Lost in the zygote: the dilution of paternal mtDNA upon fertilization

The mechanisms by which paternal inheritance of mitochondrial DNA (mtDNA) ( paternal leakage) and, subsequently, recombination of mtDNA are prevented vary in a species-specific manner with one mechanism in common: paternally derived mtDNA is assumed to be vastly outnumbered by maternal mtDNA in the zygote. To date, this dilution effect has only been described for two mammalian species, human and mouse. Here, we estimate the mtDNA content of chinook salmon oocytes to evaluate the dilution effect operating in another vertebrate; the first such study outside a mammalian system. Employing real-time PCR, we determined the mtDNA content of chinook salmon oocytes to be 3.2 x 10(9) +/- 1.0 x 10(9), and recently, we determined the mtDNA content of chinook salmon sperm to be 5.73 +/- 2.28 per gamete. Accordingly, the ratio of paternal-to-maternal mtDNA if paternal leakage occurs is estimated to be 1:5.5 x 10(8). This contribution of paternal mtDNA to the overall mtDNA pool in salmon zygotes is three to five orders of magnitude smaller than those revealed for the mammalian system, strongly suggesting that paternal inheritance of mtDNA per offspring will be much less likely in this system than in mammals.