Maternal inheritance of mitochondrial cytochrome c oxidase activity in mice

The RR mouse strain has unique restriction fragment patterns of mitochondrial DNA and shows lower activity of cytochrome c oxidase in fresh liver mitochondria than do C57BL/6 and BALB/c mice after nine weeks of age. In reciprocal F1 crosses between RR and the other two strains, the activity of crosses from RR mothers was lower than that in reciprocal crosses, although both F1 crosses had values intermediate between those of the parental strains. Cross-nursing tests demonstrated that strain differences probably are determined before or immediately after birth. Successive backcrossing was performed in order to transfer mitochondrial DNA from one strain to the nuclear background of another strain, and after eight and 10 generations, the cytochrome c oxidase activities of the backcross lines were compared with those of the maternal and paternal strains. The activity of the backcross line that started with C57BL/6 or BALB/c females and was successively backcrossed to RR males was still higher than that of RR. Similarly, the line that started with RR females and was backcrossed to C57BL/6 or BALB/c males showed lower activity than that of the paternal strain. These observations suggest that part of the strain difference of cytochrome c oxidase activity is transmitted by mitochondrial DNA, although our evidence does not exclude the possible involvement of other maternally transmitted components.