Researchers at Indiana University have also been exploring the use of synthetic gene drives based on CRISPR-Cas9 for the potential to control, alter, or suppress populations of crop pests and disease vectors, like the Aedes mosquito, a vector for malaria and the Zika virus.
Their research, published in the online journal Science Advances on May 19, shows how certain genetic and behavioral qualities in disease-carrying insect species make them resistant to genetic manipulation.
The researchers used genetic data from four populations of the flour beetle Tribolium castaneum in the study and along with statistical analysis, found that with small genetic variations within the species, along with many insects’ tendency to inbreed, the use of CRISPR technology in reducing their number is not going to be effective.
Michael J. Wade, Distinguished Professor of Biology at IU Bloomington said, “Although rare, these naturally occurring genetic variants resistant to CRISPR are enough to halt attempts at population control using genetic technology, quickly returning wild populations to their earlier, ‘pre-CRISPR’ numbers.”
What this means for controlling mosquito populations is disheartening. Introducing genes, such as a trait that causes female mosquitoes to lay fewer eggs would disappear in just a few months. This is because the male mosquito used to transmit new genes, since they don’t bite, only live about 10 days.
It seems that the “protective effect” of naturally occurring genetic modification is enough to overcome the use of “gene drives” unless a gene drive is specifically matched to the genetic background of a specific targeted population, and that would require a thorough study of the genetic variation in the target population, which can be time-consuming and costly.
Professor Wade explains that a gene drive refers to the rate of spread of a particular gene, at rates of up to 90 percent, compared to the normal 50 percent chance of inheritance that occurs in typical sexual reproduction.
Previous attempts at modifying genes in mosquitoes
In February 2016, Digital Journal reported on attempts to control the spread of the Zika virus through the use of “gene drive” to genetically modify mosquitoes. It was believed the modified mosquitoes would be incapable of carrying the Zika virus, with the modified mosquitoes out-breeding the Zika carrying insects.
And in another attempt to use genetically modified mosquitoes, A December 2016 report by Digital Journal detailed how the U.S. Food and Drug Administration (FDA) had halted the release of male mosquitoes that had been modified using gene drive in the area of Key Haven in Monroe County, Florida.
The new study from Indiana University will have great implications in the further investigations into gene drive using CRISPR technology, even though the authors don’t believe the unintended spread of modified genes across the globe will take place because of insects’ natural genetic roadblocks.
“Based on this study, anyone trying to reduce insect populations through this method should conduct a thorough genetic analysis of the target gene region to assess variation rates,” Wade said. “This will help predict the effectiveness of the method, as well as provide insight into ways to circumvent natural genetic variation through the use of Cas9 variants with an altered sequence specificity.”
