Selective propagation of functional mtDNA during oogenesis restricts the transmission of a deleterious mitochondrial variant

Though mitochondrial DNA is prone to mutation and few mtDNA repair mechanisms exist(1), crippling mitochondrial mutations are exceedingly rare(2). Recent studies demonstrated strong purifying selection in the mouse female germline(3,4). However, the mechanisms underlying the positive selection of healthy mitochondria remain to be elucidated. We visualized mtDNA replication during Drosophila oogenesis. We found that mtDNA replication commenced prior to oocyte determination during the late germarium stage, and was dependent on mitochondrial fitness. We isolated a temperature-sensitive lethal mtDNA mutation, mt:CoI(T300I), which displayed reduced mtDNA replication in the germarium at the restrictive temperature. Additionally, the frequency of mt:CoI(T300I) in heteroplasmic flies was decreased both through oogenesis and over multiple generations at the restrictive temperature. Furthermore, we determined that selection against mt:CoI(T300I) overlaps with the timing of selective replication of mtDNA in the germarium. These findings establish a previously uncharacterized developmental mechanism for selective amplification of healthy mtDNA, which may be evolutionarily conserved to limit transmission of deleterious mutations.