Mosquitoes have an infallible sense of smell to find who to bite

It is one of the recurring questions of the summer: why is it almost impossible to escape from the persecution of a mosquito?

Another is why do they itch me more?

Scientists and repellent manufacturers have long known that exhaled carbon dioxide (CO₂) or octanol, a volatile in sweat, form airborne highways for mosquitoes to reach a victim.

What they did not know and has just been discovered is that mosquitoes have more than one receptor for smells and tastes in each of their thousands of olfactory neurons.

And this makes them unique in the animal kingdom.

In 2004, researchers Richard Axel and Linda Buck received the Nobel Prize in Medicine for their discoveries about the sense of smell.

A decade earlier they had found that there are about 1,000 genes involved in the process of smelling and that they give rise to a similar number of olfactory receptors.

His work also found that each olfactory neuron expressed only one of these receptors, whose information perceived from the outside was sent as an electrical signal to the olfactory bulb, the part of the mammalian brain that processes and interprets the world of aromas.

The head of the Laboratory of Neurogenetics and Behavior at Rockefeller University (USA) Leslie Vosshall, who was a student of Richard Axel, now says that "mosquitoes have thrown away all the rules of Axel and Buck".

Vosshall directs research that began years ago focused on what the olfactory system of mosquitoes is like.

In his case, they have focused on the

species , commonly known as the dengue mosquito, as it is the vector of the virus that causes this disease.

But their pecks can also inoculate the pathogens that cause yellow fever, chikungunya, Zika fever or the Mayaro virus.

Blocking the perception of the smells of the females, the only ones that sting, of the

would also be worthy of a Nobel.

But that lockdown is not going to be easy.

Detail of an antenna of an 'Ae aegypti' mosquito seen under an electron microscope.

Fluorescent green corresponds to olfactory neurons stained with the CRISPR technique.Margo Herre

The latest research results by Vosshall and his colleagues, published in the scientific journal

, show that mosquitoes have, like other animals, a single receptor on some of their neurons.

But at least those of this species may have up to two other receptors on most neurons.

"If you're a human being and you lose a single odor receptor, all the neurons that express that receptor will lose the ability to perceive that odor," says Vosshall.

"It takes something else to kill mosquitoes because getting rid of just one receptor doesn't have any effect," she adds.

"Any future attempts to control mosquitoes with repellants or anything else must take into account how unyielding their attraction to us is," she completes.

Once the mosquito genome had been sequenced and the genes that express the olfactory receptors had been identified, the researchers used various techniques to track them down and locate them in neurons.

With the modern gene editing technique CRISPR, for example, they managed to introduce fluorescent proteins of different colors for different receptors.

Thus they could see that in many neurons more than one was activated.

They found that the neurons stimulated by the octenol in human odor were also activated by other chemical compounds derived from ammonia, the amines, which also attract insects.

“The mosquitoes have a plan B for their plan B for their plan B. For me, the system is unbreakable”

Leslie Vosshall, head of the Laboratory of Neurogenetics and Behavior at Rockefeller University (United States)

Researcher at Boston University (USA) and co-author of the study Meg Younger details the finding: "Surprisingly, the neurons to detect humans through 1-octen-3-ol [octenol] and amine receptors were not populations separated”.

In an email, his colleague Margo Herre, from Rockefeller University, expands on this: “Mosquitoes also use decanal and undecanal aldehydes [two volatile chemical compounds] and more research is needed to know the exact composition of human odor and which of the odors They detect mosquitoes.

The image that these discoveries describe is that the

have a double or triple redundancy system in which if they cannot perceive an aroma, they detect another or a third.

And if they detect all of them, the signal is amplified.

As Vosshall puts it in a note: "Mosquitoes have a plan B for their plan B for their plan B. To me, the system is unbreakable."

The finding could have far-reaching implications.

On the one hand, it would explain the repeated failures to control these mosquitoes as vectors of various pathogens.

As Younger recalls, females are hematophagous "because they need the proteins present in the blood for their eggs to mature."

There are millions of years of evolution that support their desire to snack.

The females of the 'Aedes aegypti' mosquito can transmit dengue, but also the yellow fever, chikungunya or Zika fever viruses.

In the image, a researcher at the Butantan Institute in São Paulo, who designs a dengue vaccine, shows his hand infested with mosquitoes. Diego Herculano (Getty)

Until now, the different attempts to block its olfactory receptors by genetic modification have failed, perhaps because they were based on the accepted idea of ​​specificity that a specific gene expressed only a certain receptor for each type of neuron.

The same approach would explain the relative efficacy of DEET, the repellent discovered by the US military in 1946 and which is in the composition of the vast majority of chemical repellents.

Although its mechanism is not yet clear, it is believed that N,N-Diethyl-meta-toluamide (DEET) inhibits CO₂ or lactic acid receptors, but there would still be others in the same neuron if we stopped breathing or to sweat to pass trying to go unnoticed.

It also remains to be seen whether it also happens with other species of biting mosquitoes, such as the

, the various species of anopheles, which transmit malaria, or the culex, such as the common mosquito or the tiger mosquito, which, except on rare occasions, only cause the itching nuisance.

Johns Hopkins University School of Medicine neuroscientist Christopher Potter fears so.

In 2019, in his laboratory, they verified that the fruit fly (

, used in laboratories) also had this double or triple expression of receptors in a neuron.

And in the spring of this year they published that they had found the same thing in a species of Anopheles mosquitoes.

Potter, unrelated to the current research, believes "this redundancy could be common among insects."

And he highlights from his colleague's work that “the dogma prior to this was that an olfactory neuron would only express one type of olfactory receptor;

that was the rule as far as we knew.”

But, he concludes, "Dr. Vosshall's work now suggests that a mosquito's olfactory neurons might be much more adaptive, especially toward important odors like the ones humans need to locate their hosts" needed for host search.

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