Disappearance of mtDNA During Mating of the True Slime Mold Didymium iridis

Abstract

The active, selective digestion of mitochondrial DNA (mtDNA) from 1 parent in the zygote is a possible molecular mechanism for the uniparental inheritance of mitochondria, but direct evidence has been observed in few species. In this study, we observed the behavior of mitochondria and mtDNA during mating of the myxamoeba of the true slime mold Didymium iridis. To show the selective digestion of mtDNA in the zygote, 2 myxamoebal strains of D. iridis were crossed, and the changes of mitochondria and mtDNA were observed over time by phase-contrast observation using alkaline fixation method and DAPI staining. Each myxamoeba of D. iridis contained about 30 mitochondria, and the zygote had about 60. Each mitochondrion contains rod-shaped mtDNA. About 4.5 h after mating, the fluorescence of mtDNA in about 30 mitochondria decreased simultaneously to give small spots, and then disappeared completely by 5 h after mating. In contrast, the mtDNA in the other 30 mitochondria and all of the mitochondrial sheaths remained unchanged. This is the fourth microscopic report that shows selective mtDNA disappearance. The rapid, selective disappearance of mtDNA observed in D. iridis is likely the result of selective digestion of mtDNA from 1 parent, as in other known cases of mtDNA disappearance.