Function and transmission of accessory chromosomes in Zymoseptoria tritici
Accessory chromosomes are non-essential genetic elements and defined by a presence/absence polymorphism within a population. During meiosis a number of fungal accessory chromosomes are transmitted in a non-Mendelian way. The involved exact processes are as yet unclear. The objectives of this PhD thesis are to improve our understanding of the function of the accessory chromosomes of the haploid fungus Zymoseptoria tritici and the mechanisms underlying their mitotic and meiotic transmission. This ascomycete fungus is an important pathogen of wheat causing a diphasic, hemibiotrophic infection. For the functional characterization, I used a forward genetics approach by generating isogenic whole chromosome deletion strains for each of the eight distinct accessory chromosome in the reference isolate IPO323. I could show that in contrast to other fungal accessory chromosomes, most accessory chromosomes in Z. tritici confer a fitness cost during infection of the host. Fitness costs should lead to rapid elimination of these chromosomes from the population. I therefore hypothesized that the continued maintenance of the accessory chromosomes is due to a transmission advantage during meiosis. I thus compared the meiotic transmission of the accessory chromosomes in the presence or absence of a homolog. My results revealed that unpaired accessory chromosomes of Z. tritici have a transmission advantage, as they are inherited by more than the expected 50% of the progeny. This effect is caused by the transmission of female-inherited unpaired chromosomes to all meiotic progeny instead of the expected half. This meiotic drive is most likely based on an additional DNA replication step that is restricted to the female-inherited unpaired chromosomes. This mechanism represents a completely novel aspect of meiosis and could have broader implications for the transmission of unpaired accessory chromosomes in general.
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