Female frogs (Xenopus laevis) release their eggs out into the water, where they wait for some lucky sperm to come along and fertilize them. But they don’t wait very long. Frog eggs are ticking time bombs that self-destruct after only a few hours if not fertilized. Previously, how this happened was a mystery. Now, new research from Kobe University and (in a separate paper) CNRS in France has found that the eggs die by a well-known mechanism: programmed cell death.
Nestled inside the female’s ovaries, frog eggs can hang out in a state of suspended animation for months. These eggs aren’t able to be fertilized yet, but they will be as soon as they’re laid. The hormone progesterone starts the clock during ovulation. (You may know progesterone from its role in menstruation and pregnancy in people). Progesterone brings the eggs briefly out of their suspended state, until they get to the next stopping point, from metaphase I to metaphase II of meiosis.
But even though these eggs are again frozen in time, developmentally, the countdown continues. After being laid, frog eggs start to deteriorate, and the chances a sperm cell will be successful in fertilizing it go way down. After 48 hours, the eggs are completely dead.
The finding that eggs deteriorate because of programmed cell death (called apoptosis) is not completely new. Eggs from other species, from starfish to humans, also die by apoptosis a short time after ovulation if not fertilized. But whether this also occurs in frogs was not known.
Why do we care about frogs? The frog is a very important model for development. They have huge eggs that can be studied by eye. Even extracts made from the eggs can be used to study important cellular processes that can have important consequences for understanding human diseases and causes of female infertility. Specifically in this case, if we can learn more about why frog eggs deteriorate, we can find out what goes wrong when they deteriorate abnormally fast. And in that sense, it’s pretty great that the mechanism of death seems to be conserved between us and frogs.
You might imagine that ovulated eggs could just sit around for an indefinite period of time, as long as they don’t get eaten or anything. After all, aren’t they stuck at a specific developmental stage for possibly months before ovulation? While this is true for the cell-cycle arrest inside the ovaries (metaphase I), it’s not true for the second block (at metaphase II, after ovulation).
The authors of the first paper (Tokmakov, et al.) surgically removed eggs from the ovaries, so that they wouldn’t be affected by progesterone. These progesterone-free eggs could hang out in the water for at least 72 hours (at which point, I assume the researchers got bored) without any signs of apoptosis. On the other hand, if treated with progesterone, the eggs returned to their normal suicidal selves.
Why aren’t the eggs safe in the second cell-cycle arrest? It turns out that the developmental block isn’t all that strong.
Both papers found that within 6-18 hours after ovulation, frog eggs will spontaneously escape from the block. At that point, the countdown really speeds up, since even if a sperm does come along, it won’t find the egg at the necessary stage for fertilization. As if it knows there’s no point to even trying anymore, the egg turns on the cell death program shortly after escaping arrest. Within 24 hours, the egg is dead. Interestingly, the second paper (Du Pasquier, et al.) also found that some eggs will stay inside the female after ovulation for over 24 hours. These eggs, too, will die a programmed death.
To test the hypothesis that early release from arrest jump-starts the cell death program, Tokmakov and colleagues treated newly laid eggs with a drug called roscovitine. Roscovitine interferes with the cell cycle machinery and causes the eggs to escape the developmental block much earlier than normal. Cell death was greatly accelerated in the drug-treated eggs, compared to eggs that weren’t treated. Similar results were seen in another experiment that used a different chemical.
However, the eggs didn’t start killing themselves immediately after their early escape from cell-cycle arrest. Though the drug sped things up, it still took many hours to see the signs of cell death (18 hours in the treated eggs versus 24 hours in normal eggs). So, while the authors show clearly that exiting the arrest comes before cell death, and that there may actually be a cause-and-effect relationship between the two, it can’t be the whole story.
By the way, why doesn’t evolution just find a way to make the second block stronger, so the eggs don’t have to kill themselves at all? The papers didn’t seem to offer any clues, but one guess is that, as in humans, the DNA inside the eggs will start to fall apart over time, and maybe this is more of a problem during the second block than the first. It may be in the female’s best interest to make sure only the best, freshest eggs are in the running for fertilization.
- Tokmakov, A., Iguchi, S., Iwasaki, T., & Fukami, Y. (2011). Unfertilized frog eggs die by apoptosis following meiotic exit BMC Cell Biology, 12 (1) DOI: 10.1186/1471-2121-12-56
- Du Pasquier, D., Dupré, A., & Jessus, C. (2011). Unfertilized Xenopus Eggs Die by Bad-Dependent Apoptosis under the Control of Cdk1 and JNK PLoS ONE, 6 (8) DOI: 10.1371/journal.pone.0023672