Mitochondria are inherited only from the egg, not from the sperm. The research proposed here investigates the mechanism of this inheritance. Mitochondria are semiautonomous organelles that contain their own genome and that are involved in oxidative respiration, a process that generates damaging oxygen radicals. In addition to processing energy, mitochondria can stimulate apoptosis. Mitochondrial genetic defects have been identified in a number of human diseases. Several studies suggest that inheritance of paternal mitochondria may be harmful or even fatal to the developing embryo. Recent evidence from mammals has begun to unravel the mechanism of maternal mitochondrial inheritance. It appears that during spermatogenesis, the sperm mitochondria are marked with ubiquitin, a small protein that targets molecules for degradation. Here, I propose to use a genetic approach to investigate the mechanism of paternal mitochondrial destruction in the nematode C. elegans. The study will make use of the powerful genetic and genomic tools that have been developed for this species. Specifically, we will investigate the hypothesis that paternal mitochondria are tagged during spermatogenesis for destruction inside the developing embryo. We will first randomly mutagenize worms and select for mutations that block the destruction of sperm mitochondria. We have preliminary evidence that we can obtain such mutations. The mutated genes will be mapped and their sequence determined by comparison with linked sperm specific genes; essentially all sperm specific genes in the genome are now known as a result of DNA microarray analyses. Second, in a collaborative project, we will knock out ubiquitin-associated genes that are known to be sperm specific. If these mutations block sperm degradation of sperm mitochondria, then ubiquitin is involved in the destruction of sperm mitochondria in C. elegans. Finally, in another collaborative project, we will examine sperm for the presence of ubiquitin by (i) Western analysis, and (ii) by immunocytochemistry. The results of these experiments will likely identify genes involved in the degradation of sperm mitochondria. Because all the genes in the C. elegans genome are known, and most have predicted functions, identification of genes involved in mitochondrial inheritance will help elucidate the mechanism of this inheritance. Also, studying the phenotypes of offspring with paternal mitochondria will identify potential health impacts of inheriting paternal mitochondria.
