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Mitochondrial DNA plasmids and senescence in neurospora Yang, Xiao


A survey of mitochondrial DNA plasmids was conducted in natural populations of N. intermedia and N. crassa. Many new mitochondrial plasmids, both linear and circular, were found in 37 out of 46 strains. DNA-DNA hybridization studies revealed relatedness of some plasmids in isolates from different species or from distant geographical locations. Both circular and linear plasmids showed complex concatamers. Furthermore new circular and linear plasmids appeared to arise spontaneously from the existing plasmids. All these new mitochondrial plasmids wre neutral to their hosts. However, in certain crosses of natural isolates of N. intermedia, linear and circular mitochondrial plasmids of the maternal parents wre not transmitted to their sexual progeny, in contrast to the maternal transmission of organellar genetic elements generally observed in crosses of N. crassa. The lack of plasmid transmission depended upon a strain-specific interaction and was not determined exclusively by the males. Genetic analyses revealed that suppression of plasmids was caused by plasmid-specific nuclear nonautonomous or autonomous suppressors. Paternal transmission of mitochondrial plasmids was investigated during sexual reproduction in Neurospora. Both linear and circular plasmids of male parents have been detected in sexual progeny. Also paternal leakage of mitochondrial genomic DNA has been shown. Rearrangement of mitochondrial genomes was observed in the resultant heteroplasmons. Many new senescent isolates were found among N. intermedia natural isolates. Most of these strains were shown to contain mitochondrial plasmids other than the well-studied senescence plasmids kalDNA or marDNA. The relationship of host senescence and these new plasmids was investigated. Plasmid analyses show that senescence in these strains is unlikely to be determined by mitochondrial plasmids. Genetic analyses showed that senescence in these strains is heritable through asexual and sexual cycles, and is therefore most likely caused by mutant nuclear genes.

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