Figure 2.

Tetrad analysis reveals a second mutation responsible for cell death and growth control defects. (a) Haploid parental strains and the four spores/strains of a representative tetrad derived from their cross was analyzed for growth on rich medium (YPD, top), tubular ( +/normal) or netted (N) mitochondrial morphology, sensitivity (S) to acetic acid-induced cell death, resistance (R) to growth inhibition due to low amino acids (synthetic complete dextrose medium, SCD_ME, Methods in Enzymology), and G418 resistance (indicating FIS1 gene replacement). Data are summarized for three or more independent experiments per condition in the table below; + indicates wild-type phenotype. (b) Mitochondrial morphology of each spore in tetrads no. 14 and no. 15 from (a) were visualized by MitoTracker staining.¹⁰ Arrows mark netted mitochondrial morphology. (c) Summary of genetic linkage determined by tetrad analyses as illustrated in (a). FIS1 deletion is unlinked to the cell death d gene, as the ratios of PD (parental ditype): NPD (nonparental ditype): TT (tetratype) within tetrads were 1 : 6 : 17 for fis1Δd₁ (24 tetrads for cell death and/or overgrowth on SCD_ME) and 2 : 3 : 15 for fis1Δd₂ (20 tetrads), which are close to random segregation (1 : 1 : 4, or 17% for PD (linked) versus 83% for NPD+TT (unlinked)). In contrast, cell death sensitivity was 100% linked to resistance to growth inhibition on low amino acids (SCD_ME), with ratios of 8 : 0 : 0 (fis1Δd₁) and 20 : 0 : 0 (fis1Δd₂). χ² goodness-of-fit test was used to determine whether FIS1-deletion (fis1Δ) segregated independently from mutation d (death) at the expected ratio of 1 : 1 : 4 for PD : NPD : T. As none of the p values rejected the null hypothesis, FIS1-deletion segregated independently from mutation d