Incidence and stability of infection by double-stranded RNA genetic elements in Aspergillus section flavi and effects on aflatoxigenicity

Ninety-two isolates belonging to Aspergillus sect. flavi were analyzed for double-stranded (ds) RNA via standard cellulose chromatography. Double-stranded RNA infection was detected in fungal isolates that had been in culture for long periods (5 of 26 were infected) and in those recently isolated (5 of 66 were infected). The number of dsRNA genetic elements differed among infected isolates and no two isolates contained identical dsRNAs on the basis of electrophoretic migration in agarose gels. Addition of micronutrients to culture media affected both the amount of dsRNA produced and the number of dsRNA genetic elements detected. Attempts to cure six fungal isolates of dsRNA by serial single conidial transfer, chlorate selection for nitrogen-metabolism mutants, and cycloheximide treatment, met with variable results. The frequency at which serial single conidial transfer and nitrogen-metabolism mutant (nit) selection successfully cured six Aspergillus sect. flavi isolates varied from 11 to 100% and 0 to 100%, respectively. The cycloheximide treatment was effective at curing 40% of the dsRNA-infected isolates. Comparison of aflatoxin production prior to and after dsRNA curing indicated that infection by dsRNA did not influence aflatoxin production. However, aflatoxin production by two isolates (91-031B and 91-184G) was reduced by both single conidial transfer and induction of nit mutants.