A new study published in Thursday's edition of Cell reports that a mosquito's sense of smell is more complicated than we once thought. And it may explain how this pesky insect is so good at seeking you out at a barbecue or in your bedroom, and digging its proboscis into your skin — as well as lead to new strategies to ward off the potentially deadly diseases transmitted by its bite.
Meg Younger, a neuroscientist at Boston University, is co-author of the study. When I visited her lab, she introduced me to her subjects, opening up a large incubator set to a balmy 80 degrees. Cubes, each a foot square and stretched with fine white mesh, are packed onto the shelves. Each cube is filled with 100-plus mosquitoes.
"I showed this to a friend the other day," Younger says with a smile. And her friend said it looked "like a mosquito hotel." It's safe to say that Younger is the de facto hotel manager. She places one of the mosquito-filled cubes on a lab bench, and exhales gently into it. A waft of carbon dioxide blows across the insects, and they go wild.
"They all get up and fly around and they'll do that for a few minutes," Younger explains. "And now, in this state, they're sensitized to look for other cues." Cues that would steer them to a target like the complex blend of human body odor — an aroma that's magnetic to a mosquito.
In many parts of the world, this attraction isn't merely a nuisance for humans. It's a major health problem. "The ones that prefer humans tend to be the ones that transmit diseases among humans," Younger says. These ailments include dengue, Zika, chikungunya and malaria. The latter disease alone causes over half a million deaths each year.
So scientists have attempted to break this attraction. But try as they might, the little mosquito has resisted. "They're really good at what they do," Younger says. Mutate a mosquito so it's insensitive to carbon dioxide — which primes them to scan for cues like odor — or fiddle with portions of its ability to smell and it can still zero in on people and bite them.
Younger admits it's been frustratingly hard to find chemical means of battling mosquitoes. "We've hit on certain things at random," she says, such as what led to DEET. "And if we were able to learn [more about] how mosquitoes are finding people, the more potential starting points we'll have to develop these new repellents or conversely, attractants for traps."
By peering into the mosquito's brain to decipher how it smells its surroundings, Younger and her colleagues — Leslie Vosshall of the Howard Hughes Medical Institute and Rockefeller University, Margo Herre of Rockefeller University, and Olivia Goldman of Rockefeller University — have taken steps to do just that.
Most of what we know about the neuroscience of smell comes from mice and fruit flies, where the wiring's fairly simple. Each neuron in the nose or antenna has one kind of receptor that detects a single kind of odor — say, a banana. And all the neurons with receptors for the banana smell connect to the same part of the brain.
Of course, there are hundreds of different receptors responding to countless odors. But this mechanism of one kind of receptor per neuron has been the party line for how smell generally works. Until Younger and the others started poking around inside mosquito brains, where she found that each neuron has multiple receptors that can detect multiple odors.
"I saw this and in my head, I was like, 'This is weird,'" she says. "And I just thought, 'Huh, weird is good.' Cause it's fun to study something that's new and different and it's fun to be surprised."
Younger thinks this finding that a mosquito's sense of smell is organized differently than expected (i.e., many neurons house multiple receptors instead of one) may explain why its ability to sniff people out is so tamper-proof. It gives the insect a kind of built-in redundancy in the system. For instance, Younger speculates that because humans all smell different than one another, mosquitoes may rely on this redundancy to broaden their target of what a person smells like.
This work could give researchers additional ways to thwart the bugs, like developing traps that contain new fragrance blends that are more alluring than people.
"It's an enormous study," says Josefina del Marmol, a molecular neurobiologist at the Harvard Medical School who wasn't involved with the research. She says there's more work to be done to verify, neuron by neuron, that each one actually responds to all the odors it has receptors for.
But she applauds the central finding. "It really does change a lot about what we know of how insects perceive the world," del Marmol says. "It's a lot more complex than we thought."
In her lab, Meg Younger stares at the mosquitoes darting about inside that hotel. And she can't help but marvel at the complexity tucked into a brain less than a millimeter across.
"You know, nervous systems are so powerful," she considers aloud, "that even one that's so small allows for so much."
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