NEW STUDY: Mosquitoes have neuronal fail-safes to ensure they can always smell humans

When female mosquitoes search for a human to bite, they detect a distinct cocktail of body odours that we emit into the air. These odours then stimulate receptors in the antennas of mosquitos. Scientists have attempted to make humans invisible to mosquitoes by deleting these receptors.

Even after removing an entire family of odour sensing receptors from the mosquito genome, mosquitoes continue to bite us. Now, according to a study published by Cell Press on August 18, mosquitos have evolved redundant fail-safes in their olfactory system to ensure they can always smell our scents.

"Mosquitoes are breaking all of our favorite rules about how animals smell things," says Margo Herre, a Rockefeller University scientist and one of the paper's lead authors.

An olfactory neuron in most animals is only responsible for detecting one type of odour. "If you're a human and you lose a single odorant receptor, all of the neurons that express that receptor will lose the ability to smell that smell," says Leslie Vosshall, senior author of the paper and a professor at Rockefeller University. However, she and her colleagues discovered that this is not the case in mosquitos.

"This project really started unexpectedly when we were looking at how human odour was encoded in the mosquito brain," says Meg Younger, a Boston University professor and one of the paper's lead authors.

They discovered that neurons stimulated by the human odour 1-octen-3-ol are also stimulated by amines, another type of chemical used by mosquitos to detect humans. This is unusual because all existing rules for how animals smell suggest that neurons encode odor with narrow specificity, implying that 1-octen-3-ol neurons should not detect amines.

"It was surprising that the neurons for detecting humans via 1-octen-3-ol and amine receptors were not separate populations," Younger says. This may allow all human related odours to activate "the human detecting part" of the mosquito brain, even if some of the odours are not human related.

This could allow all human related odours to activate "the human detecting part" of the mosquito brain even if some receptors are lost, acting as a fail-safe mechanism.

The researchers also used single nucleus RNA sequencing to determine which other receptors individual mosquito olfactory neurons express. "The findings provided us with a broad picture of how common receptor co-expression is in mosquitos," says Olivia Goldman, another lead author on the paper.

Other insects, according to Vosshall, may have a similar mechanism. Fruit flies have similar co-expression of receptors in their neurons, according to Christopher Potter's research group at Johns Hopkins University. "This could be a general strategy for insects that rely heavily on their sense of smell," Vosshall speculates.

Meg Younger's group hopes to learn more about the functional significance of co-expression of different types of olfactory receptors in the future.

The National Institute of Health in the United States provided partial funding for this study.

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