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Mosquitoes With Altered Smell Gene Lose Preference For Humans

By changing one gene, scientists have bred a mosquito that does not seek out the smell of humans in preference to other animals. The team behind one of the first successful attempts to genetically engineer mosquitoes believes their work not only shows what can be done with the latest genetic techniques, but also helps us better understand the insect's attraction to humans and therefore how to block it.

Lead researcher Leslie Vosshall, a Howard Hughes Medical Institute (HHMI) investigator at The Rockefeller University in New York, says in a statement:

"The time has come now to do genetics in these important disease-vector insects. I think our new work is a great example that you can do it."

"By disrupting a single gene, we can fundamentally confuse the mosquito from its task of seeking humans," she adds.

Vosshall and colleagues write about their work in a paper published online in Nature on 29 May.

Their report follows another study published recently in PLOS ONE, where researchers from the London School of Hygiene & Tropical Medicine in the UK describe how malaria-carrying mosquitoes are more strongly attracted to the smell of humans. Starting Point Was a Gene in Flies After scientists in 2007 announced they had sequenced the complete genome of Aedes aegypti, the mosquito that carries dengue and yellow fever, Vosshall switched her lab's focus from Drosophila flies to mosquitoes and set about trying to alter their genes.

From working with genetically engineered flies, she and her team already knew of a gene called orco that was important for the fly's sense of smell. So, as Vosshall explains, they started working on this gene in mosquitoes:

" ... we had some hints that mosquitoes interact with smells in their environment, so it was a good bet that something would interact with orco in mosquitoes." Genetic Engineering Tools To mutate the orco gene in Aedes aegypti, the team used "zinc-finger nucleases" (ZFNs), powerful tools that can be designed to target and cleave specific sequences of genomic DNA.

First, they injected ZFNs into mosquito embryos and when these matured, they sought out mutant individuals and used them to generate mutant strains so they could study the behavior of the orco gene in mosquitoes.

They discovered that brain cells linked to sensing odors were not as active in the genetically engineered mosquitoes. But they also found some other interesting changes. Less Preference for Human Odor Normally, when presented with a choice between humans and other animals, non-mutant Aedes aegypti mosquitoes fly toward humans, attracted by their smell.

But when Vosshall and colleagues gave their mutant mosquitoes a choice between human scent and that of guinea pigs, they did not show a preference for humans. This was the case even in the presence of carbon dioxide, which is supposed to enhance the attraction of mosquito to humans.

It appears that changing a single gene, the orco gene, disrupts the mosquito's ability to seek human prey.

However, this experiment did not establish precisely how the mutated mosquito lost the preference for human smell.

For example, did the mutated insect lose its ability detect that the guinea pig smell is not a preferred one, or did it lose the ability to discriminate that the human smell is the one to go for? Or did the altered gene cause both these changes? Response to DEET In a second part of their study, Vosshall and colleagues found that the mosquitoes with orco mutations were attracted to human skin even when it was protected by the common insect repellant DEET.

They exposed them to two human arms: one slathered in a solution of 10% DEET, and the other untreated. The insects flew equally to both arms, showing therefore that they could not smell the DEET.

However, once the mutant mosquitoes landed on the arms, they quickly flew away from the one slathered in DEET solution. Two Different Odor-Sensing Mechanisms Identified The team concluded that their experiments with DEET on human arms showed the mosquitoes are using two separate mechanisms to sense the DEET.

"One is what's happening in the air, and the other only comes into action when the mosquito is touching the skin," Vosshall explains.

There has been talk of a dual mechanism, but this is the first experiment to show it.

Vosshall's team now wants to explore how the orco protein interacts with the mosquito's smell receptors to shape its sense of smell.

"We want to know what it is about these mosquitoes that makes them so specialized for humans," she says.

"And if we can also provide insights into how existing repellants are working, then we can start having some ideas about what a next-generation repellant would look like," she adds.

In another recently published study, US researchers suggest it may be possible to use a bacterium that stops malaria parasites developing in mosquitoes.

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