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.

On the other hand, genome size does not appear to be under the same selective pressure in eukaryotes, probably due to the smaller effective population sizes in higher organisms (Lynch et al. 2006) . Although the existence of many retroviral sequences in eukaryotic genomes was already well documented (Gifford and Tristem 2003) , it has been recently found that there are also many nonretroviral RNA virus genes in eukaryotic genomes. These nonretroviral integrated RNA viruses (NIRVs) have been found in mammals (Geuking et al. 2009; Belyi et al. 2010; Horie et al. 2010; Katzourakis and Gifford 2010) , plants (Hong et al. 1998; Chiba et al. 2011) , and fungi (Taylor and Bruenn 2009) . In fungi, NIRVs are very common and have in some cases become functional genes (Koonin 2010) . In one case, the entire viral RdRp was integrated and conserved, probably conferring viral resistance to the host (Taylor and Bruenn 2009 ). It has also been suggested that NIRVs can confer resistance (Bertsch et al. 2009 ) or tolerance (Flegel 2009 ) against the original virus to plant or animal hosts. Indeed, it has been a common practice to generate commercial transgenic plants resistant to viruses by inserting full or partial viral genes into the genome of the plants of agronomic interest (reviewed in Kundu and Mandal [2001] ); these transgenic plants can be considered as artificial cases of NIRVs.