Why do we inherit mitochondrial DNA (mtDNA) only from our mothers?
You all probably know how the fertilization process works. The fastest sperm that reaches an egg cell in time gets to fuse and fertilize it. And you also might know that a sperm cell contains more mitochondria than other types of cell in your body (well, if you are a dude, of course) because it needs a lot of energy in order to get across such a long distance. However, the DNA inside mitochondria accounts for a small portion of our total DNA, containing just 37 of the 20.000-50.000 protein-coding genes in our body. Unlike nuclear DNA that comes from both parents, this one comes only from the mother!
For the reason that you inherited your mtDNA from your mother, she from hers, and so forth, you may get genetically tested to trace your maternal ancestries. But what happens with your father’s mtDNA?
Nobody fully understands how or why fathers’ mtDNA gets wiped from cells. While examining fertilization in Caenorhabditis elegans (roundworm), however, scientists observed that paternal mitochondria have an internal self-destruct mechanism that gets activated when a sperm fuses with an egg. Then, it rapidly loses its inner membrane integrity and at that instant paternal mitochondria ceases to exist and mtDNA becomes food for the zygote.
Before this research, it had been thought that maternal inheritance was orchestrated by processes in the mother’s egg cells. Large structures called autophagosomes are known to consume paternal mitochondria right after a sperm penetrates an egg. On the contrary, the researchers found that paternal mitochondria in the roundworms actually gets destroyed even before any autophagosomes reach them. They have also identified a gene, named CPS-6, that seemed to has something to do with this phenomenon. Deleting this gene caused paternal mtDNA to stay longer in the embryo, which further led to higher rates of embryonic death.
Based on this study, we can conclude that it is not good to keep sperm mitochondrial DNA in the embryo. Let’s just hope that further research will give us a more complete answer to a ‘why’ question!