Endogenous pararetroviruses of allotetraploid Nicotiana tabacum and its diploid progenitors, N. sylvestris and N. tomentosiformis

Endogenous pararetroviruses (EPRVs) represent a new class of dispersed repetitive DNA in plants. The genomes of many Nicotiana species and other solanaceous plants are rich in EPRVs. Distinct EPRV families are present in N. sylvestris (Ns) and in N. tomentosiformis (Nto), the two diploid progenitors of allotetraploid N. tabacum. Nicotiana EPRVs represent an interesting type of repetitive sequence to analyse in polyploids because of their potential impact on plant fitness and the epigenetic architecture of plant genomes. The NsEPRV family appears identical in N. sylvestris and N. tabacum, indicating little change has occurred in either species since polyploid formation. By contrast, the NtoEPRV family is larger in N. tomentosiformis than in N. tabacum, suggesting either preferential elimination from the polyploid genome or specific accumulation in the diploid genome following polyploidization. The lability of NtoEPRVs might be enhanced by a frequent association with gypsy retrotransposons. Although some EPRVs are probably benign, others are potentially pathogenic or, conversely, determinants of virus resistance. Normally quiescent EPRVs can be reactivated and cause symptoms of infection in hybrids of species that differ in their EPRV content. EPRVs that furnish immunity to the free virus exemplify the selective value of so-called 'junk' DNA. Variation in the abundance and distribution of EPRVs among related species can be useful in taxonomic and evolutionary studies.