Senescent: A New Neurospora crassa Nuclear Gene Mutant Derived from Nature Exhibits Mitochondrial Abnormalities and a “Death” Phenotype

Abstract

Navaraj, A., Pandit, A., and Maheshwari, R. 2000. senescent: A new Neurospora crassa nuclear gene mutant derived from nature exhibits mitochondrial abnormalities and a “death” phenotype. Fungi are capable of potentially unlimited growth. We resolved nuclear types from multinuclear mycelium of a phenotypically normal wild isolate of the fungus Neurospora intermedia by plating its uninucleate microconidia and obtained a strain which, unlike the “parent” strain, exhibited clonal senescence in subcultures. The mutant gene, senescent, was introgressed into N. crassa and mapped four map units to the right of the his-1 locus on linkage group VR. senescent is the first nuclear gene mutant of Neurospora derived from nature that shows the death phenotype. Death of the sen mutant occurred faster at 34°C than at 22 or 26°C. Measurements of oxygen uptake of conidia using respiratory inhibitors and the spectrophotometric analyses of mitochondrial cytochromes showed that in sen cultures grown at 34°C, cytochromes b and aa3 were present but cytochrome c was absent. By contrast at 26°C, cytochromes b and c were present but cytochrome aa3 was diminished in the late subcultures. This suggested that the sen mutation does not affect the potential to produce functional cytochromes. The deficiency of the respiratory chain cytochromes may not be the cause of death of the sen mutant because the cytochrome c and aa3 mutants of N. crassa are capable of sustained growth whereas sen is not. Possible explanations for the observations are discussed.