ELIMINATION OF PATERNAL MITOCHONDRIAL-DNA IN INTRASPECIFIC CROSSES DURING EARLY MOUSE EMBRYOGENESIS

To examine whether mtDNA is uni- or biparentally transmitted in mice, we developed an assay that can detect sperm mtDNA in a single mouse em bryo. In intraspecific hybrids of Mus musculus, paternal mtDNA was det ected only through the early pronucleus stage, and its disappearance c oincided with loss of membrane potential in sperm-derived mitochondria . By contrast, in interspecific hybrids between M. musculus and Mus sp retus, paternal mtDNA was detected throughout development from pronucl eus stage to neonates. We propose that oocyte cytoplasm has a species- specific mechanism that recognizes and eliminates sperm mitochondria a nd mtDNA. This mechanism must recognize nuclearly encoded proteins in the sperm midpiece, and not the mtDNA or the proteins it encodes, beca use sperm mitochondria from the congenic strain b6.mt(spr), which carr ies M. spretus mtDNA on background of M. musculus (B6) nuclear genes, were eliminated early by B6 oocytes as in intraspecific crosses. We co nclude that cytoplasmic genomes are transmitted uniparentally in intra specific crosses in mammals as in Chlamydomonas and that leakage of pa rental mtDNA is limited to interspecific crosses, which rarely occur i n nature.