Plasmid suppressors active in the sexual cycle of Neurospora intermedia

We have discovered that, in certain crosses of natural isolates of Neurospora intermedia, linear and circular mitochondrial plasmids of the maternal parent are not transmitted to the progeny. This contrasts with the maternal transmission of organellar genetic elements generally observed in crosses between laboratory strains and between other natural isolates. Formally, failure of plasmid transmission is a type of plasmid suppression. The present cases represent the first report of plasmid suppressors in natural populations of fungi. Strains used as female parents can transmit or not transmit plasmids depending on the strain used as male parent. Males that act to suppress in one cross fail to suppress in others. Therefore, the suppression of plasmids depends on a strain-specific interaction and is not determined exclusively by the males. Since suppression is a specific interaction we inferred that it must be genetically based and tested this hypothesis by seeking segregation of suppressed and nonsuppressed phenotypes in octads. Segregation of the original full suppression of all plasmids was indeed observed in each of the three sets of testcrosses examined. The interaction type of suppression must be initiated in ascogenous tissue during the sexual cycle. It is a nonautonomous type of suppression, affecting all descendent cells. In any one case of suppression, either one, several, or all plasmids can be lost. Both linear and circular plasmids can be eliminated by the same suppressor genotype. In addition, several strains were found to contain suppressors that act after ascospore delineation. This autonomous type of suppression has been observed previously in laboratory strains, but not in natural isolates. All the cases of plasmid suppression identified in this study involved a range of apparently neutral circular and linear plasmids. Using one senescent Kalilo strain of N. intermedia, we did not detect any case of suppression of the senescence-determining linear plasmid kalDNA.