AN INTERSPECIES HYBRID RNA VIRUS IS SIGNIFICANTLY MORE VIRULENT THAN EITHER PARENTAL VIRUS

Cucumber mosaic cucumovirus (CMV) infects a very wide range of plant s pecies (>1000 species). We recently demonstrated that a previously und escribed gene (2b) encoded by RNA 2 of the tripartite RNA genome of CM V is required for systemic virus spread and disease induction in its h osts, Herein we report that when this CMV gene is replaced by its homo logue from tomato aspermy cucumovirus (TAV), the resultant hybrid viru s is significantly more virulent, induces earlier onset of systemic sy mptoms, and accumulates to a higher level in seven host species from t hree families than either of the parents. Our results indicate that CM V and the TAV 2b protein interact synergistically despite the fact tha t no synergism occurs in double infections with the two parental virus es. To our knowledge, this is the first example of an interspecific hy brid made from plant or animal RNA viruses that is more efficient in s ystemic infection of a number of hosts than the naturally occurring pa rents. As CMV and the hybrid virus accumulated to a similar level in t he infected tobacco protoplasts, the observed synergistic responses mo st likely resulted from an increased efficacy of the hybrid virus in s ystemic spread in host plants provided by the TAV 2b protein, The rele vance of our finding to the application of pathogen-derived resistance is discussed.