How do you mate with the right person if everyone looks exactly the same? That's a problem that faces the Anopheles gambiae complex of mosquitoes, a group that comprises six identical-looking species. The solution, according to a new study, is to find a partner who can sing in perfect harmony with you.
Anopheles gambiae mosquitoes carry the malaria parasite, which kills more than a million people every year. Although the group consists of a half- dozen species, researchers can only distinguish them with molecular tests. In the West African country of Burkina Faso, two forms of one of the six A. gambiae species--called "M" and "S"-- swarm together at mating time, but they rarely mate with a mosquito of the other type. Medical entomologist Gabriella Gibson of the University of Sussex and the Natural Resources Institute of the University of Greenwich, both in the United Kingdom, wondered how they can tell each other apart.
Gibson and colleagues collected larvae of the two forms in Burkina Faso and raised them in the lab. Then they stuck a short piece of wire to their backs with beeswax and brought them within a few centimeters of another bound mosquito of the opposite sex (see picture). The pair flapped in place while a microphone recorded their "music"; mosquitoes sing by speeding up or slowing down their wing beats, which changes the frequency of their high-pitched whines. "It's very sweet," says Gibson. "When they're doing this singing thing, they're reaching their legs across to the other one, trying to do footsies."
The mosquitoes harmonized--but only with mosquitoes of the same form. A pair of two M or two S mosquitoes aligned their wing beats so that the female beat her wings twice for every three beats of the male's wings. Like a bowed violin string, the beating wings also created higher frequencies, which match when males and females are harmonizing. "If they're of the same form, they'll stick with each other with this harmonizing just for seconds on end," says Gibson. "If they're the two opposite types, they really won't come together."
The mosquitoes aren't listening to the high harmonics; they can't hear in that range. Instead, Gibson and colleagues say that the insects use a donut-shaped organ near the bottom of the antenna, the Johnston's organ, to sense when clashing tones are making the antenna vibrate oddly. When two different forms of A. gambiae try to harmonize, says Gibson, it's as "if people are trying to talk on the phone in different languages."
The work is the first to explain how the M and S forms in the A. gambiae complex remain genetically distinct--and even how these forms may one day become separate species, says mosquito evolutionary biologist Nora Besansky of the University of Notre Dame in Indiana. But medical entomologist Michel Slotman of Texas A&M University in College Station cautions that the results may not apply in other parts of Africa. In Mali, for example, M and S mosquitoes don't seem to swarm together, yet they will mate if they're introduced. So something else is keeping the forms separate in this region.