Normal meiosis in the fungus Sclerotinia sclerotiorum despite the irregular distribution of haploid chromosomes between two nuclei

Sclerotinia sclerotiorum is a phytopathogenic soilborne fungus that damages diverse crops worldwide. It has recently been discovered that this fungus distributes its 16 chromosomes irregularly between two nuclei within each haploid ascospore. Here, we study the meiotic process by tracking phenotypic segregation and recombination events in the progeny of a strain resulting from the fusion of two morphologically distinct mutants. We show that, despite the unusual chromosomal distribution into two nuclei, chromosome segregation and genetic recombination occur normally in S. sclerotiorum, likely due to a tight coordination between the two complementing nuclei during meiosis. Consistently, an atypical ascospore development process deviating from the conventional model was observed. Furthermore, we show that our co-segregation analyses method can be used to identify causal mutations in uncharacterized mutants, uncovering a role for deubiquitination in fungal development and virulence.