A mycoviral infection drives virulence and ecological fitness of the entomopathogenic fungus Beauveria bassiana.

Entomopathogenic ascomycetes are important natural regulators of insect pest populations and an increasingly adopted microbial control option. Fungal virulence in entomopathogenic ascomycetes can be modified by mycoviruses, viruses that infect fungi, whereas the possible role of these viruses on the physical and biochemical properties of the virus-containing fungal strains and on their ecological fitness has remained largely unexplored. Here, utilizing a Beauveria bassiana strain naturally infected with two mycoviruses, Beauveria bassiana partitivirus 2 (BbPV-2) and Beauveria bassiana polymycovirus 1 (BbPmV-1), we found that the mycovirus-containing strain is hypervirulent towards the experimental insect Galleria mellonella and shows major physical and biochemical changes in spore size, isoelectric point, and Pr1 activity, but even more impactful, the mycoviral infection confers a significant environmental- abiotic and biotic stress tolerance to the fungus. Hence, mycovirus infection expanded the temperature range for fungal growth and germination, and improved tolerance to osmotic stress, water stress, and UV-B radiation. Similarly, the antagonistic activity of the mycovirus-containing strain against Trichoderma harzianum was increased as compared to the mycovirus-free one. Taken together, these data suggest for the first time a mycovirus related adaptation of key traits indicators of environmental competence of a beneficial fungus, rendering these mycoviruses as potent tools for entomopathogenic fungal strain selection and development as mycoinsecticides.