Biological properties and relative fitness of inter-subgroup cucumber mosaic virus RNA 3 recombinants produced in vitro

In vitro reverse transcription of a mixture of total RNA from plants infected with the l17F or R strains of cucumber mosaic virus (CMV), representative of subgroups A and II, respectively, results in viral cDNA populations including rare recombinant RNA 3 molecules, some of which also have point mutations. The biological properties of 17 recombinants in the capsid gene or the 3' non-coding region of RNA 3 were evaluated when associated with l17F RNAs 1 and 2. Six viruses displayed deficiencies (non-viability, deficiencies for movement and/or replication, delayed infection, loss of aphid transmissibility). Nine induced symptoms close to those of l17F-CMV on tobacco and pepper plants. All recombinants bearing the movement protein (MP) of R-CMV and part or most of the capsid protein (CP) of l17F-CMV, as well as the recombinant created in vitro by exchanging the corresponding open reading frames, also induced filiformism on tobacco, but induced only faint symptoms on melon. Two recombinants induced atypically severe symptoms on both tobacco and pepper. Most of the recombinants generally accumulated to lower levels than the wild-type l17F strain in tobacco. Three recombinants, however, including one responsible for severe symptoms, accumulated to generally higher levels than l17F-CMV. When two of these were tested in co-infection experiments with l17F RNA 3, they proved to be poorly competitive, suggesting that they would be unlikely to emerge in the field.