The perfect insecticide
Scientists have tested a gene "weapon" against mosquitoes in the USA
Tatiana Pichugina, RIA Novosti
In the US state of Florida, non-biting mosquitoes with an altered genome have been released into nature. Their task is to displace wild relatives who infect people with deadly infections. The method has already been tested in several regions of the world.
Who spreads the infection
In 2009, for the first time in 70 years, 94 cases of dengue fever, a viral flu-like disease that in severe cases leads to bleeding and shock, were recorded in the city of Key West in the Florida Keys archipelago. Twenty percent of those infected die without treatment. Entomologists believe that the infection was brought by tourists, and the yellow-bordered Aedes aegypti mosquitoes spread across the islands.
These bloodsuckers are only four percent in the Florida Keys mosquito population, but they are the ones who carry the pathogens of dengue fever, chikungunya, yellow fever, Zika virus.
The number of non–imported cases of dengue is growing from year to year, and the authorities of the archipelago decided to experiment - to release genetically modified mosquitoes that will destroy wild carriers of infections. The insects were provided by the British company Oxitec with experience in the Cayman Islands, Malaysia, Panama, Brazil, which has been seeking approval for its project in America for ten years.
In mosquitoes, the genome is edited
Mosquitoes feed mainly on sweet flower nectar. But now it's time to breed, the female is looking for a male and mating. His sperm fertilizes the embryos in the womb. And in order for the larvae to develop, they need good nutrition before laying – warm blood. The female goes hunting.
A human is easy prey for a mosquito. The female is guided by light, movement, carbon dioxide from breathing, certain smells, and near – by body heat. Attacks, as a rule, in the morning and in the evening. Pierces the skin with a "stiletto" to take a blood sample. At the same time, it first injects a secret – and with it a virus.
According to WHO, about 700 thousand people in the world die annually from mosquito-borne infections. Repellents and insecticides are used to fight blood-sucking insects. This creates a risk of resistant species. Scientists suggest another way – to create a time bomb that will destroy the natural mosquito population: to bring genetically modified males in the laboratory and release them into the wild. They will mate with wild females, and they will produce offspring that are unable to reproduce.
The death gene in this case is a section of DNA where information about the mechanism that leads the larvae to death is encoded. The method has been known for a long time, in the 1970s it was even tested in the field. Lethal mutation was caused by gamma irradiation of males.
In 2000, Professor of zoology from Oxford Luke Alfie improved the technology. He inserted an artificial tTAV gene into the mosquito's DNA – an analogue of the gene of bacteria and viruses resistant to the antibiotic tetracycline. The tTAV gene produces a protein that is deadly to the mosquito in large quantities. It can be "turned on" and "turned off" using tetracycline.
Males with tTAV are raised in water, where an antibiotic is added. The gene, accordingly, does not produce a deadly protein. Then they are released into the wild – without tetracycline, they die within two days. But this is enough to mate with wild females and pass on a lethal mutation to the offspring. It turns out a very stable self-destructible system. Two years later, Alfie founded Oxitec, a subsidiary of Oxford University, which engaged in the commercialization of know-how and its improvement.
In particular, they brought out even more effective suicide mosquitoes, actually deceiving nature. The problem is that over time, the lethal gene disappears from the population because it does not have an evolutionary advantage. To effectively control the infection, new batches of mutant individuals have to be constantly released into the wild. To avoid this, scientists have made the lethal gene dominant by inserting a tool (CRISPR/Cas9) into the DNA with it, which independently edits the second chromosome of the embryo – received from the mother. An organism is born with the death gene in both chromosomes. Almost all of his offspring will also retain the mutation. This is a very powerful mechanism for promoting a gene in a population on the principle of a chain reaction.
The places where boxes with the larvae of genetically modified mosquitoes are placed are not disclosed to avoid vandalism. The Florida Keys Archipelago in the USA. Photo: Oxitec.
There is an effect
In 2010, Oxitec released genetically modified yellow-bordered mosquitoes OX513A in the suburb of Juazeiro in Brazil. Mutant males successfully competed for females. According to monitoring data, in a year the wild population decreased by 88 percent.
The largest experiment was conducted in the Brazilian city of Jacobina. From June 2013 to September 2015, 450 thousand mutant males were released there per week. The lethality gene was monitored by a fluorescent label, but the data spread turned out to be too large: the proportion of labeled individuals ranged from ten to 60 percent in the population. The results on the effectiveness of mosquito control have not yet been published.
The British have experimented in the USA before – with genetically modified cabbage moth in New York and cotton moth in Arizona. In Florida, the company met with active resistance from local residents and environmentalists. Nevertheless, the project managed to be pushed through in a referendum.
According to Nature, in April, scientists placed boxes with mutant mosquito larvae on private property in the Florida Keys, without disclosing addresses to avoid vandalism. Within three months, 12 thousand males per week will hatch from them. At the next stage, 20 million insects are expected to appear in 16 weeks.
The latest development, the OX5034 grade, was used. They are created in such a way that already at the stage of maturation in the laboratory, females die, only males remain. This eliminates the need for manual sorting of larvae. Mosquitoes are released into nature, where they pass on the death gene to their descendants. But in the new generation, only females die. Males continue to promote the lethal mutation. After ten generations, this line dies out by itself.
There are other methods of self-destruction of yellow-boredom mosquitoes. MosquitoMate tested one of them in the Florida Keys in 2017. Males infected with the bacterium Wolbachia were released into the wild. This parasite spoils the spermatozoa, and the embryos fertilized by them die. Unlike genetically modified mosquitoes, these did not cause protests of the local population.
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