Marissa A. Holmbeck, Julia R. Donner, Eugenia Villa-Cuesta, David M. Rand

Communication between the mitochondrial and nuclear genomes is vital for cellular function. The assembly of mitochondrial enzyme complexes that produce the majority of cellular energy requires the coordinated expression and translation of both mitochondrial and nuclear encoded proteins. The joint genetic architecture of this system complicates the basis of mitochondrial diseases, and mutations in both mtDNA- and nuclear-encoded genes have been implicated in mitochondrial dysfunction. Previously, in a set of mitochondrial-nuclear introgression strains we characterized a dual genome epistasis in which a naturally occurring mutation in the D. simulans simw(501) mtDNA-encoded tRNA for tyrosine interacts with a mutation in the nuclear encoded mitochondrial localized tyrosyl-tRNA synthetase from D. melanogaster. Here we show that the incompatible mitochondrial-nuclear combination results in locomotor defects, reduced mitochondrial respiratory capacity, decreased OXPHOS enzyme activity, and severe alterations in mitochondrial morphology. Transgenic rescue strains containing nuclear variants of the tyrosyl-tRNA synthetase are sufficient to rescue many of the deleterious phenotypes identified when paired with the simw(501) mtDNA. However, the severity of this defective mito-nuclear interaction varies across traits and genetic backgrounds, suggesting that the impact of mitochondrial dysfunction may be tissue specific. Because mutations in mitochondrial tRNA(Tyr) are associated with exercise intolerance in humans, this mitochondrial-nuclear introgression model in Drosophila provides a means to dissect the molecular basis of these, and other mitochondrial diseases that are a consequence of the joint genetic architecture of mitochondrial function.