Mosquitoes are not only small animals that are particularly annoying on summer nights. They are also dangerous vectors of disease and as such are the source of several hundred million cases of serious infections worldwide each year. Insecticides seem to be in short supply because the bugs have acquired a kind of resistance. Therefore, another solution had to be found to get rid of them. Genetic modification of mosquitoes seemed to be the ideal solution to eradicate entire populations of these insects. However, a few years later we realize that what was planned did not work at all and that genetically modified mosquitoes are even more resistant than before. A clear failure of a biotechnology in which all hopes were justified.
According to the World Health Organization (WHO), 390 million people each year are infected with mosquito-borne dengue fever. Public health experts fear that a veritable catalogue of serious diseases is being spread in Europe by mosquitoes. Aedes aegypti whose geographical areas extend northwards with ... global warming. Cases of chigungunya transmitted by the same insect have been recorded in Germany, Italy, Spain and metropolitan France (450 reported cases). Thus the recurrent diseases of tropical countries, malaria, dengue fever, chikingunya, typhus, young fever and today's Zika, constitute growing threats in Europe. The control of mosquitoes that carry human diseases is a global health issue. All the more so as their ability to resist insecticide treatments now threatens the prevention of epidemics.
The sterile male technique
In the face of these dangers, scientists have based all their hopes on a technique, that of the sterile male. The idea seems solid on paper. It involves genetically modifying male mosquitoes... Aedes aegypti to give them a dominant lethal gene. When the modified bugs mated with wild females, this gene greatly reduced the number of offspring they produced, and the few females that were born would be too weak to survive long. A particularly clever way, it was thought, to reduce the mosquito population in an area - up to 85 %, according to some preliminary tests.
The British firm Oxitec has quickly become the undisputed specialist in the market for the production of transgenic insects. In Brazil, in its factory in Campinas (State of Sao Paulo), the Oxford start-up, with the support of its Brazilian partner Moscamed, produces male Aedes Aegypti that transmit to their offspring a gene that inhibits the growth of larvae. Of course, the use of genetically modified organisms immediately raised questions of control and safety. Activists set out on a war footing. Many associations feared contamination of wildlife by transgenes or the imbalances of an overly brutal eradication of insects, leaving the way open for new invaders that could be even more dangerous.
Millions of modified mosquitoes
These fears remained unheeded, and Oxitec continued to work hard on developing its modified mosquito. Its mosquito strain was approved by the Food and Drug Administration, and the sesame seed was used to start large-scale experiments. In the case of Brazil, about 450,000 modified males were released into the Jacobina region of Bahia State every week for 27 months, for a total of tens of millions of insects.
American researchers from Yale University took advantage of this massive experiment to measure its effects. The Yale team studied the genomes of the genetically modified strain and of the wild species before release and again six, 12 and 27-30 months after the start of release.
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Of course, what had to happen happened happened. The genes from the transgenic insects had been incorporated into the wild population. albeit in small but very significant proportions. Indeed, the modified mosquitoes produced offspring in 3 to 4 % of the cases. However, while it was thought that these new generations would be weakened and not viable for a long time, the researchers observed quite the opposite. First surprise, the small insects that are born are not as weak as expected. Some seem to reach adulthood and reproduce. The second surprise is that although populations initially declined, numbers rebounded after about 18 months. Researchers suggest that the female mosquitoes may have learned and begun to avoid mating with the modified males.
Worse still, and this is the third surprise, the genetic experiment may have had the opposite effect and made the mosquitoes even more resistant. The native species mixed with the modified insects and the resulting crosses produced an insect with a broader, more robust gene pool, in a word: more resistant. "It's an unexpected result that's cause for concern," said the somewhat disappointed scientists. They assure that the new mosquitoes are not dangerous to humans. We're willing to believe them, but there's a great deal of doubt about their predictions.
Ethics and safety
It is undeniable that the use of living genetically modified organisms is not a harmless technology. It challenges our ethical values since it is the border between the "natural" and the "wild" that is being challenged. But it raises important safety issues. Genetic manipulation can spread to new populations that were not previously targeted, as a result of hybridization or horizontal transfer of DNA. Furthermore, if foreign DNA inadvertently enters the genetic forcing sequence when the sequence is not yet present in all individuals in the population, then this foreign DNA can spread like wildfire. If this DNA brings, by misfortune, resistance to insecticides or a better attraction to human odours, then the genetic transformation of the mosquitoes may backfire. Finally, while the consequences of genetic forcing on individuals carrying the genetic forcing sequence are relatively well understood, those at the ecosystem level are extremely difficult to estimate.
Source : Yale University