Rapid Variation in Mammalian Mitochondrial Genotypes: Implications for the Mechanism of Maternal Inheritance

Abstract

Except for strict maternal inheritance, the segregation during gametogenesis of mtDNA in animals is poorly understood. Given the high copy numbers of mtDNA in a single cell, it is difficult to explain the numerous polymorphisms that have been shown to arise rapidly both between species and among the individuals of a single species. If a simple random distribution of the mitochondria occurred between parent and progeny, no single variant could dominate the population. To delineate both the mechanism of inheritance and the shortest time scale over which such polymorphisms can be observed, we have examined the mtDNA from 15 registered Holstein cows having a common maternal ancestor. All animals were born and maintained within the genetically closed experimental herd at the University of Florida Dairy Research Unit. The maximum divergence between the two most distantly related animals is 53 years or 13 generations. Five animals in this maternal lineage have a HaeIII restriction enzyme site mapping at 37.1% on the bovine mitochondrial map (Laipis et al. 1979), whereas the other ten animals lack this restriction site. This extremely rapid variation places strict requirements on any model for the maternal inheritance of mitochondrial DNA.

DISCUSSION
Appearance of Different Mitochondrial Genotypes within a Single Maternal Lineage
Figure 1 shows HaeIII restriction enzyme digestion patterns of mtDNA from 15 maternally related Holstein cows. Five animals have fragment L (about 520 bp). Ten animals have lost the L fragment along with a 33-bp fragment (K′) and have gained a fragment, designated L*, of about...