Further studies on types A and B rat mtDNAs: cleavage maps and evidence for cytoplasmic inheritance in mammals.

We present here the cleavage maps of A and B rat mtDNA, examining particularly the legitimacy of two small recently reported EcoRI fragments. We find that the 0.13-kb fragment belongs in the map whereas the 0.09-kb fragment is of nonmitochondrial origin. A comparison of the types A and B maps shows that no major rearrangements have occurred since the time of the evolutionary segregation of the two DNAs. These data plus fragment patterns from other restriction enzyme digests, including those from additional enzymes which display differences between the two DNA types, provide no evidence for microheterogeneity in either of the mtDNA types, and also permit an estimate of 0.94 to 1.80% for the extent of sequence divergence of the two types. Nine enzymes yielded fragments all of which could be accurately sized leading to a reasonably accurate estimate of 15.85 kb for the size of rat liver mtDNA of either type. In a test of whether cytoplasmic inheritance is operative in mammals, reciprocal cross experiments with the two DNAs showed that only the female parental mtDNA was inherited with no evidence for the persistence of the male parental mtDNA in any of three tissues examined.