Genetically Eliminating Female Mosquitoes Could End Malaria

The math on eradicating malaria is simple: Kill all the mosquitoes that carry it, and you've gotten rid of the disease. Unfortunately, evolution and the insect's general hardiness has made that a nearly impossible task. But genetically-modified mosquitoes may be mankind's Trojan Horse in a centuries-long battle against nature's most deadly pest.

Female mosquitoes are the only ones that bite humans, making them the only ones that can infect us with malaria. In a lab, scientists have literally shredded the X chromosome in Amopheles gambiae mosquitoes sperm cells—a move that will ensure that all offspring are male. In an experiment detailed in Nature Communications, the technique was effective at completely eradicating a population within six generations, because of a complete lack of females.

"Shredding of the paternal X chromosome prevents it from being transmitted to the next generation, resulting in fully fertile mosquito strains that produce >95 percent male offspring," Nikolai Windbichler, a researcher at Imperial College London, wrote in the report.

To do so, the team took a gene from a slime mold and spent eight years trying to make sure that the chromosome-destroying gene ended up only in sperm cells.

Windbichler tells me it's important that the resulting offspring are fertile (albeit male), because previous attempts to intentionally distort mosquitoes' sex ratio have been naturally subverted by a little thing called natural selection—female mosquitoes have simply ended up preferring mosquitoes that weren't sterile.

Plans to keep mosquitoes from reaching sexual maturity have also been the main problem with previous attempts to genetically modify mosquitoes to destroy their own population.

In Brazil, researchers have already released, en masse, genetically modified mosquitoes to help control dengue fever. In that experiment, Aedes aegypti mosquitoes were modified to create offspring that are unable to reach sexual maturity. That's a good idea, but it's one that inherently requires a lot of upkeep: If one giant release of mosquitoes doesn't completely eradicate the population—which, let's face it, it probably won't because of existing, normal male mosquitoes—the GMO mosquitoes will naturally die off, and the population will recover.

Genetically modified males in Windbichler's experiment can pass on the X chromosome-shredding gene to their offspring, making the technique possibly self-sustaining.

"We had to make sure that the gene that destroys the X chromosome would be active only in sperm and in no other cells," he said. "Male transgenic progeny fathered by these transgenic male mosquitoes also produce Y-biased sperm so that the effect can be self-sustaining."

The technique was effective: Here's what the female population looked like in lab tests (in orange), compared to a control (in black).

Genetically modifying animals and then setting them loose in the wild to interact with existing populations seems like a recipe for disaster, but Windbichler says that it may actually be both more effective and safer than massive insecticide-spraying campaigns.

"Unlike insecticides, which can affect and kill both target but also many non-target insects, our technology is extremely species-specific," he told me. "While we think this has a lot of potential, it is very early stage … we will also have to make sure in extensive tests that all aspects of safety and ethical considerations have been addressed to our full satisfaction."

One ethical consideration that has already been considered, however, is the one where humans are actively trying to remove a part of the food chain. Turns out, mosquitoes aren't good for much of anything, and, we think (and hope) that no one will miss the malaria-carrying species if it ceased to exist.

"There are thousands of species of mosquitoes, but only a handful transmit the deadly malaria parasite to humans. These are the ones we are targeting and no, they do not play an important part of the ecosystem. Mosquito predators are mostly opportunistic general feeders and mosquitoes are only intermittently present as prey anyway (due to rainy and dry seasons)," Windbichler said.

"Therefore, it is not likely that the loss of a particular mosquito species (among the many unaffected mosquito species) will have significant consequences for the food chain. What we do know is that every minute a child dies of malaria, so this is what drives us," he continued.