I6 and N5 are naturally occurring tobravirus isolates that produce symptoms in herbaceous plants similar to those induced by strains of tobacco rattle virus (TRV). In immunosorbent electron microscopy tests, however, they reacted with antisera to particles of pea early-browning virus (PEBV), not TRV. Furthermore, these tests indicated that I6 was related to the British serotype of PEBV and N5 to the Dutch. Pseudo-recombinant isolates were produced by reassortment of the genome parts of I6 or N5 with those of TRV, in any combination, but not in most combinations with those of PEBV. However, I6 RNA-2 was replicated in plants inoculated also with RNA-1 from an isolate of the British serotype of PEBV, but the PEBV RNA-1 was imperfectly packaged by I6 coat protein, and the virus particles seemed to have only limited stability. Nucleic acid hybridization experiments showed that the RNA-1 sequences of both I6 and N5 were similar to those of TRV strains. I6 RNA-2 contained sequences resembling those of the British serotype of PEBV, but with some TRV-like sequences at the 3′ and 5′ ends, whereas N5 RNA-2 contained more extensive TRV-like 3′ and 5′ ends flanking sequences that were related, but perhaps not closely, to those of the Dutch serotype of PEBV. Thus, the RNA-2 species of I6 and N5 were recombinant molecules that contained sequences typical of both TRV and PEBV, and which probably had separate but similar evolutionary origins. As a result of their hybrid nature, I6 and N5 were part of the gene pool and had the pathogenicity of TRV, while possessing the serological properties of PEBV.


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