A Distinct Endogenous Pararetrovirus Family in Nicotiana tomentosiformis, a Diploid Progenitor of Polyploid Tobacco

A distinct endogenous pararetrovirus (EPRV) family corresponding to a previously unknown virus has been identified in the genome of Nicotiana tomentosiformis, a diploid ancestor of allotetraploid tobacco (Nicotiana tabacum). The putative virus giving rise to N. tomentosiformis EPRVs (NtoEPRVs) is most similar to tobacco vein clearing virus, an episomal form of a normally silent EPRV family in Nicotiana glutinosa; it is also related to a putative virus giving rise to the NsEPRV family in Nicotiana sylvestris (the second diploid progenitor of tobacco) and in the N. sylvestris fraction of the tobacco genome. The copy number of NtoEPRVs is significantly higher in N. tomentosiformis than in tobacco. This suggests that after the polyploidization event, many copies were lost from the polyploid genome or were accumulated specifically in the diploid genome. By contrast, the copy number of NsEPRVs has remained constant in N. sylvestris and tobacco, indicating that changes have occurred preferentially in the NtoEPRV family during evolution of the three Nicotiana species. NtoEPRVs are often flanked by Gypsy retrotransposon-containing plant DNA. Although the mechanisms of NtoEPRV integration, accumulation, and/or elimination are unknown, these processes are possibly linked to retrotransposon activity.