For the first time, researchers have used the Crispr-Cas9 gene-editing tool in an attempt to disrupt the ability of mosquitoes Aedes aegypti to recognize the dark targets that insects explore to determine if they are hosts.
We have known that mosquitoes have been attracted to dark colors since the late 1930s. Until now, however, the molecular mechanism by which these insects visually perceived their targets was largely unknown. A new study, led by a team from the University of California (Santa Barbara), is the first to decipher aspects of how these mosquitoes use vision to find targets to bite.
However, "The better we understand how they perceive humans, the better we can control the mosquito in an ecological way “, said Yinpeng Zhan, lead author of the study published in the journal Current Biology.
These mosquitoes hunt during the day, mainly at dawn and dusk. Insects rely on several senses to locate a potential blood source. A simple cloud of carbon dioxide is normally enough to attract them. “They can also detect some of the organic signals of our skin such as heat and humidity “, underlines Craig Montell, co-author of the study. "But if there is no suitable host, the mosquito will fly directly towards the target that seems closest represented in its visual field by a dark spot “.
For this work, Zhan and his team focused on mosquitoes of the species Aedes aegypti , also called tiger mosquitoes. These are the main vectors of dengue fever, Zika virus, chikungunya and yellow fever. Females, in search of the blood they need to lay their eggs, infect tens of millions of people every year .
Researchers suspected that one of the five photosensitive proteins expressed in the eyes of these mosquitoes could be the key to eliminating its ability to visually search for human hosts by detecting dark colors. So they started using the CRISPR/Cas9 gene-editing tool to knock out the most abundant vision protein in insect photoreceptor cells:the rhodopsin protein Op1.
The researchers then collected about ten adult females to release them into a cage with a black circle and a white circle inside. Result:The females behaved in the same way as usual in all experimental trials. In other words, they all went towards the black circle.
The team then focused on the Op2 protein, the rhodopsin most closely linked to Op1. Again, the "mutant insects" behaved normally. It was only when the researchers eliminated the two opsins that the insects showed behavioral abnormalities, displaying no preference between the circles even after exposure to CO2. By eliminating two of this mosquito's light-sensing receptors, the researchers eliminated its ability to visually target hosts .
To find out, the researchers set up several tests. All genetic groups displayed positive phototaxis, moving toward light after being exposed to CO2. The manipulated insects could also distinguish shadows and movements. Their ocular morphology was healthy. Also, the lack of target recognition was not due to retinal degeneration. Finally, the insects displayed the same activity when exposed to light. In other words, they were not blind, but simply unable to distinguish dark colors .
Researchers have not yet exposed the engineered mosquitoes to hosts. Future work could thus make it possible to understand whether or not this visual impairment affects the ability of mosquitoes to feed on blood. However, these findings could have broad implications.
“Vision plays an important role in detecting a potential host for blood-sucking insects such as mosquitoes. All serve as vectors of human disease. And this study is the first to uncover the molecular mechanisms behind this behavior “, points out Craig Montell.
This work may one day inform future mosquito population control strategies . If females were unable to see their hosts, they would have a harder time finding the blood needed to develop their eggs. As a result, their populations could then decline.
