| Research Abstract |
The fluorescence of mtDNA stained with DAPI in half of the mitochondria decreased simultaneously to give small spots, and then disappeared completely about 1 h after nuclear fusion, while the other mt-nucleoids and all of the mitochondrial sheaths remained unchanged in Physarum polycehalum. PCR analysis of single zygote cells confirmed that the loss was limited to mtDNA from one parent. The mt-nucleoid protein also dispersed soon after the digestion of mtDNA. The dispersed mt-nucleoid protein was eliminated from mitochondria. The vacant mitochondrial sheaths were gradually eliminated by 60 h after mating. Using six mating types, the transmission patterns of mtDNA were examined in all possible crosses. In 39 of 60 crosses, strict uniparental inheritance was confirmed in accordance with a hierarchy of relative sexuality. In the other crosses, however, mtDNA from both parents was transmitted to plasmodia. The ratio of parental mtDNA was estimated to be from 1:1 to 1:10^<-4>. Nevertheless, the matA hierarchy was followed. In these crosses, the mtDNA was incompletely digested, and mtDNA replicated during subsequent plasmodial development. We conclude that the rapid, selective digestion of mtDNA promotes the uniparental inheritance of mitochondria ; when this fails, biparental inheritance occurs. We used DNA zymography assays with native PAGE and SDS-PAGE to investigate the DNase activities in young zygotes, and identified two novel mitochondrial DNase activities a 50 kDa Ca^<2+>-dependent DNase, and a 13-kDa Mn^<2+>-+dependent DNase. We propose that the regulation of the mitochondrial nuclease by some unknown factor is the key to digestion of the mtDNA from one parent.
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