Each year millions around the world are infected by dengue, chikungunya, and Zika viruses. The principal culprit behind the transmission of these deadly diseases is the mosquito vector, Aedes aegypti. Conventional methods of pest control have so far fallen short.

To curb the spread of A. aegypti, researchers at the University of California, San Diego (UCSD), have now developed a CRISPR-based molecular genetic control system called precision-guided sterile insect technique (pgSIT) that alters insect genes to generate flightless female and sterile male mosquitoes. The pgSIT system can be deployed effectively at any stage in the life cycle of the mosquito.

The authors used mathematical models to empirically demonstrate that once released, male A. aegypti mosquitoes sterilized using the pgSIT system can compete, suppress, and eliminate fertile mosquito populations in the wild. The pgSIT system is not limited to restricting mosquito populations, it can be adapted to different vectors to curb transmissible diseases in a safe, confinable, and reversible manner, the authors claim.

These findings are reported in the Nature Communications article, “Suppressing mosquito populations with precision guided sterile males.”

Zach Adelman, PhD, professor at the department of entomology at Texas A&M University, who is not involved in the study said, “Definitely, an important contribution and very well-designed study. The pgSIT system they describe represents a potentially valuable public health intervention product, and I look forward to following its progress as the authors transition from the lab to the field.”

Omar Akbari looking at mosquitoes
Omar Akbari, PhD, an assistant professor of cell and developmental biology at UCSD, heads a laboratory that is studying the basic genetics and physiology of mosquitoes with the overarching goal of developing synthetic biology–inspired genetic control technologies for reducing the burden of mosquito-borne diseases on humans. [UCSD Publications / Erik Jepsen]
The corresponding author of the paper, Omar Akbari, PhD, professor at the department of biological sciences at UCSD, said, “pgSIT is a new scalable genetic control system that uses a CRISPR-based approach to engineer deployable mosquitoes that can suppress populations. Males don’t transmit diseases. So, the idea is that as you release more and more sterile males, you can suppress the population without relying on harmful chemicals and insecticides.” Akbari is co-founder of Agragene, a biocontrol company that uses the pgSIT system to suppress invasive crop pests.

Gene drive systems have been used in the past decade that pass on genetic mutations indefinitely from one generation to the next to curb transmission of mosquito-borne diseases. The pgSIT system differs from these earlier gene drive systems: it is self-limiting and is not predicted to persist or spread in the environment. These safety features bode well for the widespread acceptance of the new technology, following in the footsteps of Florida’s momentous decision last year to approve the release of genetically modified mosquitoes in an effort to control viral infections spread through A. aegypti.

The system improves upon established radiation and chemical-based pest control methods through increased precision and scalability. Akbari said, “pgSIT eggs can be shipped to a location threatened by mosquito-borne disease or developed at an on-site facility that could produce the eggs for nearby deployment. Once the pgSIT eggs are released in the wild, typically at a peak rate of 100–200 pgSIT eggs per A. aegypti adult, sterile pgSIT males will emerge and eventually mate with females, driving down the wild population as needed.”

To generate sterile male mosquitoes Akbari’s team target the β-tubulin gene that is specifically expressed in mosquito testes and is essential for spermatogenesis and male fertility. On the other hand, the team targets the myosin heavy chain (myo-fem) gene that is almost exclusively expressed in female pupae and is essential for female flight, to render female mosquitoes flightless.

UCSD postdoctoral scholar Ming Li, PhD, is first author of the paper describing pgSIT. [SOURCE: Akbari Lab, UC San Diego]
The authors showed in first-generation crosses that produce mosquitoes with one β-tubulin mutant allele, two lines out of ten have sterile males due to immotile sperms whereas female mosquito fertility is not affected. On the other hand, in myo-fem crosses all female mosquitoes in the first generation are flightless but males retain normal flight.

The authors showed the inability to fly significantly reduces female mating ability, blood consumption, and survival as the flightless females get trapped on the water surface following the emergence of the adult from the larvae (eclosion). They performed Sanger sequencing on genomic DNA to confirm the presence of the mutations at the targeted loci in subsequent generations.

The authors noted, “Going forward, pgSIT may provide an efficient, safe, scalable, and environmentally friendly alternative next-generation technology for wild population control of mosquitoes resulting in wide-scale prevention of human disease transmission.”

Funding for this work was provided by DARPA, the National Institutes of Health, the U.S. Army Research Office, and the Innovative Genomics Institute.