Rapid, Selective Digestion of Mitochondrial DNA in Accordance With the matA Hierarchy of Multiallelic Mating Types in the Mitochondrial Inheritance of Physarum polycephalum

Abstract

Although mitochondria are inherited uniparentally in nearly all eukaryotes, the mechanism for this is unclear. When zygotes of the isogamous protist Physarum polycephalum were stained with DAPI, the fluorescence of mtDNA in half of the mitochondria decreased simultaneously to give small spots and then disappeared completely ∼1.5 hr after nuclear fusion, while the other mitochondrial nucleoids and all of the mitochondrial sheaths remained unchanged. PCR analysis of single zygote cells confirmed that the loss was limited to mtDNA from one parent. The vacant mitochondrial sheaths were gradually eliminated by 60 hr 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.

Semi-in vitro detection of Mg2+-dependent DNase that specifically digest mitochondrial nucleoids in the zygote of Physarum polycephalum

“…1 B). As reported previously 15 , mt-nucleoids in several mitochondria became short and disappeared in zygotes 5 h after plating (Fig. 1 A m–o).…”

“…Both strains have no mitochondrial fusion plasmids, so mitochondrial fusion doesn’t occur. When AI35 and DP246 were crossed, previous studies showed that mt-nucleoids in mitochondria derived from DP246 were selectively digested in zygotes 23 and mtDNA derived from AI35 was selectively inherited to plasmodium 15 . AI35 and DP246 contained 15.5 ± 1.2 and 15.7 ± 1.4 mitochondria per cell, respectively (Supplementary Fig.…”

“…Uniparental inheritance of mitochondria has also been reported in isogamous species 13 – 17 . In the true slime mold Physarum polycephalum and the basidiomycete fungus Cryptococcus neoformans , the uniparental mtDNA is selectively digested before elimination of mitochondria during zygote maturation 15 , 16 . Recently, in C. neoformans , an endonuclease G (NUC1), which has been reported to be involved in maternal inheritance in D. melanogaster 8 and C. elegans 18 , was found to be inessential for the selective digestion of mtDNA for uniparental inheritance 16 .…”

“…In P. polycephalum , haploid myxamoebae act as isogametes. Two different mating types of myxamoebae fuse to form diploid zygotes and then develop into vegetative multinuclear plasmodium after repeated mitotic cycles without cell division 15 . P. polycephalum has oval shaped mitochondria and mitochondrial fusion doesn’t occur unless mitochondria have the mitochondrial fusion plasmid 19 .…”

“…Such long mt-nucleoids help to monitor the digestion of mt-nucleoids during uniparental inheritance. After mating of myxamoebae, the long rod-shaped mt-nucleoids in approximately half of the total mitochondria become shorter and are then lost, a process that is easy to observe 15 . In addition, we have developed a method for the isolation of mitochondria from plasmodium, which contains structurally intact mt-nucleoids.…”