Characterization and prevalence of a circular mitochondrial plasmid in senescence-prone isolates of Neurospora intermedia.

Genetic and molecular analyses of the phenomenon of senescence-i.e., irreversible loss of growth and reproductive potential upon subculturing-in Neurospora intermedia strain M1991-60A, collected from Maddur in southern India, showed the presence of plasmid pMaddur1, which is homologous to the senescence-inducing circular mitochondrial plasmid, pVarkud. Maternal inheritance of senescence in M1991-60A correlated to the formation of variant pMaddur1, its subsequent insertion into mitochondrial (mt)DNA and the accumulation of defective mtDNA with the pMaddur1insert. PCR-based analyses for similar plasmids in 147 natural isolates of Neurospora from Maddur showed that nearly 40% of the strains had pMaddur1 or pMaddur2 that shared 97-98% sequence homology with pVarkud and pMauriceville. Nearly 50% of the strains that harbored either pMaddur1 or pMaddur2, also contained a circular Varkud satellite plasmid (pVS). Size polymorphism maps to the cluster of PstI sites in the non-coding region. Whereas senescence of nearly 40% of N. intermedia strains may be due to pMaddur, the presence in seven strains of pVS but not pMaddur and the absence of either of these two plasmids in other senescence-prone isolates suggests yet undiscovered mechanisms of senescence in the Maddur strains.