| Research Abstract |
Mitochondria contain their own genomes, which are transmitted either uniparentally or maternally in diverse taxa of sexual eukaryotes, including higher plants, mosses, ferns, algae, fungi, and animals, including humans. Mitochondria are known to be inherited strictly maternally in many species. However, the molecular mechanism underlying uniparental inheritance remains unclear.
When zygotes of the isogamous protest Physarum polycephalum were stained with 4',6-diamidino-2-phenylindole (DAPI), fluorescence from half of the total mitochondrial nucleoids (mtDNA-containing areas) decreased simultaneously to small spots about 1 h after nuclear fusion. The fluorescence disappeared completely after another 30 min, while the other nucleoids and all of the mitochondrial sheaths remained unchanged. PCR analysis of single zygote cells confirmed that the loss was limited to the mtDNA from one parent. The vacant mitochondrial sheaths were gradually eliminated by 60 h after mating. Multiple mating typ
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es occur in Physarum. Transmission patterns of mtDNA were examined in all possible crosses between 16 strains with matA1, matA2, matA11, matA12, matA15 or matA16. In 39 of 60 crosses, strict uniparental inheritance was confirmed in accordance with a hierarchy of relative sexuality of multiallelic matA. We have confirmed that active, selective digestion of mtDNA from one parent in Ulva compressa and Silene latifolia.
The active, selective digestion of mtDNA from one parent is a possible molecular mechanism for the uniparental inheritance of mtDNA. We identified two novel nucleases using DNA zymography with native-PAGE and SDS-PAGE. One is a Ca^<2+>-dependent, high-molecular-weight nuclease complex (about 670 kDa), and the other is a Mn^<2+>-dependent, high-molecular-weight nuclease complex (440-670 kDa) ; the activity of the latter was detected as a Mn^<2+>-dependent, 13-kDa DNase band on SDS-PAGE. All mitochondria isolated from myxamoebae had mt-nucleoids, whereas half of the mitochondria isolated from the zygotes at 12 h after mixing had lost the mt-nucleoids. The activity of the Mn^<2+>-dependent nuclease in the isolated mitochondria was detected at least 8 h after mixing of two strains. The timing and localization of the Mn^<2+>-dependent DNase activity matched the selective digestion of mtDNA. Less
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