Mosquito Sperm Follow Their “Noses” to an Egg

A female mosquito readies herself for a blood meal. Photograph by U.S. Centers for Disease Control and Prevention/PHIL/Corbis

Insects are plenty strange: Certain individuals eat their mates, doting parents bury offspring with carcasses, and sometimes dinner means sucking liquefied organs through a straw.

Now it gets even weirder. Researchers have found that mosquito sperm contain proteins that enable them to smell their way to a waiting egg.

Many insects have these proteins—known as odorant receptors—on their sperm, as well as in their antennae, where they use them to sniff out dinner. But researchers weren’t sure what these odor proteins did when they appeared on sperm.

A new study—published online February 3 in the journal Proceedings of the National Academy of Sciences—shows that the proteins on the sperm cells of Anopheles gambiae, the mosquito that carries malaria, respond to chemical cues and tell the sperm when it’s time to get a move on. (See “Nazi Scientists May Have Plotted Malaria Mosquito Warfare.”)

“Female [mosquitoes] mate only once in their life,” says researcher Laurence Zweibel of Vanderbilt University in Nashville, Tennessee. And they store that sperm for the rest of their lives in special organs, he explains. Female mosquitoes use only some of the sperm each time they have a blood meal to create an egg.

“[Then] the sperm have to go from this inactive to active state,” says Zweibel. “You don’t want the sperm, which has a limited amount of fuel in the tank, to be beating its tail unnecessarily without any egg around. You need to have a mechanism where you only turn on the engine of the car when you’re getting ready to start the race.”

That mechanism, it turns out, are these odorant receptor proteins, which Zweibel and his team showed cause a strong response in the sperm to chemicals from a female mosquito’s ovaries. It’s the first time these proteins have been shown to have a function outside of the sensory system in insects.

Get a Move On

Not only do the sperm use their sense of smell as an on-off switch, they may also be able to follow their “noses” to the egg. “The female produces these signals to say to the sperm, ‘OK, I need some of you guys to get your act together,'” Zweibel explains. The researcher and his team also believe those chemical signals tell the sperm where to go—a kind of “here I am, come get me” signal, he says.

Zweibel’s lab colleagues stumbled on this mechanism during their many years of trying to develop mosquito repellents by disrupting an adult mosquito’s sense of smell. “Not to turn it off,” he says, “but to turn it hyper on.” He likened the method to overwhelming a human by shouting at her at the top of his lungs. “I call it the New York system,” the native New Yorker added. (See “New Mosquito Repellant Made, ‘Better Than Anything Else.'”)

A similar technique could render mosquitoes sterile by overwhelming the odor receptors on their sperm. “These processes have to be done at the right time in the right amount to provide this activation. Too much, too little, or at the wrong time” could make successful mosquito reproduction impossible, Zweibel says.

Finally, the study could have implications beyond just the malaria mosquito. Odor receptors have been found on insect sperm belonging to bugs of all kinds. Now that researchers know a mosquito sperm can “smell,” Zweibel thinks it’s just a matter of time before scientists find similar mechanisms on other species’ sperm.

“We’re looking at every insect we can get our hands on,” the researcher says. “We’ve looked in ants, bees, and we see [these proteins] all over the place.”

Rachel Kaufman