Zwiebel and colleagues scanned the mosquito genome looking for genes similar to those that generate fruit fly odorant receptors, proteins that project from nerve cells and initiate a biochemical cascade when they encounter certain molecules in the air. The four candidates the team found were all active in the antennae and mouthparts of the mosquito, where its sense of smell resides. Significantly, one of the genes the team isolated was active only in females¿the mosquito gender that bites¿and its activity dropped off sharply 12 hours after a blood meal. Previous studies have found that a female's sense of smell is dulled after feeding on human blood. Zwiebel says he and co-workers have now isolated a total of 30 possible receptors, and he expects to find anywhere from 60 to 100 in the end.
"Understanding the switch in the mosquito nose is just step one," he notes. Individual receptors generally bind to a range of molecules with varying strengths. A longer and more difficult task, he says, will be to figure out how a mosquito's brain processes the signals that various receptors send. Controlling malaria will require an international effort, Zwiebel stresses, and "we hope that by identifying these sorts of genes¿ we'll be able to help."