Mitochondria: Form and Function

After an egg is fertilized, the resulting zygote contains bothmaternal and paternal mitochondria. With few exceptions, however, mitochondrial DNA (mtDNA) is transmitted solely through thematernal lineage. Howdo cells dispose of paternalmitochondria?New research from two independent groups provides evidence that paternal mitochondria are actively and rapidly degraded by autophagy, a highly regulated process that digests and recycles intracellular components. Sato and Sato (2011) follow fluorescently labeled paternal mitochondria in newly fertilized C. elegans oocytes and simultaneously monitor autophagosome formation. Immediately after fertilization,maternal autophagosomemembranes accumulate near the site of sperm entry and appear to engulf paternal mitochondria. As embryogenesis proceeds, mitochondria and autophagosomes disperse throughout the cytosol,withmaternal autophagosomesassociating specificallywithpaternalmitochondria. Time-lapse microscopy reveals that paternal mitochondria are engulfed by maternal autophagosomes, which are subsequently degraded by lysosomes. In a related study, Al Rawi et al. (2011) examine recently fertilized oocytes fromM.musculus andC. elegans, providing evidence that maternal autophagy of paternal mitochondria is an evolutionarily conserved event. Previous reports show that ubiquitin marksmitochondria inmammalian sperm, and it is likely that this signal is read by thematernal autophagymachinery inmice. However, the identity of the signal that marks paternal mitochondria for destruction in C. elegans remains unknown. To begin to address this question, Sato and Sato (2011) examine the fate of paternal mitochondria in embryos lacking autophagy. In these embryos, paternal mitochondria persist, but theymaintain their compact, granular morphology and do not increase in number.Whether this distinct structure or a heretofore-unidentified sperm-specific mark underlies their selective elimination is a question for the future. Sato, M., and Sato, K. (2011). Science. Published online October 13, 2011. 10.1126/science.121033. Al Rawi, S., et al. (2011). Science. Published online October 27, 2011. 10.1126/science.1211878.