Diversity of double-stranded RNA viruses within populations of entomopathogenic fungi and potential implications for fungal growth and virulence

Naturally occurring isolates as well as several culture collection isolates of the entomopathogenic fungi, Metarhizium anisopliae, Beauveria bassiana; and Metarhizium flavoviride were assessed for the presence of double-stranded RNA (dsRNA). DsRNA was present in the mycelium of 28 of 73 isolates (38.4%) of M. anisopliae and 2 of 12 isolates (16.7%) of B. bassiana collected from southern Ontario soil samples. DsRNA was also present in mycelium of 2 of 3 isolates of ill. anisopliae and 5 of 6 isolates of M. flavoviride obtained from culture collections. DsRNA banding patterns were variable and, in total, eighteen dsRNA banding patterns were identified, containing from one to eight bands ranging from 5.2 to 0.5 kbp in size. Partially purified virion preparations from selected dsRNA-containing isolates provided similar banding patterns to dsRNAs extracted from mycelium. Comparison of 12 dsRNA-containing isolates with 10 dsRNA-free isolates indicated dsRNAs were not responsible for the large variation in growth rate, conidiospore production, and insect virulence observed in M anisopliae. However, a comparison between two isogenic strains, one with dsRNA and one without, showed a significantly increased growth rate and conidiospore production on potato dextrose agar (PDA) amended with dodine, as well as significantly increased virulence, in the dsRNA-free isolate. These results indicate that dsRNA infections may not be entirely latent in these fungi.