Abstract

The haploid genome of the pathogenic fungus Zymoseptoria tritici is contained on \"core\" and \"accessory\" chromosomes. While 13 core chromosomes are found in all strains, as many as eight accessory chromosomes show presence/absence variation and rearrangements among field isolates. We investigated chromosome stability using experimental evolution, karyotyping and genome sequencing. We report extremely high and variable rates of accessory chromosome loss during mitotic propagation in vitro and in planta. Spontaneous chromosome loss was observed in 2 to >50 % of cells during four weeks of incubation. Similar rates of chromosome loss in the closely related Z. ardabiliae suggest that this extreme chromosome dynamic is a conserved phenomenon in the genus. Elevating the incubation temperature greatly increases instability of accessory and even core chromosomes, causing severe rearrangements involving telomere fusion and chromosome breakage. Chromosome losses do not impact the fitness of Z. tritici in vitro, but some lead to increased virulence suggesting an adaptive role of this extraordinary chromosome instability.