That’s how it goes in theory. So far, only one biotech company, the British firm called Oxitec, has conducted trials of modified mosquitoes in the field. Oxitec’s target is Aedes aegypti, originally an inconsequential mosquito species in the forests of North Africa but now a worldwide carrier of dengue fever.
Manipulating male mosquitoes raised from eggs, Oxitec injects a gene that makes a lethal protein in the bugs’ bodies. The transgenic males don’t die because the gene is inactivated by tetracycline (a common antibiotic), which is part of their diet in the lab. Once released, the modified males round up most of the local females. The offspring of these matings express the lethal protein in their cells, and they die well before maturity because there’s no tetracycline antidote in their environment.
Field trials of this method in the Cayman Islands (2009-10), Panama (2014) and Brazil (ongoing) showed good results, the company says. For several months, the process knocked back local populations of mosquitoes by 80 percent or more. The next Oxitec release may soon take place in the Florida Keys if U.S. environmental agencies approve. The area has seen a recent smattering of dengue fever.
A drawback of this method is that mosquitoes must be released repeatedly because the lethal genetic effect persists only for one generation. On the other hand, that can also serve as a brake if something goes wrong.
Windbichler’s group at Imperial College has come up with a longer-lasting system called the X-shredder, referring to the mosquito’s X chromo-
some. The subjects of the experiments are Anopheles mosquitoes, which transmit malaria.
Again, the males are altered, this time with a transgene for an enzyme that cuts up DNA. In mosquitoes, as in humans, the sex-determining chromosomes are X and Y. When a male mosquito produces sperm, just one of these two chromosomes is copied. If a sperm cell contains an X, the X-shredder enzyme slices it up, in effect aborting the cell, while sperm cells containing the Y chromosome can proceed to fertilization. Because female mosquitoes always produce an egg with an X chromosome, the mosquitoes that survive are all XY — only males.
Windbichler’s technology, if scaled up, could establish the X-shredder gene in half the males of a given population. As they mated with a shrinking supply of females, the X-shredder males would push the population lower until there are no female mates left. Windbichler says it will be some time before he can test the mosquitoes in the field, though. “We have to make sure that all aspects of safety and ethical considerations have been addressed,” he says.
Scientists have learned that genetic tinkering exacts a cost on an organism, as if the weight of the inserted transgene drags it down. A modified mosquito is less fit than its natural counterpart. This can be overcome temporarily with raw numbers, but all things being equal, natural selection will eventually scrub the gene from the population. However, all things are not equal in the world of biotechnology.
A new tool, CRISPR-Cas9, is revolutionizing the field by allowing researchers to edit the genomes of just about any species, and to outfox natural selection in the process. The tool harnesses a DNA-cutting enzyme, Cas9, from the same family of enzymes as the X-shredder. Guided by specially tailored RNA molecules, the gene for the enzyme can be inserted anywhere on the chromosomes, and it delivers an attached transgene to the site, like a locomotive pulling a freight car.
Inserting a CRISPR-Cas9 complex — made up of the cutting gene, the RNA guide and the transgene — on a mosquito chromosome immediately snips out a place on the complementary chromosome. (All chromosomes come in pairs.) Then the complex copies itself.
Thus, the mosquito gets a double dose, ensuring it will pass along the transgene since both chromosomes have been altered. And when it reproduces, not only does the offspring reliably receive a copy of the transgene, but that inherited copy immediately makes a copy of itself, too. So unlike Windbichler’s shredder, which reaches only half the males in a mosquito population, CRISPR-Cas9 drives its transgene through the entire horde, thanks to internal duplication. Hence the term gene drive for this prospective new weapon in pest control.