Insecticides have been used for centuries to counteract widespread pest damage to valuable food crops. Eventually, over time, beetles, moths, flies and other insects develop genetic mutations that render the insecticide chemicals ineffective.
Escalating resistance by these mutants forces farmers and vector control specialists to ramp up use of poisonous compounds at increasing frequencies and concentrations, posing risks to human health and damage to the environment since most insecticides kill both ecologically important insects as well as pests.
To help counter these problems, researchers recently developed powerful technologies that genetically remove insecticide-resistant variant genes and replace them with genes that are susceptible to pesticides. These gene-drive technologies, based on CRISPR gene editing, have the potential to protect valuable crops and vastly reduce the amount of chemical pesticides required to eliminate pests.
Still, gene-drive systems have come under scrutiny with concerns that once they are released into a population they could continuously spread unchecked.
University of California San Diego geneticists have now developed a solution to this concern. Publishing in the journal Nature Communications , School of Biological Sciences Postdoctoral Scholar Ankush Auradkar and Professor Ethan Bier led the creation of a new genetic system that converts insecticide-resistant forms of mutated insect genes back to their natural, native form. The novel system is designed to spread the original "wild type" version of the gene using the biased inheritance of specific genetic variants known as alleles and then disappear, leaving only a population of insects with the corrected version of the gene.
"We have developed an efficient biological approach to reverse insecticide resistance without creating any other perturbation to the environment," said Bier, a professor in the Department of Cell and Developmental Biology, of the self-eliminating allelic drive, or "e-Drive." "The e-Drive is programmed to act transiently and then disappear from the population."
As described in the paper, the researchers created a novel genetic "cassette," a small group of DNA elements, and inserted it inside fruit flies as a proof-of-concept technology that could be applied to other insects. They developed the e-Drive to target a gene known as the voltage gated sodium ion channel, or vgsc, which is required for proper nervous system functioning.
The e-Drive cassette is designed to spread through CRISPR gene editing and features a guide RNA that binds to a Cas9 DNA protein and makes a cut at the targeted vgsc insecticide resistant gene site. The gene is then switched out for a native copy of the gene that is susceptible to insecticides.
Per the study, when insects carrying the cassette are introduced into a target population, they mate randomly and transmit the e-Drive cassette to their offspring. To maintain control of the e-Drive's spread, the researchers imposed a fitness check on those carrying the cassette, either through limited viability or fertility. The cassette was inserted on the X-chromosome and reduced the mating success of males, resulting in reduced offspring. The frequency of the cassette in the population eventually declines through each generation until it fully vanishes from the population.
In laboratory experiments all of the offspring were converted to native genes in eight-to-10 generations, which took about six months in flies.
"Because insects carrying the gene cassette are penalized with a severe fitness cost, the element is rapidly eliminated from the population, lasting only as long as it takes to convert 100 percent of the insecticide-resistant forms of the target gene back to wild-type," said Auradkar.
The researchers note that the self-eliminating nature of the e-Drive means it can be introduced and re-introduced as needed, and as different types of pesticides are used. The researchers are now developing a similar e-Drive system in mosquitoes to help prevent the spread of malaria.
In addition to Auradkar and Bier, the coauthors of the Nature Communications paper included their close collaborators Rodrigo Corder of the Institute of Biomedical Science, University of São Paulo; and John Marshall of the Innovative Genomics Institute, who performed sophisticated mathematical modeling that revealed important hidden features of the e-Drive system, including its ability to efficiently cull a class of individuals in which the drive process did not occur.
