As mosquitoes and mosquito-borne illnesses like dengue fever spread across the world, researchers say that a key strategy to prevent these illnesses may be dissuading the insects from biting their victims in the first place. But while scientists have uncovered the ways that odor and heat help mosquitoes find humans, little is known about the role of taste once they've landed.
In a new study, Yale researchers have now shown how different tastes are encoded by neurons in mosquitoes and how they influence biting, feeding, and egg-laying. They also identify compounds in human sweat that increase biting behavior in mosquitoes as well as bitter compounds that suppress egg-laying and feeding behaviors - and reveal new insights into why some humans might be more enticing to mosquitoes than others.
These findings, they say, could inform methods to halt or reduce mosquito biting in the future.
The study, which was published Oct. 16 in Nature, focused on the Asian tiger mosquito, a species once limited to Southeast Asia but now found across six continents.
When the researchers offered mosquitoes human sweat samples, they found that the mosquitoes showed strong biting preferences for some samples over others.
"This mosquito is capable of spreading many diseases, including dengue and chikungunya," said senior author John Carlson, the Eugene Higgins Professor of Molecular, Cellular, and Developmental Biology in Yale's Faculty of Arts and Sciences. "And it's outcompeting other mosquito species, so it could be an even greater problem in our future."
To better understand the species' taste abilities, the researchers first took 46 different taste compounds - including sugars, salts, bitter compounds, and amino acids - and observed how neurons in the mosquito's taste organ responded to them. They found that some compounds, like sugars, excited many of the neurons. But surprisingly, some compounds actually inhibited neuronal activity.
"We've done a lot of research on taste in the fruit fly, and we haven't seen this kind of widespread inhibition before in flies," said Carlson. "Having these two different responses - excitation and inhibition - gives mosquitoes an expanded ability to encode taste, meaning they can likely differentiate a wide variety of tastes."
After they also examined how different taste compounds affected mosquitoes' biting, feeding, and egg-laying behaviors, the researchers found that different tastes promoted or suppressed different behaviors.
Having these two different responses - excitation and inhibition - gives mosquitoes an expanded ability to encode taste, meaning they can likely differentiate a wide variety of tastes.
John Carlson
For instance, some bitter compounds reduced mosquitoes' feeding behavior but had no effect on egg-laying. And while salt and some amino acids typically found in human sweat had no effect on biting when presented separately, they promoted an increase in biting when combined.
"And that nuance makes sense to us," said lead author Lisa Baik, a postdoctoral associate in Carlson's lab who led the work. "There are a lot of places in nature that have salt and a lot of places that have amino acids, but humans have both together on our skin. So maybe the mosquito is able to identify the combination and recognize our skin as a good place to bite."
Additionally, when the researchers offered mosquitoes human sweat samples, they found that the mosquitoes showed strong biting preferences for some samples over others.
"We think this could be part of the reason why some of us get bitten by mosquitoes a lot more than others," said Carlson. "Some people may just taste better to mosquitoes."
Together, the findings help describe how mosquitoes that have landed decide whether to bite or fly away. That information, the researchers say, may help identify compounds that can sway mosquitoes to leave rather than bite.
"Our study may be helpful in identifying compounds that protect us from mosquito bites in a new way," said Carlson. "Such compounds could be extremely useful, especially as climate change expands the range of mosquitoes and the diseases they spread."
