A striking aspect of "Zymoseptoria tritici" genetics is the presence of many dispensable chromosomes. Eight of chromosomes could be lost with no visible effect on the fungus and thus are dispensable. Dispensable chromosomes have been found in other fungi but they usually occur at a low frequency and typically represent single or a few chromosomes. Dispensable chromosomes have originated by ancient horizontal transfer from an unknown donor, that was followed by extensive genetic recombination, a possible mechanism of stealth pathogenicity and exciting new aspects of genome structure.