Homeostatic responses regulate selfish mitochondrial genome dynamics in C. elegans

Selfish genetic elements have profound biological and evolutionary consequences. Mutant mitochondrial genomes (mtDNA) can be viewed as selfish genetic elements that persist in a state of heteroplasmy despite having potentially deleterious consequences to the organism. We sought to investigate mechanisms that allow selfish mtDNA to achieve and sustain high levels. Here, we establish a large 3.1kb deletion bearing mtDNA variant uaDf5 as a bona fide selfish genome in the nematode Caenorhabditis elegans. Next, using droplet digital PCR to quantify mtDNA copy number, we show that uaDf5 mutant mtDNA replicates in addition to, not at the expense of, wildtype mtDNA. These data suggest existence of homeostatic copy number control for wildtype mtDNA that is exploited by uaDf5 to hitchhike to high frequency. We also observe activation of the mitochondrial unfolded protein response (UPRmt) in animals with uaDf5. Loss of UPRmt results in a decrease in uaDf5 frequency whereas constitutive activation of UPRmt increases uaDf5 levels. These data suggest that UPRmt allows uaDf5 levels to increase. Interestingly, the decreased uaDf5 levels in absence of UPRmt recover in parkin mutants lacking mitophagy, suggesting that UPRmt protects uaDf5 from mitophagy. We propose that cells activate two homeostatic responses, mtDNA copy number control and UPRmt, in uaDf5 heteroplasmic animals. Inadvertently, these homeostatic responses allow uaDf5 levels to be higher than they would be otherwise. In conclusion, our data suggest that homeostatic stress response mechanisms play an important role in regulating selfish mitochondrial genome dynamics.