Human Reproduction, Vol. 15, No. suppl_2, pp. 229-234, 2000
© 2000 European Society of Human Reproduction and Embryology
Mitochondrial DNA segregation in the developing embryo
Montreal Neurological Institute and Department of Human Genetics, McGill University Montreal, Canada
Correspondence: lTo whom correspondoence should be addressed at: Montreal Neurological Institute 3801 University Street, Montreal, Quebec, Canada. E-mail: eric{at}ericpc.mni.mcgill.ca
Mitochondrial (mt)DNA is strictly maternally inherited in mammals; new mutations thus segregate along maternal lineages without the benefit of homologous recombination with mtDNA of paternal origin. Despite the high mtDNA copy number (
100 000 or more) in mature oocytes, and despite the relatively small number of cell divisions during oogenesis, mtDNA sequence variants segregate rapidly between generations. This paradoxical behaviour has been ascribed to the presence of a mtDNA bottleneck in oogenesis or early embryogenesis. The nature and size of this bottleneck have been the subject of much controversy. This review argues that segregation of mtDNA sequence variants in the female germline occurs primarily during mitosis in the oocyte precursor population. Segregation is rapid because the precursor cells (primordial germ cells and oogonia) contain a relatively small number of mtDNA templates (the bottleneck) and because the replication of mtDNA is under relaxed control. For the most part, the process appears similar in mice segregating polymorphic sequence variants and in human pedigrees segregating pathogenic point mutations. In particular, there is no evidence for selection against high levels of pathogenic mtDNA point mutations in oogenesis, in early embryonic development, or in fetal development, thus suggesting that efficient respiratory chain function is not critical until post-natal life. These results have important practical implications for clinical genetics.
Key words: maternal inheritance/mitochondrial disease/mitochondrial bottleneck/mitochondrial DNA/oogenesis
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