Waldron and Roberts isolated cold-sensitive mutants in the fungus Aspergillus nidulans by mutagenizing conidia with a chemical mutagen. While most cold-sensitive mutants showed normal Mendelian inheritance patterns, one mutant designated cs67 exhibited non-Mendelian segregation in crosses with wild-type strains. All progeny from individual perithecia were either uniformly cold-sensitive or uniformly non-cold-sensitive, independent of nuclear genetic markers. Heterokaryon tests confirmed that cold-sensitivity segregated independently of nuclear markers, with colonies within samples showing either the cold-sensitive phenotype or lacking it entirely. These results satisfied genetic criteria for extranuclear inheritance. The authors note that cs67 produces an altered cytochrome spectrum at restrictive temperatures, suggesting mitochondrial origin similar to other extranuclear mutations in Aspergillus species. The findings indicate that cold-sensitivity enrichment could effectively identify mitochondrial mutants in fungal systems.

Key findings

• The cs67 cold-sensitive mutant in Aspergillus nidulans exhibits non-Mendelian inheritance patterns in sexual crosses, with all progeny from individual perithecia being either cold-sensitive or wild-type
• Cold-sensitivity in cs67 segregates independently of nuclear genetic markers in both sexual crosses and heterokaryon tests, satisfying genetic criteria for extranuclear inheritance
• The cs67 mutant produces altered cytochrome spectra at restrictive temperatures, strongly indicating a mitochondrial origin for the mutation
• Cold-sensitivity can serve as an effective enrichment method for identifying mitochondrial mutants in fungi