Trade-off between accumulation potential and transmission efficiency in hypovirus variants infecting phytopathogenic fungi

The high mutation rates of RNA virus replication generate genetically diverse virus variants in infected hosts. However, the effects of mutations on viral fitness and adaptability remain understudied in fungal-virus pathosystems. In this study, a novel hypovirus (single-stranded positive-sense RNA genome, family <i>Hypoviridae</i>), designated Valsa pyri hypovirus 1-α (VpHV1-α), and two of its shorter, less prevalent variants (VpHV1-β and VpHV1-γ), that contain different internal deletions in the N-terminal coding region of the viral protein, were identified in phytopathogenic <i>Valsa pyri</i> fungal strains. Repeated subculture of a fungal strain infected with VpHV1-α produced VpHV1-β and VpHV1-γ, demonstrating that VpHV1-β and VpHV1-γ were generated by the deletion of the VpHV1-α genome. Compared to VpHV1-α and VpHV1-β, VpHV1-γ, which has a larger deletion, attenuated fungal growth and pathogenicity while accumulating to higher levels. However, it exhibited lower vertical transmission efficiency through spores. Intriguingly, unlike VpHV1-α and VpHV1-β, VpHV1-γ showed restricted horizontal transmission via hyphal anastomosis. This restriction was associated with the induction of programmed cell death and transcriptional activation of vegetative incompatibility-related genes upon VpHV1-γ infection. Additionally, VpHV1-γ infection upregulated key components of RNA silencing (<i>AGO3</i> and <i>DCL2</i>). Our results reveal a trade-off between viral accumulation and transmission efficiency, influencing the spread and persistence of VpHV1 variants in fungal populations. These findings provide new insights into viral evolution and host adaptation in natural fungal-virus ecosystems.IMPORTANCEStudies on mycoviruses are significant for advancing our understanding of viral evolution and host-pathogen interactions. In this study, we identified and characterized a novel hypovirus (VpHV1) infecting the plant-pathogenic fungus <i>Valsa pyri</i>. VpHV1 exists as three viral variants (α, β, and γ). Notably, the γ variant, the least prevalent and shortest due to an internal genomic deletion, exhibited unique phenotypic traits: enhanced viral accumulation and symptom severity but impaired horizontal and vertical transmission. Intriguingly, infection by the γ variant induces programmed cell death during hyphal anastomosis with an isogenic fungal strain, thereby preventing viral transmission. This vegetative incompatibility-like reaction may represent a previously unknown defense mechanism in filamentous fungi, functioning to restrict viral spread within genetically homogeneous populations. Our findings demonstrate that transmission ability is a critical selective factor in viral evolution and adaptation within host populations.