Both the TAV 2b protein and WAII-CMV 2b protein were able to promote the recombination and/or selection of RNA 3 in the passage plants, but not in the inoculated plants. To determine when the recombinant RNA 3s could first be detected, a time-course experiment was done involving three viruses: C1C2 T2B T3, C1C2 T2BC2a T3 and C1C2 W2B T3. The appropriate plasmid combinations were inoculated onto 10 N. glutinosa plants and sap was then passaged onto healthy N. glutinosa plants for 10 passages, at 6-day intervals. The inoculated leaves of the passage plants were detached 3 days after inoculation, total RNAs were extracted and the RNAs were analysed by northern blot hybridization. These analyses showed that the recombinant RNAs were only detected from the fourth passage and onwards in plants infected with C1C2 T2B T3 or C1C2 T2BC2a T3, or from the sixth passage and onwards in plants infected with C1C2 W2B T3 (Supplementary Figure 1A, data not shown) . These results indicated that selection of the recombinant RNA 3s is a slow process and therefore it seems more likely that the 2b proteins were selecting for specific recombinants, rather than being involved in the generation of the recombinant RNA 3s. This is also consistent with the different subcellular localization of the replication proteins and the 2b protein (49) (50) (51) and the lack of detectable interaction of the replicase proteins with the 2b protein (52) . In addition, virus containing the TAV 2b protein showed selection of the recombinant RNA 3s two passages earlier than did virus containing the WAII-CMV 2b protein. Therefore, it seems likely that the source of the 2b protein also affected the rate of selection.