Real-time assessment of mitochondrial DNA heteroplasmy dynamics at the single-cell level.

Mitochondrial DNA (mtDNA) is present in multiple copies within cells and is required for mitochondrial ATP generation. Even within individual cells, mtDNA copies can differ in their sequence, a state known as heteroplasmy. The principles underlying dynamic changes in the degree of heteroplasmy remain incompletely understood, due to the inability to monitor this phenomenon in real time. Here, we employ mtDNA-based fluorescent markers, microfluidics, and automated cell tracking, to follow mtDNA variants in live heteroplasmic yeast populations at the single-cell level. This approach, in combination with direct mtDNA tracking and data-driven mathematical modeling reveals asymmetric partitioning of mtDNA copies during cell division, as well as limited mitochondrial fusion and fission frequencies, as critical driving forces for mtDNA variant segregation. Given that our approach also facilitates assessment of segregation between intact and mutant mtDNA, we anticipate that it will be instrumental in elucidating the mechanisms underlying the purifying selection of mtDNA.

Adaptive introgression of a visual preference gene.

Visual preferences are important drivers of mate choice and sexual selection, but little is known of how they evolve at the genetic level. In this study, we took advantage of the diversity of bright warning patterns displayed by Heliconius butterflies, which are also used during mate choice. Combining behavioral, population genomic, and expression analyses, we show that two Heliconius species have evolved the same preferences for red patterns by exchanging genetic material through hybridization. Neural expression of regucalcin1 correlates with visual preference across populations, and disruption of regucalcin1 with CRISPR-Cas9 impairs courtship toward conspecific females, providing a direct link between gene and behavior. Our results support a role for hybridization during behavioral evolution and show how visually guided behaviors contributing to adaptation and speciation are encoded within the genome.

Cristae-dependent quality control of the mitochondrial genome.

Mitochondrial genomes (mtDNA) encode essential subunits of the mitochondrial respiratory chain. Mutations in mtDNA can cause a shortage in cellular energy supply, which can lead to numerous mitochondrial diseases. How cells secure mtDNA integrity over generations has remained unanswered. Here, we show that the single-celled yeast Saccharomyces cerevisiae can intracellularly distinguish between functional and defective mtDNA and promote generation of daughter cells with increasingly healthy mtDNA content. Purifying selection for functional mtDNA occurs in a continuous mitochondrial network and does not require mitochondrial fission but necessitates stable mitochondrial subdomains that depend on intact cristae morphology. Our findings support a model in which cristae-dependent proximity between mtDNA and the proteins it encodes creates a spatial "sphere of influence," which links a lack of functional fitness to clearance of defective mtDNA.