Maternal inheritance of mouse mtDNA in interspecific hybrids: segregation of the leaked paternal mtDNA followed by the prevention of subsequent paternal leakage.

The transmission profiles of sperm mtDNA introduced into fertilized eggs were examined in detail in F1 hybrids of mouse interspecific crosses by addressing three aspects. The first is whether the leaked paternal mtDNA in fertilized eggs produced by interspecific crosses was distributed stably to all tissues after the eggs'' development to adults. The second is whether the leaked paternal mtDNA was transmitted to the subsequent generations. The third is whether paternal mtDNA continuously leaks in subsequent backcrosses. For identification of the leaked paternal mtDNA, we prepared total DNA samples directly from tissues or embryos and used PCR techniques that can detect a few molecules of paternal mtDNA even in the presence of 10(8)-fold excess of maternal mtDNA. The results showed that the leaked paternal mtDNA was not distributed to all tissues in the F1 hybrids or transmitted to the following generations through the female germ line. Moreover, the paternal mtDNA leakage was limited to the first generation of an interspecific cross and did not occur in progeny from subsequent backcrosses. These observations suggest that species-specific exclusion of sperm mtDNA in mammalian fertilized eggs is extremely stringent, ensuring strictly maternal inheritance of mtDNA.