Molecular manipulation of the mating-type system and development of a new approach for characterizing pathogen virulence in Pyrenophora tritici-repentis

The ascomycete Pyrenophora tritici-repentis (Ptr) is an important fungal pathogen worldwide that causes tan spot of wheat. The fungus is self-fertile because each isolate contains both mating type (MAT) idiomorphs. In this work, we developed knockouts of the MAT genes in Ptr and tested fertility of the knockout strains and outcrossing between the knockout strains carrying the opposite mating type. The fungal strains with deletions of either MAT1-1-1 or MAT1-2-1 did not form mature pseudothecia making them functionally heterothallic. The cross between the heterothallic strains of the same isolate (86-124) was fully fertile with the only difference compared to the homothallic wild type strain being the slightly lower percentage of pseudothecium formation. However, the cross between 86-124 (race 2, ToxA-containing isolate) and DW5 (race 5, ToxB-containing isolate) was partially fertile and had fewer mature pseudothecia. Furthermore, most mature asci produced only two or four instead of eight functional ascospores. A collection of ascospores from this cross was obtained and genotyped for the presence of the ToxA, ToxB and MAT genes as well as simple sequence repeat markers. The segregation of these genes and markers and recombination of different allele types at these loci was observed. This work clearly demonstrates that the fungus requires both MAT genes for sexual production and can undergo outcrossing and sexual recombination. It also establishes a new and practical way for further characterizing fungal virulence in Ptr through the development of segregating fungal populations and subsequent genetic analysis.