Step-Wise Elimination Of Alpha-Mitochondrial Nucleoids And Mitochondrial Structure As A Basis For The Strict Uniparental Inheritance In Cryptococcus Neoformans

In most sexual eukaryotes, mitochondrial (mt) DNA is uniparentally inherited, although the detailed mechanisms underlying this phenomenon remain controversial. The most widely accepted explanations include the autophagic elimination of paternal mitochondria in the fertilized eggs and the active degradation of paternal mitochondrial DNA. To decode the precise program for the uniparental inheritance, we focused on Cryptococcus neoformans as a model system, in which mtDNA is inherited only from the alpha-parent, although gametes of a- and alpha-cells are of equal size and contribute equal amounts of mtDNA to the zygote. In this research, the process of preferential elimination of the mitochondria contributed by the alpha-parent (alpha-mitochondria) was studied by fluorescence microscopy and single cell analysis using optical tweezers, which revealed that alpha-mitochondria are preferentially reduced by the following three steps: (1) preferential reduction of alpha-mitochondrial (mt) nucleoids and alpha-mtDNA, (2) degradation of the alpha-mitochondrial structure and (3) proliferation of remaining mt nucleoids during the zygote development. Furthermore, AUTOPHAGY RELATED GENE (ATG) 8 and the gene encoding mitochondrial endonuclease G (NUC1) were disrupted, and the effects of their disruption on the uniparental inheritance were scrutinized. Disruption of ATG8 (ATG7) and NUC1 did not have severe effects on the uniparental inheritance, but microscopic examination revealed that alpha-mitochondria lacking mt nucleoids persisted in Delta alpha tg8 zygotes, indicating that autophagy is not critical for the uniparental inheritance per se but is responsible for the clearance of mitochondrial structures after the reduction of alpha-mt nucleoids.