Having a restful weekend at the cottage ruined by the irritating din of mosquitoes orbiting your head is something almost all cottage-goers can relate to, but what if those mosquitos couldn’t find you? For the first time, researchers have used cutting-edge gene-editing technology to knock out a mosquito’s ability to see humans. Their findings were published earlier this summer in the journal, Current Biology.
“We want to understand the various ways that mosquitoes find humans to ultimately devise approaches that will interfere with their ability to detect humans,” said Craig Montell, a biologist at the University of California, Santa Barbara, and one of the study’s authors.
The researchers performed their experiments on Aedes aegypti mosquitoes that spread diseases like yellow and dengue fever, chikungunya, and zika virus. “It’s really a huge health burden,” says Montell. As many as 400 million people are affected by these diseases each year, which have historically occurred in tropical, subtropical, and temperate zones around the world.
“The other issue is that Aedes aegypti is an invasive mosquito. It’s spreading to new locations all the time because of climate change and worldwide travel,” says Montell. Populations of the mosquito have been detected in Canada, and with a warming climate, more of the pests will be able to spread northward. “There’s a lot of misery from this mosquito, so there’s a lot of interest in controlling it.”
Aedes aegypti are typically on the prowl during the day and are most active at dawn and dusk. Their preferred prey is humans, and they initially sense people from the carbon dioxide exhaled from our breath. After sensing the carbon dioxide, they become excited and flock towards dark colours. They then use cues like humidity, warmth, and smell to hone in on their prey.
This new study used the cutting-edge gene-editing tool called CRISPR to knock out the visual receptors important for mosquito sight. “We all have light receptors called rhodopsins, and that’s how we see,” explains Montell. “In the case of Aedes aegypti, there are five of them in their eyes.”
First, the scientists knocked out a gene that was responsible for a light receptor called Op1, the most common rhodopsin. To test the effect on the mosquito’s vision, they placed the genetically modified mosquitoes in a box with either a white dot or a black dot. They then placed a steady stream of carbon dioxide into the box, which excited the mosquitoes, who would then typically flock towards the dark spot.
However, when they placed the mosquitos in the box that had Op1 removed, it didn’t seem to have a discernible effect on the mosquitoes’ vision. “When we found that Op1 was not compromised in this behaviour, the next thing we did was knock out the opsin that was most related to Op1 called Op2,” explains Montell. “Again, we were disappointed because there was no effect. But then we made a double mutant between Op1 and Op 2, and now their ability to find the dark spot over the light spot was eliminated.”
The scientists then performed several other tests to show that the mosquitoes weren’t just blind, and were in fact, just unable to discern between the light and dark cues.
While the study was performed using white and dark spots as a proxy to examine how the mosquitoes’ eyes were affected, Montell says he is fairly certain that the manipulated vision would still apply their ability to detect humans, and future studies could confirm that in the field. Montell is also interested in identifying the other mechanisms and receptors that mosquitoes use to find humans such as smell, which he hopes will lead to innovations on how to control their populations. “We’d like to get rid of them before they get a chance to spread disease,” he says.