Some of the female mosquitoes that spread dengue and yellow fever transitioned from animal-biting insects into deadly vectors of human disease by acquiring “love” for human body odour, a new study suggests.
“We provide the ideal lifestyle for mosquitoes. We always have water around for them to breed in, we are hairless and we live in large groups,” said Leslie B Vosshall from the Rockefeller University in US.
“They’ve acquired a love for human body odour, and that’s a key step in specialising on us,” she said.
To understand the evolutionary basis of this attraction, Vosshall and her colleagues examined the genes that drove some mosquitoes to prefer humans in Rabai, Kenya.
Their findings suggest that human-loving mosquitoes are attracted to our scent.
Scientists had earlier observed two distinct populations living just hundreds of meters apart.
Black mosquitoes, a subspecies called Aedes aegypti formosus, tended to lay its eggs outdoors and preferred to bite forest animals.
Their light-brown cousins, Aedes aegypti aegypti, tended to breed indoors in water jugs and mostly hunted humans.
To zero in on the genes responsible for the human-loving mosquitoes’ preference, researchers crossbred the mosquitoes, creating thousands of genetically diverse grandchildren.
And then they sorted those mosquitoes based on their odour preference and compared the two groups.
“We knew that these mosquitoes had evolved a love for the way we smell,” Vosshall said.
Researchers looked specifically for genes that had higher levels of expression in the human-loving insects’ antennae. These structures contain proteins called odorant receptors that pick up different scents.
They found 14 genes strongly linked to liking humans, but one odour receptor gene – Or4 – stood out.
“It’s very highly expressed in human-preferring mosquitoes,” Vosshall said.
The researchers guessed that Or4 must be detecting some aroma in human body odour.
The switch from preferring animals to humans involves a variety of behaviour adjustments: Mosquitoes had to become comfortable living around humans, entering their homes, breeding in clean water found in water jugs instead of the muddy water found in tree holes.
“There’s a whole suite of things that mosquitoes have to change about their lifestyle to live around humans,” Vosshall said.
The study was published in the journal Nature.