Fate of mitochondrial DNA in human-mouse somatic cell hybrids (density gradient centrifugation-ethidium...

Several hybrid lines between human and mouse somatic cells, containing one or two complements of mouse chromosomes and a reduced complement of human chromosomes, have been examined for the presence of mouse and human mitochondrial DNAs. For this analysis, advantage was taken of the fact that these two types of mitochondrial DNA have a buoyant density difference in CsCl gradients of 0.008 g/cm(3). In all the hybrid clones analyzed, which retained an average number of human chromosomes estimated conservatively to vary from 5 to 23, only mitochondrial DNA of mouse character was detected. It seems likely that either repression of relevant human genes by the mouse genome or loss of human chromosomes is responsible for these results. If the latter explanation is true, since chromosome loss under the conditions used here was substantially a random process, one would have to assume that the activity of nuclear genes distributed in many chromosomes is required for the survival of mitochondrial DNA.