THE DYNAMICS OF MITOCHONDRIAL PLASMIDS IN A HAWAIIAN POPULATION OF NEUROSPORA-INTERMEDIA

We analysed the distribution of mitochondrial plasmids among 82 Neruos pora intermedia isolates from Hawaii; 74% of the isolates carried the neutral circular plasmid Han-2, whereas 38% contained the linear senes cence-causing plasmid kalDNA. The distributions of the two plasmids ar e independent. There is no significant difference between the Kauaian population of 1972 and that of 1976. To further examine the reasons fo r this frequency distribution we studied the transmission of both Hawa iian plasmids through the maternal parent in a large series of crosses using non-Kalilo isolates as conidial parents. Plasmids can be lost d uring the sexual cycle. The Han-2 plasmid is transmitted more efficien tly than kalDNA. No clear cases of autonomous or non-autonomous plasmi d suppression were observed, so loss can be considered accidental. One Kalilo strain proved to be ineffectual as a maternal parent, and this reduced its ability to transmit kalDNA to the next generation. The dy namic balance of plasmids in natural populations over time is probably a result of the interplay of many forces, including those described i n this work and those from several other studies on Neurospora plasmid s.