Do you have the bad luck that all the mosquitoes bite you? Does it matter if you wear long pants or cover your arms? Do insects laugh at the smell of citronella? It turns out that these animals are perfect human sniffing machines.
Female mosquitoes, which are the ones that bite, search for their victim guided by a unique cocktail of body odors that each of us emits. The amount of carbon dioxide that we exhale when we breathe is especially attractive to them because this gas does not mix immediately with the air, but remains in suspension for some time. Lactic acid , also present in exhalation and sweat , also attracts them. These are therefore some of the scents that stimulate mosquito receptors. And if they didn’t have receptors, wouldn’t mosquitoes bite us?
A group of scientists has done just that: remove the receptors of these bloodsuckers to make us undetectable to their radar. The bad news is that, even so, by removing a whole family of odor-sensitive receptors from the mosquito genome, insects continue to find ways to bite us. The reason is that these animals have developed security mechanisms in their olfactory system to ensure that they can always detect our scents.
In most animals, each olfactory neuron is responsible for detecting only one type of odor. “If you’re a human and you lose a single odorant receptor, all the neurons that express that receptor will lose the ability to smell that odor,” says Leslie Vosshall of the Howard Hughes Medical Institute, a Rockefeller University professor and lead author of the paper. This is not the case with mosquitoes. For them, losing one or more receptors does not imply a decrease in their ability to pick up human odors . According to the researchers, this could be a survival mechanism.
“You have to try harder to kill mosquitoes because getting rid of just one receptor doesn’t have any effect,” says Vosshall. “Any future attempts to control mosquitoes through repellants or anything else have to take into account how unyielding their attraction to us is.”
The study has focused on the Aedes aegypti mosquito, which can transmit arboviruses such as dengue by sucking our blood. It is believed that this insect evolved to bite humans because of our proximity to fresh water, which is where they lay their eggs. According to the researchers, unraveling how the brain of these insects processes our smell could be useful to intervene in some way and reduce the spread of diseases such as malaria, dengue and yellow fever .
What they saw in the research was that neurons stimulated by the human odor 1-octen-3-ol are also stimulated by amines, another type of chemical that mosquitoes use to seek out and bite humans. This is unusual, since by all existing rules of animal smell, neurons encode odor with narrow specificity, suggesting that 1-octen-3-ol neurons should not detect amines.
“Surprisingly, the neurons to detect humans through 1-octen-3-ol and amine receptors were not separate populations,” says Meg Younger, a professor at Boston University and one of the paper’s lead authors. This could allow all human-related odors to activate “the human-sensing part” of the mosquito brain even if some of the receptors are lost, acting as a safety mechanism.
The team also used single-core RNA sequencing to see what other receptors individual olfactory neurons in mosquitoes express. “The result gave us a broad view of how common receptor co-expression is in mosquitoes ,” says Olivia Goldman, another lead author on the paper.
Receptor co-expression could also be a mechanism present in other insects. In fact, researchers at Johns Hopkins University recently reported that fruit flies have a similar co-expression of receptors on their neurons. Vosshall said a general approach could be made for insects that rely heavily on their sense of smell.
There is still a lot of work ahead. One of the team’s next goals is to see if the olfactory system of other mosquitoes, especially those that do not feed on blood, present receptor co-expression.
Referencia: Herre, M., V. Goldman, O. et al. 2022. Non-canonical odor coding in the mosquito. Cell. DOI: 10.1016/j.cell.2022.07.024.
