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

Mitochondrial DNA (mtDNA) exists in many copies per cell, with cell-to-cell variability in mutation load, which is known as heteroplasmy. Developmental and age-related expansion of heteroplasmic mtDNA mutations contributes to the pathogenesis of mitochondrial and neurodegenerative diseases. Here, we describe an approach for in situ sequence-specific detection of single mtDNA molecules (mtDNA-single-molecule fluorescent in situ hybridization [smFISH]). We apply this method to visualize and measure mtDNA and heteroplasmy levels in situ 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 by a mathematical bottleneck model, predisposing individual neurons to a high mutation load. However, both during neurogenesis and oogenesis, mtDNA segregation is accompanied by purifying selection, promoting wild-type (WT) over pathogenic mtDNA. mtDNA-smFISH thus elucidates how developmental cell-fate transitions, accompanied by changes in cell morphology, behavior, and metabolism, can shape the transmission and selection of deleterious mtDNA variants.