Step-wise elimination of α-mitochondrial nucleoids and mitochondrial structure as a basis for the strict uniparental inheritance in Cryptococcus neoformans

Abstract

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 a-parent, although gametes of a-and α-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 α-parent (α-mitochondria) was studied by fluorescence microscopy and single cell analysis using optical tweezers, which revealed that α-mitochondria are preferentially reduced by the following three steps: (1) preferential reduction of α-mitochondrial (mt) nucleoids and α-mtDNA, (2) degradation of the α-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 α-mitochondria lacking mt nucleoids persisted in Δatg8 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 α-mt nucleoids. Mitochondria and chloroplasts are organelles responsible for ATP production via respiration and photosynthesis, respectively. In addition, these organelles function as centers for diverse cellular processes, including lipid biosynthesis, amino acid metabolism, and cell death. Mitochondria and chloroplasts contain their own genomes, which are thought to be vestiges of their bacterial ancestors 1. Unlike nuclear DNA, one or two copies of which are present per cell, hundreds to thousands of copies of mitochondrial (mt) DNA are present in individual cells. Usually, each of the cells maintains only one genotype of mtDNA (homoplasmy), which is inherited from the maternal parent. Maternal inheritance of mitochondrial (chloroplast) genomes is commonly observed in sexual eukaryotes, including animals, plants, fungi, and protists 2-4. Uniparental/maternal inheritance was once considered to be a result of the dilution of the paternal contribution because the paternal gametes (sperm) are much smaller than maternal gametes (eggs) and contribute a limited amount of cytoplasm to the progeny 5. However, this simple dilution model has been challenged by many findings 6. Currently, various mechanisms have been proposed to explain the active elimination of paternal

Current Perspectives on Uniparental Mitochondrial Inheritance in Cryptococcus neoformans

“…This observation led to the conclusion that the α nucleus leaves the parental cell and travels through the conjugation tube to join the a nucleus in the parental a cell. Recently, Nishimura et al [ 24 ] reported that the a nucleus is located inside of the conjugation tube. However, the images presented in Nishimura’s report do not allow unequivocal distinction of the assumed “zygote” from other neighboring cells.…”

“…This observation suggests that uneven cytoplasmic mixing is unlikely to be the driving force of UMI. Contrary to the aforementioned model, Nishimura et al reported that a conjugation tube can originate from the a cell [ 24 ]. Thus, it remains to be established if the α nucleus migrates through the conjugation tube to meet the a nucleus on the other side of the zygote and if migration of organelles, including nuclei and mitochondria, follows the same dynamics of cytoplasm mixing.…”

“…As mentioned previously, Sun et al showed that the α mitochondria extend into the conjugation tube based on the labelling of nuclear encoded mitochondrial protein Hem15. Nishimura et al reported an opposite observation based on the labelling of nuclear encoded mitochondrial protein Atp2 [ 24 ]. In this latter study, in a cross between an α strain labeled with Atp2-mCherry and a nonlabelled a strain, the mCherry labeled mitochondria were specifically restricted to the α parental cell and did not extend into the conjugation tube.…”

“…Autophagy does not play a role in mitochondrial inheritance in C. neoformans either. Nishimura et al have shown that disruption of ATG8 (responsible for the creation of the autophagosome vesicle membrane [ 38 ] and NUC1 (an endonuclease involved in mtDNA degradation) do not have an effect on the preferential inheritance of MAT a mtDNA inheritance in Cryptococcus [ 24 ] ( Table 1 ). Consistently, we found that a unilateral cross of wild type with either atg3 Δ or aif1 Δ mutant defective in autophagy-related E2-like conjugation enzyme and mitochondrial nuclease respectively yielded the same UMI pattern as a cross between two wild type partners.…”

Beyond the “selfish mitochondrion” theory of uniparental inheritance: A unified theory based on mutational variance redistribution

“…In mammals, sperm mitochondria enter the egg at fertilization, but the zygotic ubiquitin-mediated proteolysis and autophagy machinery removes paternal mitochondria shortly after syngamy. [15,16] Basidiomycete yeast Cryptococcus neoformans, producing morphologically identical gametes of two mating types (a or α), eliminates mitochondrial nucleoids of the α parent post-fertilization, and then degrades the remaining α-mitochondrial structures through autophagy, [17] and there is similar nuclease-dependent mtDNA degradation in slime mould Physarum polycephalum. [18] Male fruit flies remove their mitochondria from maturing sperm before fertilization.…”