Suppression of cytoplasmic senescence in Neurospora.

We have shown that senescence in Kalilo strains of Neurospora, caused by a linear mitochondrial plasmid called kalDNA, is suppressible by existing variants of the nuclear genome. The suppressors are manifested by 4:4 segregation of senescence and immortality in asci from crosses between senescent female strains and males chosen from non-senescent candidate stocks. In one case of suppression, the asci also show segregation at the plasmid level. There is a reduction of kalDNA to barely detectable levels in the four ascospores showing immortality, so this suppressor evidently influences the maintenance of the plasmid itself. In the other case of suppression, the phenotypic segregation is not correlated with segregation at the plasmid level, and all eight ascospores in the asci show both free and inserted forms of kalDNA. This suggests that the suppression genotype provides a way of tolerating the presence of the plasmid rather than diminishing it. However, the allele f, which provides an analogous kind of suppression for the cytoplasmic mutation poky, does not suppress Kalilo or Maranhar senescence. Suppression is hence shown to be a possible option for host strains to combat the plasmid in nature, but no examples of suppressors were found in a limited survey of natural isolates. In addition, we have shown that long-lived, presumably non-senescent, strains do not arise by suppressor mutation, but lose senescence plasmid DNA by another mechanism.