New study figures out why mosquitos' targeting system is 'essentially unbreakable'

We all know how sensitive insects, especially mosquitoes, can smell and how they can sting us. No matter what we do, there is no escape from them. According to a study, it seems that getting rid of them will be inevitable.

The study indicates that female mosquitoes that had their human-scent receptors blocked/removed could still smell people. This development also demonstrates that insects have a more sophisticated olfactory system than was previously known.

The study was published in Cell yesterday.

Under normal conditions, female mosquitoes rely on the cocktail of smells that humans and other animals emit to detect the blood they need to nourish their eggs. They pick up scents through olfactory neurons located primarily on their antennae, which detect and transmit scent information to the brain. But now, the result is pretty surprising.

“At first glance, mosquito olfaction makes no sense. The way the mosquito organizes its sensation of smell is completely unexpected,” says Leslie Vosshall, the Robin Chemers Neustein Professor at The Rockefeller University and Chief Scientific Officer of the Howard Hughes Medical Institute.

“But for the mosquito, it makes perfect sense. Every neuron that interprets smell is redundant in such a way that the olfactory system is essentially unbreakable. This may explain why we haven’t found a way to break mosquitoes’ attraction to humans."

There are a lot of overlaps

Scientists often think of the brain processing smells with a 1:1:1 (one-neuron-one-receptor-one-glomerulus model) system for insects and mammals. Each olfactory neuron expresses one odor receptor that communicates with one cluster of nerve endings, known as a glomerulus.

However, it is observed that some species have almost the same number of olfactory receptors as glomeruli. For instance, honeybees have 180:160, or tobacco hornworms have 60:70.

The complexity continues to the brain, the team found. “In mosquitoes, we saw a lot of overlap in the brain,” says Margaret Herre, the co-author of the study.

Meg Younger, a former postdoc in Vosshall’s lab and now an assistant professor at Boston University made a surprising discovery. Although the 1:1:1 rule dictated that mosquitoes should have one neuron, receptor, and glomerulus for smelling body odor and a separate scheme for carbon dioxide, Younger working with Margaret Herre - co-author of the study - found evidence of individual odor neurons with multiple different receptors.

Can we understand their biting behaviors?

As a result, the researchers say that they hope to understand how the mosquito brain processes human odor could be used to intervene in biting behavior, and they aim to reduce the spread of mosquito-borne diseases.

“One major strategy for controlling mosquitoes is to attract them to traps to remove them from the biting population. If we could use this knowledge to understand how human odor is represented in the mosquito antennae and brain, we could develop blends that are more attractive to mosquitoes than we are. We could also develop repellants that target those receptors and neurons that detect a human odor,” said Younger.

Study abstract:

Aedes aegypti mosquitoes are a persistent human foe, transmitting arboviruses including dengue when they feed on human blood. Mosquitoes are intensely attracted to body odor and carbon dioxide, which they detect using ionotropic chemosensory receptors encoded by three large multi-gene families. Genetic mutations that disrupt the olfactory system have modest effects on human attraction, suggesting redundancy in odor coding. The canonical view is that olfactory sensory neurons each express a single chemosensory receptor that defines its ligand selectivity. We discovered that Ae. aegypti uses a different organizational principle, with many neurons co-expressing multiple chemosensory receptor genes. In vivo electrophysiology demonstrates that the broad ligand sensitivity of mosquito olfactory neurons depends on this non-canonical co-expression. The redundancy afforded by an olfactory system in which neurons co-express multiple chemosensory receptors may increase the robustness of the mosquito olfactory system and explain our long-standing inability to disrupt the detection of humans by mosquitoes.