Sorry, but there won’t be any pictures of frogs having sex in this post. Why not? Because frogs (Xenopus laevis) are external fertilizers; the females lay their eggs in a pond, and then the males release sperm out into the open water.
In just plain pond water, frog sperm will slow down within minutes. This is a serious problem for a sperm cell that wants to fertilize an egg, because it will never make it through the thick jelly layer of the egg if it doesn’t keep its speed up. Research from Nathan Tholl, Douglas Chandler and colleagues at Arizona State University recently showed that there is a chemical in the egg jelly that keeps sperm going strong until they get to the egg surface. The research was published in the journal The Biological Bulletin. The full article is available online here.
Fertilization is an intricate dance between sperm and egg. These specialized cells are responsible for bringing together the genetic information from two individuals in order to make a new individual. Any mistakes will lead to sterility. The first step of this dance is that the two partners must find each other and this requires communication.
In general, the egg sits and the sperm goes toward it. But sperm are just cells, they don’t have eyes or brains, so how do they find their target?
In many animals, the egg is coated with chemicals that attract the sperm. In sea urchins, two chemicals are produced in the jelly layer of the egg: speract and resact. Together, these chemicals tell the sperm which way to swim and they kick the sperm into high gear so it can keep swimming faster. In mammals, progesterone in the cumulus layer of the egg (equivalent to the jelly layer in frogs and urchins) guides sperm toward the egg and keeps it swimming.
In sea urchins and mammals, sperm swimming is activated by chemicals in the egg jelly. But in frogs, the sperm are first activated by the shock of hitting fresh pond water. The difference between the salt levels inside the frog and in the water turn the sperm on so they can swim toward the eggs. They have to get there fast, though, because the sperm will run out of juice after about 5 or 10 minutes.
It has been known since at least the 70s that if you remove the jelly layer of a Xenopus egg, it can’t be fertilized by sperm. One reason for this is that the jelly contains a chemical called allurin that attracts the sperm to it. Another is an unknown factor on the surface of the egg that helps in the fertilization process.
The authors of the new study found that the egg jelly does another job as well: it keeps the sperm swimming. They extracted the jelly components into a liquid they call “egg water” and found that it not only extends the amount of time that the sperm are able to swim, it also causes sperm to swim the correct way. Without the egg water, sperm didn’t always swim so well. They kind of just flapped their tails around without moving forward. Allurin wasn’t able to make them swim any better, so the chemical must be something else.
Xenopus sperm don’t swim like human sperm. They spin around in a helix while swimming forward, and these two different movements have to be coordinated in just the right way, or the sperm won’t be able to move forward quickly enough to get through the thick jelly layer and, eventually, the egg. Check out videos of frog sperm here.
We still don’t know the identity of the sperm ‘swim extender’ is, or how it controls the way the sperm swim. In fact, very little is actually known about frog egg jelly. Hopefully, future research will discover more about how eggs and sperm communicate through the chemicals and proteins that surround them.
- Study Guide About the Life Cycle of a Frog (brighthub.com)
- Unwanted sex change: A common pesticide disrupts male development(nittygrittyscience.com)
- Xenbase: A Xenopus biology and genomics resource (www.xenbase.org)