any selective advantage to using in trans the protein produced from the NIRV as a functional viral protein? How will a virus population evolve in such a situation of functional redundancy? Finally, how do such large-scale genomic perturbations affect the regulation of expression and accumulation of all other viral components? To our knowledge these questions have not been addressed in any study, while given the prevalence of NIRVs in natural host populations and biosafety issues linked to the use of transgenic plants expressing viral genes for resistance, it is a highly relevant topic.