(A) Schematic overview of the assumed sexual process between two parental strains of Z. tritici (Alexopoulos et al., 1996; Kema et al., 2018) with one supernumerary chromosome shared and therefore paired (blue/orange) and one supernumerary chromosome unique to one strain (blue checkered) and unpaired in the zygote. The spermatial nucleus is transferred from the male partner via the trichogyne to the ascogonium of the female partner, resulting in plasmogamy and a dikaryon with two separate nuclei. Prior to karyogamy, the chromosomes are replicated and thus comprise each two chromatids when meiosis is initiated by pairing of homologous chromosomes. In meiosis I, homologous chromosomes are segregated, followed by chromatid separation in meiosis II. A subsequent mitosis results in the production of eight ascospores contained within one ascus. The expected segregation of chromosomes according to Mendelian law of segregation is shown - which for unpaired chromosomes is 4:0. (B) Schematic illustration of the distribution of supernumerary chromosomes present in the parental strains exemplified for five of nine different crosses performed in this study. Parental strain IPO323 contains eight supernumerary chromosomes (chr14-21, blue). Parental strain IPO94269 contains six supernumerary chromosomes with homologs in IPO323 (chr14, chr15, chr16, chr17, chr19, and chr21 in orange). The IPO323 chromosomes chr18 and chr20 are not present in IPO94269. We used a set of IPO323 chromosome deletion strains to generate an additional unpaired chromosome (as examples IPO323∆chr14 X IPO94269, IPO323∆chr15 X IPO94269, IPO323∆chr18 X IPO94269, IPO323∆chr20 X IPO94269 to demonstrate the one to three unpaired chromosomes and the five to six paired chromosomes present in the different crosses). Orange and blue indicate chromosomes that are shared between both strains. Checkered orange and checkered blue indicate chromosomes that are unique to one parent and therefore unpaired in the zygote.