Mycovirus prevalence is linked to increased virulence benefits in the fungal entomopathogen Beauveria bassiana.

Numerous studies have reported the occurrence of mycoviruses in fungal strains of medical-veterinary, phytopathological, and entomological relevance, with an expanding literature on their phenotypic effects. However, their prevalence within fungal populations remains poorly understood. If mycoviral infections were to confer adaptive advantages to their fungal hosts, positive selection could favour their persistence within fungal populations, potentially leading to an increased prevalence of beneficial viral symbionts. Addressing this gap is essential for improving mycovirus-based biocontrol strategies. Here, using a collection of 11 B. bassiana strains, we investigated whether mycovirus prevalence is associated with increased virulence. Six out of the 11 strains harbored single or mixed mycovirus infections, including Beauveria bassiana victorivirus 1 (BbVV-1), partitivirus 2 (BbPV-2), and polymycovirus 1 (BbPmV-1). In these mycovirus-harbouring strains, mycovirus-free (MFr) isogenic lines were generated from the wild mycovirus-infected (WMI) strains, and virulence against Galleria mellonella larvae was compared. In general, mycovirus presence was associated with increased virulence. To quantify mycovirus prevalence within clonal populations, 480 clones (80 per strain) were analysed by qRT-PCR. Clonal analyses revealed marked strain-dependent variation, ranging from 10.0% to 92.5% prevalence. Importantly, regression analysis demonstrated a strong positive relationship between mycovirus prevalence and the increase in mortality observed in WMI relative to MFr isogenic lines (R2 = 0.879), indicating an approximately proportional association. Collectively, these findings provide the first evidence that mycovirus prevalence within B. bassiana populations can confer a selective advantage through increased virulence highlighting the importance of population-level characterization for the selection and preservation of fungal biocontrol agents.