Progressive loss of the chromosome 1L arm and loss of the distal 1R region. ( A ) Germline PCR analysis of newly established clonal lines derived from TX607(3) at ∼150 fissions (see Figures 1A and 1B for PCR primer positions). The germline C3ΔRFB allele was absent in all the 15 lines and the B rDNA homolog was also missing in the subclone 15. PCR primers 15F and 15R span the Cbs element at the 3′ end of the micronuclear rDNA locus in intact micronuclear chromosomes. B: wild-type B rDNA strain, CU428; C: wild-type C3 rDNA strain, SB1934. ( B ) PCR analysis of micronuclear-limited DNA fragments from the left and right arms of the five micronuclear chromosomes. While most of the analyzed Cbs junctions have not been precisely positioned on their respective micronuclear chromosome, the primer sets used to examine chromosomes 4 and 5 map close to micronuclear telomeres (E. Hamilton and E. Orias, personal communication). W: wild-type B rDNA strain CU428, M: TX607(3) subclone 15. Lane 1 (L): 100 bp DNA ladder. ( C ) Loss of 1L and 1R markers during prolonged propagation of TX607(3) subclone 15. Top panel: map depicting the relative order of chromosome 1 Cbs junctions that were subjected to PCR analysis in wild-type (CU428) and mutant (TX607(3) subclone 15) strains (CEN: centromere, Tel: telomere). TX607(3) subclone 15 (derived from TX607(3) late passage) was serially propagated and subjected to PCR analysis at early (E: ∼70 fissions) and late [late 1 (L1): ∼150, late 2 (L2): ∼250] fissions (bottom panels).