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In South America there lives a mosquito that has become popular for being one of the main vectors of malaria: the Anopheles darlingi. AIn addition to other factors such as climate change, which is increasing its geographical expansion and population density, this insect has already developed capabilities that improve its conditions to survive. According to a study published in the journal Science and led by the Harvard TH Chan School of Public Health, the Anopheles darlingi of the region They are evolving to resist insecticides, which could make them more difficult to eliminate and therefore complicate the control of malaria, a disease that has increased on the continent.
While in Africa genomic studies of the different Anopheles who live there, in Latin America there was a void, explains Dr. Jacob Tennessen, co-author of the study, through a video call. This, he adds, motivated them to ally with researchers from Colombia, Venezuela, Brazil, Peru, Guyana and French Guiana who sent them samples of mosquitoes captured in 16 different places. After generating the complete genome sequence of 1,094 adult females, they were able to verify that, in effect, resistance to insecticides has been generated.
Initially, what they looked for in the genomes were “indications of selection,” says Tennessen. That is, genes that were evolving faster than the rest. Although this did not necessarily point to resistance to insecticides, when they put a magnifying glass on these genes, they found that they belonged to a class of enzymes—called P450—that are basically responsible for detoxifying the foreign substances that the insect encounters. “From other species that have already evolved in response to insecticides, we know that they increase the production of P459 enzymes. These molecules become more frequent and break down dangerous substances faster than it takes an insecticide to kill mosquitoes,” explains the scientist. They generate resistance.
Although the study did not validate on a large scale what happens with Anopheles darlingi In real life, one of its researchers, John Bernard Dushiman, did do a mini experiment in French Guiana. He collected some mosquitoes in a bottle, exposed them to insecticides, and timed how long it took for them to die. “By tracking the genotypes, we did see that one of these P450 genes influenced the lifespan of the mosquitoes,” says Tennessen.
Also, he adds, “they suspect that insecticides for agricultural use could be influencing this evolution.” Being careful to insist that it is not a certainty, the biologist comments that, since they know the place from which the mosquitoes came – cities, forests, wetlands, grasslands, agricultural and mining areas -, “the most intense signals detected were usually found in places where there was agricultural activity.”
Mosquitoes that travel in envelopes
From the University of Colombia, at its Bogotá headquarters, Dr. Martha Lucia Quiñones explains what the adventure of capturing the mosquitoes whose genome was analyzed in Harvard laboratories is like. Those from this country, he comments, were collected in Chocó, in the middle of Atrato, and in Guainía, around the border with Venezuela. “What you do,” he says, “is go to those areas, wait until 6 or 7 at night, get up blue jean and suck out the mosquitoes with a vacuum cleaner, which is like a small tube with a hose,” he remembers. They call this the “human attractant” technique.
The next morning, the mosquitoes are exposed to insecticides and sorted into those that died and those that did not. In the case of this study, each of the individuals was dried in a silica gel pellet after the experiment and thus traveled to Bogotá. Later, they made it to Boston, in the United States, to the Tennessee laboratory, stuffed in the ball, a bag and an envelope, along with other preserved mosquitoes.
Of the Chocó group, Quiñones had already reported resistance to insecticides since the 1990s. “Since that time, that very focused population of Anopheles darlingi “It had resistance to DDT, which, furthermore, some time later, we saw was cross-resistance with pyrethroids,” he says. DDT—or dichloro diphenyl trichloroethane—is an insecticide that was banned globally because its bioaccumulation generated a toxic risk, while pyrethroids are a group of synthetic insecticides that are still used for agricultural, domestic, and public health issues.
“We had the bioassays that verified the resistance,” says Quiñones, as well as the biochemical mechanisms that explained it. But, together with the alliance with Harvard, they were able to clearly see the entire DNA sequence of these mosquitoes. In fact, since the more than 1,000 genomes that were made were left in an open database, which Quiñones or any other scientist can access, she is already asking questions that she hopes to solve with this data. “That population of Anopheles darlingi Chocó, for example, in addition to having resistance, can surely have many other things that are different from other populations.” It is something that this immense base of information will allow you to explore.
Like good scientists, however, Tennessen and his team prefer to be measured about their scope. Yes, it is a milestone for biological science and efforts to combat malaria. “But in reality, the bridge that would allow us to take our research and link it with public policies would be to further investigate how these changes are really affecting the ability of mosquitoes to resist different insecticides,” says the researcher.
