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November 14, 2014

Why Do Mosquitoes Have A Taste For Human Blood?

Chuck Bednar for redOrbit.com - Your Universe Online

Disease-spreading female mosquitoes weren’t always reliant upon human blood in order to nourish their eggs – a genetic tweak caused them to be more sensitive to the smell of people, according to new research appearing in Thursday’s edition of the journal Nature.

Ancestors of the yellow fever mosquito were more likely to target furry forest-dwelling creatures, Carolyn McBride, an assistant professor in Princeton University’s Department of Ecology and Evolutionary Biology and the Princeton Neuroscience Institute, and her colleagues report in the new study. That behavior changed thousands of years ago, and McBride’s team set out to discover the genetic reasons why the insects came to prefer human blood.

They discovered that the mosquitoes contained a version of the odor-detecting gene AaegOr4 in its antennae which is specially tuned to sulcatone, a compound prevalent in human odor. McBride and her colleagues found that the gene is more abundant and more sensitive in the modern form of the yellow fever mosquito than in its ancestral form.

“They’ve acquired a love for human body odor, and that’s a key step in specializing on us,” Leslie B. Vosshall, head of the Rockefeller University Laboratory of Neurogenetics and Behavior and a researcher at Howard Hughes Medical Institute, said in a statement. “It was a really good evolutionary move. 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.”

Image Above: Researchers report that the yellow fever mosquito sustains its taste for human blood thanks in part to a genetic tweak that makes it more sensitive to human odor. The human-preferring 'domestic' form of the mosquito (right) contains a version of the odor-detecting gene AaegOr4 in its antennae that is highly attuned to sulcatone, a compound prevalent in human odor. The researchers found that this gene is more abundant and more sensitive in the domestic form than in its ancestral 'forest' form (left), which prefers the blood of non-human animals. Credit: Carolyn McBride, Department of Ecology and Evolutionary Biology and the Princeton Neuroscience Institute

The research began in 2009, when McBride was working as a postdoc in Vosshall’s lab. The two of them and their fellow investigators traveled to Rabai, Kenya, to investigate two distinct populations of mosquitoes – black mosquitoes (Aedes aegypti formosus), which laid eggs outdoors and preferred to bite forest creatures, and light-brown mosquitoes (Aedes aegypti aegypti), which tended to breed indoors and hunt humans.

Vosshall, McBride and their fellow investigators collected mosquito larvae from tree holes in the forest and water-filled containers in homes in an attempt to determine whether or not the two groups of insects still existed. They raised the insects under laboratory conditions and found that those collected indoors tended to be light brown and typically opted to bite humans instead of guinea pigs, while the forest ones were black and preferred biting the rodents.

In order to determine what genes were responsible for giving the light-brown mosquitoes a taste for human blood, the research team crossbred them with the black ones to create thousands of genetically diverse grandchildren. Those mosquitoes were then classified based on their odor preference, and the scientists compared the two groups. They found a total of 14 genes linked to liking humans, but none stronger than the odor receptor AaegOr4.

They determined that this gene must be detecting some aroma in human body odor, so to determine which one, they recruited volunteers and had them wear pantyhose for a period of 24 hours. The stockings were then placed into a machine designed to separate the scent they contained into the individual chemicals that comprise body odor. Ultimately, they pinpointed sulcatone, a substance which helps give humans their distinctive odor.

“The more we know about the genes and compounds that help mosquitoes target us, the better chance we have of manipulating their response to our odor,” McBride said, explaining that while scent is not the only reason that the yellow fever mosquitoes prefer dining on humans, it does play a predominant role in the behavior.

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