PLASMID DIVERSITY IN SENESCENT AND NONSENESCENT STRAINS OF NEUROSPORA

A sample of 171 natural isolates of Neurospora crassa and Neurospora intermedia was tested for senescence. Of these, 28 strains senesced within the duration of the experiment. These senescent strains, together with a selection of nonsenescent strains, were examined for the presence of mitochondrial plasmids. This was done by digesting mitochondrial DNA preparations with proteinase K, and running these samples on agarose gels. Most of the strains examined, both senescent and nonsenescent, contained plasmids, many of them new. Some new plasmids were linear, as inferred from their resistance to 5' exonuclease and sensitivity to 3' exonuclease. New circular plasmids were also found. Some strains carry several plasmids, and mixtures of circular and linear elements were common. A cross-homology study was performed on a sample of plasmid-bearing strains, and several cases of apparent relatedness were found, some between strains from distant geographical locations. Linear plasmids homologous to the maranhar linear senescence plasmid were quite common. A new member of the LaBelle circular plasmid homology group was found. In the sample tested for homology, no strains contained elements related to the kalilo linear senescence plasmid. The relationship of the new plasmids to senescence is not known. In addition to plasmid monomers, several different types of derivatives were found. The kalilo linear plasmid was found to occur in linear and circular forms of low mobility, presumed to be giant concatamers, and, in some strains, variant sibling structures and ladders of short derivatives were found. Circular plasmids also gave rise to extensive ladders on electrophoresis, probably representing different relaxation states and head-to-tail concatameric series. Some such forms migrated more slowly than mitochondrial DNA. One unique type of plasmid modification observed was a pair of linear elements that had apparently arisen de novo which showed homology to a circular plasmid.