Replacement of Bovine Mitochondrial DNA by a Sequence Variant Within One Generation

Inheritance of mitochondrial DNA (mtDNA) in Holstein cattle was characterized by pedigree analysis of nucleotide sequence variation. mtDNA was purified from leukocytes of 174 individuals representing 35 independent maternal lineages, and analyzed for nucleotide sequence variation by characterization of restriction fragment length polymorphism and direct sequence determination. These data revealed 11 maternal lineages in which leukocytes from some individuals seemingly were homoplasmic for the reference mtDNA sequence at nucleotide 364, whereas those from other individuals were homoplasmic for a sequence variant at this position. Both alternative alleles were detected in all branches of these 11 lineages, suggesting that mutation at nucleotide 364 and fixation of the variant sequence occurred frequently in independent events. Thirteen instances were detected of mother-daughter pairs in which leukocytes of each of the two animals seemingly were homoplasmic for a different allele at nucleotide 364, demonstrating the bovine mitochondrial genome can be replaced completely by a nucleotide sequence variant within a single generation. The two alternative sequences seemingly arose de novo at similar frequency, ruling out replicative advantage or other selective bias as the explanation for rapid fixation of mutations at nucleotide 364. Another instance of intralineage sequence variation was detected at nucleotide 5602. This variation was detected in only one of the lineages examined, and evidently arose within three generations.