Environmentally friendly CRISPR technology could check crop pests

Ensuring that we and future generations have enough to eat in the face of climate change and the resulting migrations of species involves the necessary evil of killing the insects that destroy our crops. Conventional and environmentally harmful chemical pesticides are species-independent and often kill harmless insects and gradually decrease in effectiveness as pests develop resistance. Technologies that selectively control or kill pest species offer a safer option.

One such invasive insect pest is Drosophila suzukii, which poses a threat to agricultural yields, especially the production of fruits such as berries, cherries, plums and grapes, in Western countries. So far the impact of control measures to limit the spread of D. suzukii has been less than optimal.

An article published in GEN Biotechnology (“Precisely Guided Sterile Males Suppress Populations of an Invasive Crop Pest”) reports the development of a programmable CRISPR-based method that, if deployed at scale in the wild, could replace the fertile male. D. suzukii with sterile counterparts, effectively, specifically and safely curbing this pest population. Although developed in D. suzukii, the authors claim that the method can be modified to target other pest species, avoiding the need for environmentally unfriendly pesticides.

Omar Akbari, PhD, professor in the department of cell and developmental biology at the University of California, San Diego’s School of Biological Sciences, is the study’s corresponding author.

The study, conducted by scientists at the University of California, San Diego (UCSD), including co-lead authors Nikolay Kandul, PhD, Junru Liu, PhD, and Anna Buchman, PhD, and corresponding author Omar Akbari, PhD, uses a precision-guided and temperature-inducible Sterile Insect Technique (pgSIT), reported by the team in a previous study, to generate sterile, but fit and competitive. D. suzukii males with a slightly reduced lifespan. Through empirical experimentation and mathematical modeling, the researchers demonstrated that the repeated release of neutered males can be quickly and successfully stopped or eliminated. D. suzukii populations (Akbari’s team has used pgSIT in a previous study to sterilize mosquitoes.)

“It’s an evolutionarily secure and stable system,” said Akbari, a professor in the department of cell and developmental biology in UCSD’s College of Biological Sciences. “The system does not lead to uncontrolled spread or persist in the environment, both important safety features that will help it gain approvals for use.”

“This builds on previously published work,” said Rodolphe Barrangou, PhD, editor of The CRISPR magazine. “The study illustrates how next-generation CRISPR-based gene drives can be used to control crop pests. The data convincingly show that this approach has technical advantages and enables population suppression by deploying competitive sterile males. Importantly, this proof-of-concept study provides a basis for thinking about commercial applications.”

The pgSIT system edits essential genes that determine sex and fertility. The authors used two transgenics D. suzukii strains, one expressing Cas9 in germ and somatic cells and the other expressing guide RNAs targeting genes essential for female survival and male fertility, such as sex lethal (Sxl), double sex (dsx), and the transformer (tra). The technique produced up to 100% sterile males with females either killed or converted into intersex flies.

The authors demonstrated that the pgSIT system was effective when sterile pgSIT males were released in ratios comparable to approaches using conditional female lethal strains and lower than release ratios recommended for SIT paradigms (sterile insect technique ) conventional. In addition, conventional SIT programs use DNA-damaging agents such as ionizing radiation to generate sterilized males. This limits the fitness and mating competitiveness of released males.

Survival curve
Survival curves show reduced longevity of pgSIT D. suzuki males compared to wild-type males. [adapted from Kandul NP, et al, GEN Biotechnology, 2022]

The authors showed the combination of pgSIT D. suzukii with wild-type males significantly decreased egg hatch rate, indicating that sterile males are competitive and successfully compete with wild-type males to mate with females. The authors also showed that increasing the release ratios of pgSIT males to wild-type males increased the rate of population decline.

“The pgSIT technique is an attractive way to monitor D. suzukii populations,” said Kutubuddin Molla, PhD, a scientist at the National Rice Research Institute in Cuttack, India, and editor of the The Plant Cell magazine “However, before environmental release, we must be careful to avoid any anticipated risks and unintended ecological consequences. If we could develop a system to recover genetic unity, we would have more confidence in the environmental release of insects gene drivers”.

Barrangou agreed. “We need to be aware and cautious with practical deployment and ensure that appropriate studies are carried out to assess the safety of the release and the ecological impact and consequences,” he said. “Depending on the scale and pattern of release, the result is presumably population reduction and control rather than elimination per se.”

Akbari said his team has developed pgSIT for several species in the past four years and hopes to use it as a platform technology to safely control the spread of other pests in the real world.

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