Unequal Variation in the Two Genome Parts of Tobraviruses and Evidence for the Existence of Three Separate Viruses

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Abstract
Hybridization experiments using complementary DNA copies showed that 14 tobravirus isolates could be divided into three groups, comprising ten of tobacco rattle virus (TRV) serotype I-II, one of TRV serotype III, and three of pea early-browning virus (PEBV), respectively. RNA from members of each group has much nucleotide sequence homology with RNA of other members of the same group but little or none with RNA from members of the two other groups. The ten TRV serotype I-II isolates share extensive sequences in their RNA-1 species, but show much diversity in their RNA-2 species and in the antigenic relatedness of their particles. However, homology was found between the RNA-2 sequences of isolates that are serologically closely related. Because strain CAM (TRV serotype III) is as different from isolates of TRV serotype I-II in nucleotide sequence, antigenic specificity and biological properties as is PEBV, it is now considered to be a separate virus and a return to its original name, pepper ringspot virus, is recommended.