Mixed infection in tomato to ensure frequent generation of a natural reassortant between two subgroups of Cucumber mosaic virus.

Cucumber mosaic virus (CMV) is divided into subgroups I (sub-I) and II (sub-II). Isolate CMV-PF, which had been isolated from tomato by single lesion isolation (SLI), was classified in sub-II by serology, but it had the pathogenicity of sub-I CMVs. Sequence comparisons and phylogenetic analyses showed that CMV-PF was actually a reassortant CMV isolate containing RNAs 1 and 2 from sub-I and RNA 3 from sub-II (I, I, and II). A subgroup-specific reverse transcription-polymerase chain reaction (SS-RT-PCR) was devised to detect reassortants between sub-I and sub-II. Six primer sets specific to genomic RNAs were designed for each subgroup, and those primer sets successfully amplified their target RT-PCR products. Of 38 natural CMV isolates, two isolates (nos. 9-7 and 27-3) contained mixed RNAs of both subgroups. SLI-progenies of 9-7 were subsequently isolated by SLI using Vigna unguiculata and analyzed by SS-RT-PCR; they were found to be a reassortant containing RNAs 1 and 2 from sub-I and RNA 3 from sub-II (I, I, and II). Although natural reassortants between CMV subgroups were thought to be infrequent based on previous observations, we readily found two isolates of a reassortant (I, I, and II) in different tomato fields, implying that tomato may act as a reservoir of mixed infection in the generation of a CMV reassortant perhaps by aphids.