Nuclear Genetic Polymorphism in Pyrenophora tritici-repentis Strains for ToxA and ToxB Effector Genes

Pyrenophora tritici-repentis is a phytopathogenic fungus, which forms specific phytotoxins encoded by the ToxA and ToxB genes. The aim of the study was to identify the genetic nuclear polymorphism on the ToxA and ToxB genes as evidence of the heterokaryotic state of P. tritici-repentis strains. The novelty of the study was evaluation of the proportion of nuclei with the ToxA or ToxB gene relative to the total pool of nuclei with the reference gene Act1 by quantitative PCR for the detection of heterokaryons. Twenty-one P. tritici-repentis strains from three geographical populations, as well as mitotic (conidial) and meiotic generations of individual strains (103 subclones), were analyzed in the study. The proportion of nuclei with the ToxA gene in 70% of P. tritici-repentis ToxA+ strains from the Kazakhstan population was in the range of 0.24–0.65, indicating their heterokaryotic state. In the Greek population, 20% of P. tritici-repentis ToxB+ strains characterized by the proportion of nuclei with the ToxB gene much more than 1 were detected, while at least 40% of the strains were also heterokaryotic. Probably the proportion of nuclei with effector genes is a genetically inherited trait. The low polymorphism of the proportion of nuclei with effector genes among the mitotic generation was revealed. At the same time, a broader variability of the proportion of nuclei with ToxA among the meiotic generation was found. The functional role of nuclear polymorphism for the adaptation of P. tritici-repentis isolates in the population requires further study.