Insecticides play a central role in efforts to counter the global impact of mosquito-borne malaria and other diseases, which cause an estimated 750,000 deaths each year. These insect-specific chemicals, which cost more than $100 million to develop and bring to market, are also critical in controlling insect-related crop damage that poses a food security challenge.
But in recent decades, many insects have genetically adapted to be less sensitive to the effectiveness of insecticides. In Africa, where durable, insecticide-treated bed nets and indoor spraying are important weapons in the fight against malaria, many mosquito species across the continent have evolved insecticide resistance, reducing the effectiveness of these key measures. In certain areas, climate change is expected to exacerbate these problems.
Biologists at the University of California have now developed a method that reverses insecticide resistance using CRISPR/Cas9 technology. A team that included UC Santa Barbara researchers Craig Montell and Menglin Li, researchers Bhagyashree Kaduskar, UC San Diego’s Raja Kushwah and Professor Ethan Bier of UCSD’s Tata Institute for Genetics and Society (TIGS) used the gene editing tool to replace an insecticide-resistant gene in fruit flies with the normal insecticide-sensitive form. Their achievement, described in Nature Communications, could significantly reduce the amount of insecticides used.
This strategy could be used to reverse the resistance of mosquito disease vectors that spread devastating diseases that affect hundreds of millions of people every year.”
Craig Montell, Professor of Molecular, Cellular, and Developmental Biology, UC Santa Barbara
“This technology could also be used to increase the proportion of a naturally occurring genetic variant in mosquitoes that makes them more resistant to transmission or malaria parasites,” said Bier, UCSD professor of cell and developmental biology and senior author of the paper.
The researchers used a modified type of gene drive, a technology that uses CRISPR/Cas9 to cut genomes at specific points and propagate specific genes throughout a population. When a parent passes genetic elements to its offspring, the Cas9 protein cuts the other parent’s chromosome at the appropriate point and the genetic information is copied to that point, so that all offspring inherit the genetic trait. The new Gene-Drive includes an add-on that Bier and his colleagues previously developed to affect the inheritance of simple genetic variants (aka alleles) by simultaneously cutting an unwanted genetic variant (e.g. insecticide-resistant). . and replacing with the preferred variant (e.g., insecticide sensitive).
In the new study, the researchers used this “allelic drive” strategy to restore genetic susceptibility to insecticides, similar to what insects in the wild did before they developed resistance. They focused on an insect protein known as the voltage-gated sodium channel (VGSC), which is a target for a widespread class of insecticides. Resistance to these insecticides, often referred to as knockdown resistance or “kdr,” results from mutations in the vgsc gene that no longer allow the insecticide to bind to its VGSC protein target. The authors replaced a resistant kdr mutation with its normal natural counterpart, susceptible to insecticides.
Starting with a population consisting of 83% kdr (resistant) and 17% normal (insecticide sensitive) alleles, the allelic drive system reversed this proportion to 13% resistant and 87% wild-type in 10 generations. Bier also notes that adaptations that confer insecticide resistance come with an evolutionary cost, making these insects less fit in the Darwinian sense. Combining the gene drive with the selective advantage of the more appropriate wild-type gene variant results in a highly efficient and collaborative system, he says.
Similar allelic drive systems could be evolved in other insects, including mosquitoes. This proof-of-principle adds a new method to pest and vector control toolboxes as it could be used in combination with other strategies to improve insecticidal or parasite-reducing measures to stem the spread of malaria.
“Through these allelic replacement strategies, it should be possible to achieve the same level of pest control with far less use of insecticides,” Bier said. “It should also be possible to design self-eliminating versions of allelic drives that are programmed to act only transiently in a population to increase the relative frequency of a desired allele and then disappear.” Such locally acting allelic drives could be reapplied if needed to increase the abundance of a naturally occurring preferred trait, with the ultimate endpoint being that no GMO remains in the environment.
“An exciting possibility is to use allelic drivers to introduce new versions of the VGSC that are even more sensitive to insecticides than wild-type VGSCs,” Montell suggested. “This could potentially allow even smaller amounts of insecticides to be introduced into the environment to control pests and disease vectors.”
University of California – Santa Barbara
Kaduskar, B., et al. (2022) Reversal of allelic drive insecticide resistance in Drosophila melanogaster. nature communication. doi.org/10.1038/s41467-021-27654-1.