Environmentally induced differential amplification of mitochondrial populations

Resistance to the drug rutamycin, an inhibitor of mitochondrial ATPase, has been shown to be cytoplasmically inherited in a mouse fibroblast line (TL) on fusion of the cytoplast (enTL) with a nucleated recipient A9 [Lichtor & Getz (1978) Proc. Natl. Acad. Sci. U.S.A. 75, 324-328]. The cytoplasmic hybrid (cybrid) so formed may be readily grown in the presence [CY(+)] or absence [CY(-)] of rutamycin. The ATPase of TL mitochondria is similarly resistant to rutamycin whether grown in the presence or absence of antibiotic. The ATPase of CY(+) mitochondria is resistant to rutamycin, but CY(-) mitochondrial ATPase is sensitive to rutamycin. Nevertheless, CY(-) can be readily grown in rutamycin after a brief lag. The pH optima of mitochondrial ATPase are 8.0 for A9 and CY(-) cells and 7.5 for TL cells, whereas the pH optimum for CY(+) spans the optima of A9 and TL. The TL mitochondrial NADH-cytochrome c reductase is resistant to rotenone, whereas that of A9 mitochondria is sensitive to this agent. CY(-) and CY(+) mitochondria are sensitive and resistant respectively to rotenone. Growth of cybrids in rutamycin for 2 weeks results in a 2-3-fold increase in mitochondrial mass, measured on the basis of electron microscopic morphometry, mitochondrial membrane enzyme assays, mass of cardiolipin, and quantification of mitochondrial DNA. These data suggest that the cybrid harbours two populations of mitochondria and that the proportions of the two populations dramatically influence morphology, growth and mitochondrial phenotype in the cybrid. Selective pressure appears to induce these changes through the differential amplification of mitochondria.