Purifying selection during maternal inheritance of mammalian mtDNA depends on autophagy and bottleneck size

Mammalian mitochondrial DNA (mtDNA) is transmitted asexually without recombination and accumulates mutations at a high rate, which eventually should cause a mutational meltdown. Two processes operating in the maternal germline, the genetic bottleneck and purifying selection, are counteracting this decline but the exact molecular mechanisms and their possible link remain incompletely understood. To address this, we investigated the role of autophagy and mtDNA copy number in shaping purifying selection during maternal mtDNA transmission. Using a carefully designed breeding strategy in mice expressing a proofreading-deficient mitochondrial DNA polymerase, we generated animals carrying random mtDNA mutations and simultaneously introduced moderately decreased or increased mtDNA copy number, or impaired autophagy. Mutation patterns in control animals closely resembled those observed in humans, showing strong purifying selection against non-synonymous mutations, particularly in oxidative phosphorylation (OXPHOS) genes. Our recent work provides new insight by identifying autophagy as a key mediator of germline purifying selection of mtDNA. Moreover, we demonstrate that mtDNA copy number directly influences the efficiency of purifying selection, revealing that these two processes are functionally interconnected.