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Transmission of kalilo DNA in senescent strains of Neurospora intermedia Myers, Carolyn J.


Senescence, the progressive loss of growth potential culminating in death, is common among Kauaian strains of Neurospora intermedia. Senescence is initiated by the insertion of kalilo DNA into the mitochondrial DNA. Mitochondrial DNA molecules carrying the insert accumulate and death occurs when the insert is equimolar with the mitochondrial DNA. The inserted form of kalilo DNA is referred to as mtlS-kalDNA. Studies on the somatic transmission of mtlS-kalDNA in ascospore series have revealed that kalilo DNA is capable of assuming new locations within the mitochondrial DNA. It is proposed that these novel insertions originate from intramitochondrial movement and an autonomous form of kalilo DNA, mtFF-kalDNA, is predicted to be an intermediate in movement. Novel insertion of kalilo DNA appears to depend on the form of mtlS-kalDNA transmitted sexually. If a mutagenic insert is transmitted, senescence is initiated at the onset of vegetative growth of the ascospores and no novel insertions are detected. The lifespans of these ascospores are quite short, death occurring in 10 subcultures or less. Transmission of a nonmutagenic insert delays the onset of senescence until either a novel insertion or a rearrangement of the transmitted insert occurs. The lifespans of these ascospores usually exceed 10 subcultures and are variable. Information obtained from tetrad analysis has revealed that novel insertion of kalilo DNA may also be under the influence of the host genome. A senescent Kauaian strain was identified which shows some but not all characteristics of kalilo senescence. In this strain and its derivatives, the behaviour of mtlS-kalDNA is erratic and in, some cultures the characteristic mitochondrial biochemical deficiencies, normally accompanying kalilo senescence, are not observed. It is suspected that kalDNA is not responsible for senescence in this strain and its derivatives but rather some other unknown factor is affecting the normal growth patterns of these cultures. Kauaian strains were surveyed for the presence of dsRNA to determine whether kalDNA has a viral origin. Only one senescent strain contains detectable amounts of dsRNA which was not homologous with a kalDNA probe. The survey identified six nonKauaian strains which contain dsRNA and seven dsRNA species were delineated. Although the presence of dsRNA is not relevant to kalilo senescence, analysis of dsRNA in a genetically-well defined organism like Neurospora may give insight into the significance of dsRNA in fungi in general.

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