Temporal Change, Frequency, and Spatial Distribution of Mating Types of Bipolaris maydis in China Provide Indirect Evidence for Potential Sexual Reproduction

The heterothallic fungus Bipolaris maydis, the causal organism of southern corn leaf blight, is a destructive pathogen of corn and is considered to predominantly reproduce asexually, as no perfect reproductive stage has been discovered under natural conditions. In this study, a multiplex PCR with mating type-specific primers was used to investigate the temporal change and spatial distribution of mating types in several B. maydis populations in China. Mating type monitoring demonstrated that both mating types co-existed throughout the entire monitoring period across the seven monitoring locations, with mating type frequencies skewing significantly (p < 0.05) to MAT1-2 during the period of 2015 to 2020 and being practically symmetric after 2020, implying that the two populations were approaching equilibrium. Both mating type isolates were detected within the same leaf or field, affording a high theoretical likelihood for the two mating type isolates to cross sexually. In addition, both MAT1-1 and MAT1-2 isolates were detected from large-scale geographical regions in China, with mating type frequencies approaching a null hypothesis ratio of 1:1 in most populations, suggesting frequency-dependent selection consistent with sexual reproduction. Conclusively, our results provided indirect evidence supporting the potential for sexual reproduction in B. maydis under natural conditions, although no sexual morphs have yet been discovered in the field.