Single-molecule mitochondrial DNA imaging reveals heteroplasmy dynamics shaped by developmental bottlenecks and selection in different organs in vivo

Mitochondrial DNA (mtDNA) occurs in many copies per cell, with cell-to-cell variability in mutation load, known as heteroplasmy. Developmental and age-related expansion of pathogenic mtDNA mutations contributes to mitochondrial and neurodegenerative disease pathogenesis. Here, we describe an approach for in situ sequence-specific detection of single mtDNA molecules (mtDNA-smFISH). We apply this method to visualize and measure in situ mtDNA and heteroplasmy levels at single-cell resolution in whole-mount Drosophila tissue and cultured human cells. In Drosophila, we identify a somatic mtDNA bottleneck during neurogenesis. This amplifies heteroplasmy variability between neurons, as predicted from a mathematical bottleneck model, predisposing individual neurons to a high mutation load and degeneration. However, both during neurogenesis and oogenesis, mtDNA segregation is accompanied by purifying selection, promoting wild-type over mutant mtDNA. mtDNA-smFISH thus elucidates novel mechanisms whereby developmental cell-fate transitions, accompanied by changes in cell morphology, behaviour and metabolism, will shape disease-relevant and tissue-specific transmission and selection of mtDNA mutations.