Starvation-induced cell fusion and heterokaryosis frequently escape imperfect allorecognition systems to enable parasexual interactions in an asexual fungal pathogen

Asexual fungi include important pathogens of plants and other organisms, and their effective management requires understanding of their evolutionary dynamics. Genetic recombination is critical for species adaptability and could be achieved via heterokaryosis and the parasexual cycle in asexual fungi. Here, we investigate the extent and mechanisms of heterokaryosis in the asexual plant pathogen Verticillium dahliae. We used live-cell imaging and genetic complementation assays of tagged V. dahliae strains to analyze the extent of nonself vegetative fusion, heterokaryotic cell fate and nuclear behavior. An efficient CRISPR/Cas9-mediated system was developed to investigate the involvement of autophagy in heterokaryosis. Under starvation, nonself fusion of germinating spores occurs frequently regardless of the previously assessed vegetative compatibility of the partners. Supposedly "incompatible" fusions often establish viable heterokaryotic cells and mosaic mycelia, where nuclei can engage in fusion or transfer of genetic material. The molecular machinery of autophagy has a protective function against destruction of "incompatible" heterokaryons. Our results suggest an autophagy-mediated trade-off between parasexual interactions for genetic exchange and allorecognition systems possibly for mycelial protection from parasitic elements. Our study reveals unexpected capacity for heterokaryosis in V. dahliae and suggests, therefore, important roles of parasexuality in the evolution of asexual fungi.