When female mosquitoes are looking for a human to bite, they smell a unique
cocktail of body odors that we emit into the air. These odors then stimulate
receptors in the mosquitoes' antenna. Scientists have tried deleting these
receptors in attempts to make humans undetectable to mosquitoes.
However, even after knocking out an entire family of odor-sensing receptors
from the mosquito genome, mosquitoes still find a way to bite us. Now, a
group of researchers, publishing in the journal Cell on August 18, found
that mosquitoes have evolved redundant fail-safes in their olfactory system
that make sure they can always smell our scents.
"Mosquitoes are breaking all of our favorite rules of how animals smell
things," says Margo Herre, a scientist at Rockefeller University and one of
the lead authors of the paper.
In most animals, an olfactory neuron is only responsible for detecting one
type of odor. "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 of the Howard Hughes Medical Institute and
a professor at Rockefeller University and the senior author of the paper.
But she and her colleagues found that this is not the case in mosquitoes.
"You need to work harder to break mosquitoes because getting rid of a single
receptor has no effect," says Vosshall. "Any future attempts to control
mosquitoes by repellents or anything else has to take into account how
unbreakable their attraction is to us."
"This project really started unexpectedly when we were looking at how human
odor was encoded in the mosquito brain," says Meg Younger, a professor at
Boston University and one of the lead authors of the paper.
They found that neurons stimulated by the human odor 1-octen-3-ol are also
stimulated by amines, another type of chemical mosquitoes use to look for
humans. This is unusual since according to all existing rules of how animals
smell, neurons encode odor with narrow specificity, suggesting that
1-octen-3-ol neurons should not detect amines.
"Surprisingly, the neurons for detecting humans through 1-octen-3-ol and
amine receptors were not separate populations," says Younger. This may allow
all human-related odors to activate "the human-detecting part" of the
mosquito brain even if some of the receptors are lost, acting as a
fail-safe.
The team also utilized single-nucleus RNA sequencing to see what other
receptors individual mosquito olfactory neurons are expressing. "The result
gave us a broad view of just how common co-expression of receptors is in
mosquitoes," says Olivia Goldman, another lead author of the paper.
Vosshall thinks that other insects may have a similar mechanism. Christopher
Potter's research group at Johns Hopkins University recently reported that
fruit flies have similar co-expression of receptors in their neurons. "This
may be a general strategy for insects that depend heavily on their sense of
smell," says Vosshall.
In the future, Meg Younger's group plans to uncover the functional
significance of the co-expression of different types of olfactory receptors.
Reference:
Herre M, Goldman OV, Lu T et al. Non-canonical odor coding in the mosquito.
Cell. 2022; 185:1–20.
DOI: 10.1016/j.cell.2022.07.024