Mutations in mating-type genes of the heterothallic fungus Podospora anserina lead to self-fertility

The heterothallic fungus Podospora anserina has two mating-type alleles ter med mat+ and mat-. The mat+ sequence contains one gene, FPR1, while mat- co ntains three genes: FMR1, SMR1, and SMR2. FPR1 and FMR1 are required for fe rtilization, which is followed by mitotic divisions of the two parental nuc lei inside the female organ. This leads to the formation of plurinucleate c ells containing a mixture of parental mat+ and mat- nuclei. Further develop ment requires a recognition between mat+ and mat- nuclei before migration o f the mat+/mat- pairs into specialized hyphae in which karyogamy, meiosis, and ascospore formation take place. FPR1, FMR1, and SMR2 control this inter nuclear recognition step. Initial development of the dikaryotic stage is su pposed to require SMR1; disruption of SMR1 results in barren perithecia. In a systematic search for suppressors restoring fertility, we isolated 15 su ppressors-all of them mutations in the mating type genes. These fmr1, smr2, and fpr1 mutants, as well as the strains disrupted for FMR1, SMR2, and FPR 1, are weakly self-fertile. They are able to act as the male partner on a s train of the same mating type and give a mixture of biparental and uniparen tal progeny when crossed with a wild-type strain of opposite mating type. T hese observations lead us to propose that SMR2, FMR1, and FPR1 act as activ ators and repressors of fertilization and internuclear recognition function s.