Multivesicular Bodies (MVBs) function to degrade sperm mitochondria after fertilization in Drosophila

Almost all animals contain mitochondria of maternal origin only. However, the exact mechanisms underlying this phenomenon are still unclear. We investigate the fate of paternal mitochondria after fertilization in Drosophila. We previously demonstrated that sperm mitochondria is eliminated in a stereotypical process dubbed paternal mitochondrial destruction (PMD). This process is initiated by intra-egg vacuolar network, which display common features of both endocytic and autophagic machineries. These vesicles, which resemble multivesicular bodies (MVBs), associate with sperm plasma membrane and secrete their intraluminal vesicles into sperm flagellum. Subsequently, sperm mitochondria breaks into smaller fragments, which are then sequestered by autophagosomes for degradation in lysosomes. Interestingly, sperm mitochondria is ubiquitinated upon fertilization, suggesting a mechanism evolved to distinguish between sperm and egg mitochondria. To identify the repertoire of sperm mitochondrial ubiquitinated proteins, we now devise an approach that combines whole fly stable isotope labeling by amino acids (SILAC) together with ubiquitinated protein enrichment followed by mass spectrometry. Furthermore, to uncover egg-derived mechanisms underlying mitochondrial targeting after fertilization, we subjected intra-egg MVBs for proteomics analysis. Fascinatedly, MVBs are enriched with proteins related to innate immune pathways, hinting towards an involvement of ‘defense’ mechanisms exerted by the egg for recognition and clearance of paternal mitochondria upon fertilization. Collectively, our work shall shed light on this evolutionary conserved process, and may provide insight as to why this process is highly conserved.