Everyone knows that you get half your genes from your mom and half from your dad. But that’s not the whole story. You also get another kind of DNA, mitochondrial DNA, that only comes from Mom. Men have mitochondria, too, and mitochondrial DNA is in their sperm. So, why doesn’t it get into a fertilized egg?
The maternal-only transmission of mitochondria has long been a mystery. It’s a now you see it, now you don’t situation: the sperm’s mitochondria go into the fertilized egg, but they disappear within minutes. Now, a paper published in the journal Science has found the answer. Machinery within the egg literally eats up the sperm’s mitochondria through a process called autophagy.
The researchers discovered the mechanism for destroying Dad’s mitochondria in the worm, C. elegans. But before you go thinking that this is just some weird thing worms do (after all, their sperm do look pretty weird), they were able to show that the same thing happens in mice. My guess is if you looked in other animals, you’d find the same thing, too. The group’s results show that the way paternal mitochondria are eliminated was conserved by evolution over millions of years.
How exactly does it work?
In worms, sperm don’t only bring along mitochondria that need to get destroyed, but a bunch of other organelles that are needed for the sperm to work…but no one really knows what they do. By looking inside the fertilized egg, the researchers were able to see structures called autophagosomes being recruited to the extra stuff carried in by the sperm.
Autophagosomes are the cell’s garbage disposals. They engulf whatever is targeted for destruction and digest it. The researchers looked for known marks on the mitochondria and other organelles that typically signal they are doomed to die . They found these marks on the other junk, but not on the mitochondria, even though the mitochondria get eaten up the same way. How the cell knows to eat the sperm mitochrondia, too, is still an unsolved mystery.
But what about in mice? Scientists have known since 1999 that sperm mitochondria in the mouse have marks on them for destruction. But when and how the destruction occurs wasn’t known. Having these doomed-to-death marks on them just means they’ll be eliminated somehow, but there are other ways besides autophagy to do this. They wanted to know whether autophagy is used to eat up sperm mitochondria in mice the same way it does in worms.
The authors of the current study switched out mice for worms under their microscope and found the same kind of autophagy likely accounts for the sperm’s mitochondrial destruction as in the worm. Besides the generic marks of destruction, the sperm mitochondria also had specific marks for autophagic destruction.
Why study this in the worm instead of one of our closer relatives?
The answer to this is simple: worms are easier. They’re easier to get lots of embryos from (you don’t have to kill the mom to get them because she lays eggs!), and they’re easier to do cool genetic manipulations in.
Worm scientists have a much bigger toolbox available to them to study many different kinds of processes inside cells. They can mark different proteins or cell components with fluorescent colors, and can even easily delete genes and proteins from the cell. These things are still much trickier to do with mice and other mammals.
But even though they’re just microscopic worms, they can still tell us a lot about ourselves.
Al Rawi S, Louvet-Vallée S, Djeddi A, Sachse M, Culetto E, Hajjar C, Boyd L, Legouis R, & Galy V (2011). Postfertilization Autophagy of Sperm Organelles Prevents Paternal Mitochondrial DNA Transmission. Science (New York, N.Y.) PMID: 22033522