SATELLITE RNA OF CUCUMBER MOSAIC CUCUMOVIRUS SPREADS EPIDEMICALLY IN NATURAL-POPULATIONS OF ITS HELPER VIRUS

Three hundred thirty-eight isolates of cucumber mosaic cucumovirus (CM V), sampled from natural populations in six areas of Spain between 198 9 and 1996, were screened for the presence of satellite RNA (satRNA). The frequency of CMV isolates with satRNA approached 1.00 in Valencia least Spain between 1990 and 1994 where a tomato necrosis epidemic ind uced by CMV+satRNA had started in 1986 and was smaller north and west of this area in 1992 and 1993. After 1994, satRNA almost disappeared f rom all CMV populations. Genetic typing of satRNA variants was done by ribonuclease protection assay, and from these data, genetic distances were estimated for any pair of satRNA variants. CMV-satRNA population s were highly diverse, containing 0.07865 nucleotide substitutions per site on average. Data also showed that the whole compared set of 100 satRNA variants form a single population that is not structured accord ing to place, year, host plant, or strain of helper virus (HV). This i s in sharp contrast with the metapopulation structure of the Spanish C MV population. Thus, the genetic structure and dynamics of populations of CMV and its satRNA are not coupled. This shows that CMV-satRNA spr eads epidemically, as a hyperparasite, in the population of its HV. Th is conclusion is relevant to the use of CMV-satRNA as a biocontrol age nt of CMV.