Scientific Method / Science & Exploration

Fighting malaria by engineering flies to smell like mosquitos

Scientists isolate and test the olfactory genes of a mosquito to find out …

Mosquitos are still a leading cause of the spread of malaria and other diseases in many parts of the world. While we know that mosquitos find humans through smell, the details of their olfactory abilities hadn't been worked out. A new study in Nature describes how researchers isolated the olfactory genes of mosquitos and expressed them in fruit flies to see how each responds to certain smells. They found that mosquitos are attracted to certain chemicals in human breath and sweat, even in small amounts—knowledge they could use to build better repellents and traps.

The type of mosquito tested, A. gambiae, is the biggest spreader of malaria in sub-Saharan Africa. The fact that mosquitos detect humans through smell is well-known, but the molecular basis of their ability to sniff us out is not, even though the mosquito genome was sequenced several years ago. 

Using the genome sequence, the researchers determined that there are a total of 79 odorant receptor genes that help this particular variety of mosquito smell. These genes were individually transferred to fruit flies, another species that detects its food through olfaction. Each mosquito odorant receptor gene was expressed in an olfactory neuron; of the 79 genes transferred, 50 remained functional in the fruit fly.

Each of the altered fruit flies were then exposed to 110 different odors to see how the mosquito olfactory genes caused them to respond to various smells. This data was then cross referenced with a previous study on the olfactory responses of genetically normal fruit flies to isolate the effects of the mosquito's odorant receptors.

Scientists found that, of the 50 mosquito genes, there were a handful that made the fruit flies respond very strongly to certain smells, while mostly ignoring others. For example, one gene, called AgOr2, made the fruit fly very attentive to indole, a chemical that is responsible for 30 percent of the "volatile headspace," or scent, of human sweat. Another mosquito gene attracted the fruit fly to the smell of various microflora that grow on human skin.

Not only did the individual genes enable dramatic responses, but the smell didn't need to be strong to get a reaction from the altered fruit flies—they could smell the chemicals in tiny concentrations. 

Overall, scientists found that there were five genes that produced a strong, almost tunnel-vision response in the fruit flies. The five genes controlled responses to human breath, human sweat, plants that repel mosquitos (such as citronella), and a chemical that induces egg-laying in females.

The rest of the mosquito odorant receptors were responsible for lukewarm responses to other chemicals present during the daily life of a bug. For example, the esters produced by overripe fruits would elicit a reaction from a particular receptor, but it wasn't sufficiently sensitive to allow the fruit fly to distinguish between the ester and a chemically related acid.

The researchers suggest that the new knowledge of mosquitos' olfactory receptors could help in creating new mosquito deterrents, screens that can mask the particular human chemicals that mosquitos love, or traps that releases those chemicals so that mosquitos mistake it for a person. Not only would this neutralize a minor annoyance in your backyard, but it could help prevent the spread of malaria and other diseases in developing countries. 

Nature, 2010. DOI: 10.1038/nature08834

Listing image by NASA

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