Hypovirulence-associated Mycovirus Epidemics Cause Pathogenicity Degeneration of Beauveria bassiana in the Field

Abstract

Background The entomogenous fungus Beauveria bassiana is used as a biological insecticide worldwide, but its application is affected by pathogenicity degeneration in the field. Previous studies showed that multiple factors contribute to this phenomenon. Mycovirus infection causes hypovirulence of phytopathogenic fungi and mycoviruses have been used for plant disease biocontrol. However, it remains unknown whether mycovirus epidemics are key factors causing hypovirulence of B. bassiana naturally in the field. Methods A mycovirus Beauveria bassiana chrysovirus 2 (BbCV2) we have previously identified was employed to clarify its impact on the pathogenicity of host fungi Beauveria bassiana against the larvae of insect pest Ostrinia furnacalis. Wild strains of B. bassiana were collected from different geographic locations in Jilin Province, China, to clarify the epidemic and genetic diversity of the mycovirus BbCV2. The serological analysis was conducted by preparing polyclonal antibody against a BbCV2 coat protein, to determine whether the it can dissociate outside the host fungi cells and subsequently infect new hosts. Transcriptome analysis was used to reveal the interactions between viruses and hosts. Results We found that themycovirus BbCV2 possessed efficient and stable horizontal and vertical transmission capabilities and caused B. bassiana hypovirulence. Surprisingly, this virus was prevalent in the field as a core virus in wild B. bassiana strains, without obvious genetic differentiation. The serological results showed that the virus could not only replicate within the host cell, but also dissociate outside to infect new hosts. Transcriptome analysis revealed decreased expression of genes related to insect epidermis penetration and toxin metabolism in B. bassianacaused by mycovirus infection. Conclusion B.bassiana infected by hypovirulence-associated mycovirus can spread the virus to new host strainsafter infecting insects, and causethe virus epidemics in the field. The findings confirmed that mycovirus infection may be an important factor affecting the pathogenicity degradation of B. bassiana in the field.