Selective and continuous elimination of mitochondria microinjected into mouse eggs from spermatids, but not from liver cells, occurs throughout embryogenesis

Exclusion of paternal mitochondria in fertilized mammalian eggs is very str ingent and ensures strictly maternal mtDNA inheritance. In this study, to e xamine whether elimination was specific to sperm mitochondria, we microinje cted spermatid or liver mitochondria into mouse embryos. Congenic B6-mt(spr ) strain mice, which are different from C57BL/6J (BG) strain mice (Mus musc ulus domesticus) only in possessing M. spretus mtDNA, were used as mitochon drial donors. B6-mt(spr) mice and a quantitative PCR method enabled selecti ve estimation of the amount of M. spretus mtDNA introduced even in the pres ence of host M m. domesticus mtDNA and monitoring subsequent changes of its amount during embryogenesis. Results showed that M. spretus mtDNA in sperm atid mitochondria was not eliminated by the blastocyst stage, probably due to the introduction of a larger amount of spermatid mtDNA than of sperm mtD NA into embryos on fertilization. However, spermatid-derived M. spretus mtD NA was eliminated by the time of birth, whereas liver-derived nl. spretus m tDNA was still present ill most newborn mice, even though its amount introd uced was significantly less than that of spermatid mtDNA. These observation s suggest that mitochondria from spermatids but not from liver have specifi c factors that ensure their selective elimination and resultant elimination of mtDNA in them, and that the occurrence of elimination is not limited to early stage embryos, but continues throughout embryogenesis.