1. Nondisjunction was assayed in MCY387 a diploid strain in the SK1 strain background with the genotype MAT a / MAT α leu2/leu2 his4::HIS3/HIS4 his3Δ200/his3Δ200 CAN1 s /can1 r CYH2 s /cyh2 r ade2-101/ADE2 lys2/lys2 ura3-52/ura3-52 trp1Δ1/trplΔl. MCY387 is heterozygous for the recessive mutations can1 r and cyh2 r which confer resistance to canavanine and cycloheximide respectively. Thus one-quarter of all spores are resistant to both drugs. Spores that receive one copy of chromosome III are His − . Disomic spores that receive HIS4 and his4::HIS3 are His + . The rate of missegregation of chromosome III is given by the number of His + Can r Cyh r spores divided by the number of Can r Cyh r spores. MCY387 was transformed with a library of wild-type yeast genomic DNA fragments in the URA3 leu2-d vector pHR81 [
2. Nehlin J. O., Carlberg M., Ronne H., Gene 85, 313 (1989);
3. ] and plasmid copy number was controlled by selection for URA3 or LEU2. Plasmids that interfered with meiosis or sporulation were subjected to transposon mutagenesis [
4. Strathman M., et al., Proc. Natl. Acad. Sci. U.S.A. 88, 1247 (1991);
5. ] to map the region responsible for the meiotic effect. The NDJ1 open reading frame was identified in three plasmids that caused increased nondisjunction with URA3 selection (∼30 copies per cell) and prevented sporulation with LEU2 selection (∼100 copies per cell).