As per the recent paper published in the journal Current Biology, scientists for the first time ever were able to make humans invisible in the eyes of Aedes aegypti mosquitoes using the gene-editing tool Crispr-Cas9. Aedes aegypti commonly known vector predominantly found in tropical, subtropical and temperate regions of the world that hosts and transmits dengue virus, yellow fever virus, chikungunya virus and the zika virus. They use dark visual cues to hunt, experts, exploited this aspect of the Ae. aegypti, by wiping out two of that mosquito’s light-sensing receptors, thus eradicated its ability to visually target hosts.

“Nobody has studied this before,” said Neha Thakre, a postdoctoral researcher at the University of California, San Diego, who studies Crispr as a mosquito control tool. Thakre, who was not involved with the research, said she saw the study as a “great start” to understanding what controls mosquito vision.

The female mosquito quests for blood, as they require to lay eggs and thereby pass on the viruses, which are capable of infecting tens of millions of people each year with flaviviruses that lead to dengue, yellow fever and Zika. These pests hunt in the morning, when the sun’s out, at dusk or dawn, unlike the Anopheles mosquito that infects its host during the night.

“The better we understand how they sense the human, the better we can control the mosquito in an eco-friendly manner,” said Yinpeng Zhan, a postdoctoral researcher at the University of California, Santa Barbara, and the lead author on the paper.    

These insects are attuned via various senses to detect blood, a minuscule gust of carbon dioxide, gives an indication of someone or something has just respired in the vicinity, making the mosquito hyperactive.

“They can also detect some of the organic cues from our skin,” such as heat, humidity and stench, said Craig Montell, a neurobiologist at the University of California, Santa Barbara, and an author on the study. But if there is no suitable host, the mosquito will fly straight to the closest-seeming target: a dark spot. 

Quite a few experiment, to detect mosquito vision take place in wind tunnels, large chambers that cost tens of thousands of dollars. In previous experiments too, the species where placed in the wind tunnel and given a whiff of carbon dioxide; chose to fly to a dark spot over a white one.

However, Montell’s lab lacks a wind tunnel, so Zhan designed an inexpensive setup — a cage with a black circle and a white circle inside, costing less than100$ and presented the same results. In the spring of 2019, Zhan conducted spot tests in the cage. Jeff Riffell, a biologist at the University of Washington, along with Claire Rusch, a graduate student, and Diego Alonso San Alberto, a postdoctoral fellow, ran the same experiment in the fall using a wind tunnel to double-check the original results.

How was it done?

Montell and Zhan fostered the thought that, out of the five light-sensing proteins expressed in the mosquito’s eye one held the key to eradicate its ability to visually seek out human hosts by sensing dark colors.

First, they decided to knock out the rhodopsin protein Op1. Op1, the most widely expressed vision protein in the mosquito’s compound eyes, seemed the best candidate for interfering with the mosquito’s vision. Zhan injected the mutation into thousands of tiny mosquito eggs using a tool with a special needle with a very tiny tip.

After his wee mutants had grown into adults, Zhan sucked 10 or so females into a tube using a mouth-controlled aspirator. With each group, he held his breath, walked over to the cage and released the females with one big exhale.

The Op1 mutants behaved exactly like the wild-type Aedes aegypti: After huffing carbon dioxide, they flew directly to the black dot in the cage. Montell and Zhan tried again, this time knocking out Op2, a closely related rhodopsin. Still, the Op2 mutants showed no meaningful decline in their vision.

But when the researchers knocked out both proteins, the mosquitoes whizzed around aimlessly, showing no preference between the white circle and black circles. They had lost their ability to seek dark-colored hosts.

This raised the next set of questions: whether the mosquitoes were complete blind or just blind to people.

For the answers, Montell and Zhan ran a series of tests to see how the double mutants responded to light.

First, they tested whether the double mutants would move toward light. Next, they connected electrodes to the double mutants’ eyes to measure if the eyes displayed voltage changes in response to light. Finally, they placed the double mutants in rotating cylinders with vertical black and white stripes to see if the insects would walk in the direction of the moving stripes. The double mutants passed all three tests, although they had a weaker response than the wild types in the last two tests.

The mosquitoes were not completely blind, after all. “My first transgenic mosquito,” Zhan said proudly. “We had a happy ending.”

The new paper could inform future strategies to control mosquito populations. If female mosquitoes were unable to see hosts, they would have a harder time finding the blood required for their eggs to develop. “The population would crash,” Montell said.