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Abstract:
Z. tritici can reproduce clonally by mitosis, as well as to reproduce sexually when two strains of opposing mating types cross. A single meiotic reproduction results in eight ascospores in a single ascus. In addition to core chromosomes that are indispensable, Z. tritici also possesses accessory chromosomes which are dispensable and show presence-absence polymorphisms among isolates. The accessory chromosomes are maintained by meiotic drive of female-inherited chromosomes which leads to non-random inheritance of chromosomes inherited from the female parent in a cross during sexual reproduction. This mechanism requires additional DNA replication which is either happening prior to meiosis or during it. To distinguish between the proposed models, I designed crossing experiments of Z. tritici strains with disomic accessory chromosomes (two homologues of differentiable genotype). If DNA replication happens prior to meiosis the disomic homologues of the female parent can be overrepresented within one ascus. If DNA replication happens during meiosis random inheritance would always present both genotypes of the female parental strain within a single ascus. Using in planta crossing experiments I was able to show that strains disomic for accessory chromosomes can produce ascospores, whereas trisomic strains used failed to produce ascospores. I also showed that the is no effect of repeat induced point mutations (RIP), a DNA defence mechanism in fungi which mutates duplicated DNA premeiotically, on whole chromosome duplications. To compare my results to strains which have lost the RIP mechanism I created deletion mutants in both mating types of Z tritici of rid gene homologues previously described as responsible for RIP in Neurospora crassa. I also tried to develop an in vitro crossing protocol for Z. tritici strains involving multiple different growth media, plating techniques and growth conditions to present an alternative to the lengthy process of in planta crossing experiments.
