The extreme precision with which some species of mosquitoes hunt humans may be a result of their unusual olfactory system, which has an internal backup for detecting human scent.
Mosquitoes can sense CO2 or sweat from humans using unique chemoreceptors on their antennae and maxillary palps, the insect’s paired sensory appendage.
A new study led by researchers at Boston University and Rockefeller University explains why mosquitoes are so good at detecting us, even when researchers genetically turn off human-specific chemoreceptors.
According to the study, at least one species of mosquito, Aedes aegypti, has a completely different way of organizing its olfactory system than most animals.
Using CRISPR as a gene-editing tool, the researchers created mosquitoes whose olfactory neurons would express fluorescent proteins and glow under the microscope when certain odors were present. This allowed the researchers to see how different scents stimulated the olfactory system.
This suggests that A. aegypti binds several olfactory sensory receptors to a single neuron, a process called coexpression.
According to the team, this defeats a fundamental principle of olfactory science, which states that each neuron has only one chemoreceptor.
That’s surprisingly strange, says Boston University neuroscientist and lead author Meg Younger. “This is not what we expected.”
“The core belief in smell is that the sensory neurons, to us in our noses, each express one type of olfactory receptor,” says Younger.
This axis is valid for the honey bee (Apis mellifera), the tobacco hornworm (Manduca sexta), and the fruit fly (Drosophila melanogaster), which have approximately the same number of chemosensory receptors as the olfactory glomeruli. (The glomeruli are spherical structures in the brain that receive olfactory signals.)
However, A. aegypti has at least twice as many receptors as the glomeruli, a “striking similarity,” the researchers write.
The results indicate an unconventional olfactory system that simultaneously expresses multiple sensory receptors within individual neurons.
The researchers concluded that “the redundancy of an olfactory system… may increase the robustness of the mosquito olfactory system and explain our long-standing inability to disrupt human recognition by mosquitoes.”
The temptation to feed on blood is strong, as female mosquitoes must feed on the blood of humans or animals to reproduce.
A long-term goal of research is to develop better mosquito repellents that effectively mask human odor or create attractants that draw mosquitoes to your food.
The ability of mosquitoes to seek out humans makes them effective vectors for viral diseases such as dengue, Zika, yellow fever, and chikungunya. In total, these viruses kill about 700,000 people each year.
