In addition to their DNA complement, sperm and egg cells (oocytes) also contain cellular components such as mitochondria. However, sperm do not contribute any mitochondria to the resultant offspring, despite being entirely engulfed by the ooctye during fertilization. In fact, genealogies can be traced through the female line using mitochondrial DNA. What happens to the paternal mitochondria? According to researchers from several French institutions, right after fertilization the oocyte quickly digests all the paternal organelles, including the mitochondria.
Although eggs and sperm contribute equal amounts of DNA to the zygote, the egg contributes the bulk of the cellular material within that zygote. This is not surprising, considering that a human oocyte contains about 85,000 times as much volume as its partner, the human sperm. It’s not just a matter of dilution though. The male organelles are actively digested by the oocyte. This was confirmed by the disappearance of all paternal mitochondrial DNA in the zygote. When the sperm-digesting (spermophagy) machinery was artificially disrupted, the resulting offspring retained both maternal and paternal mitochondria.
Although these experiments were conducted in tiny worms called nematodes, the scientists saw evidence of the same processes in mice, indicating that the disintegration of paternal organelles may be universal. The authors speculate that this mechanism evolved to spare zygotes from mitochondria derived from cells with a very high metabolic rate (sperm) and consequently a higher than usual mutation rate. In any case, by shutting down the spermophagy machinery, scientists can now create offspring with both maternal and paternal mitochondria. That should yield some interesting results.