Is CRISPR/Cas9 a Viable Solution for the Control of Crop Insect Pests?

In my article “A Breakthrough In Biotechnology And The Future Of The Fruit Industry,” I described the precision genome editing technology, called CRISPR/Cas9 (clustered regularly interspersed short palindromic repeats/CRISPR associated-nuclease) and its applications in crop plant improvement. Here, I would like to cover how the CRISPR/Cas9 technology has been repurposed as a gene drive system that may become a potentially viable solution for managing crop insect pests.

The Gene Drive Process
What is CRISPR/Cas9-based gene drive and how can it be used for insect pest control?

Gene drive is a term that describes a process in which a particular gene or genetic element is spread quicker than what is expected according to genetic principles in a sexually reproducing population. CRISPR/Cas9-based gene drive is an engineered gene drive that can drive a rapid spreading of a CRISPR/Cas9-altered gene in a target population.

 

Figure 1. Spotted wing drosophila adult flies (A), genitalia (B) and dissected reproductive tissue (C) of wild type female, male (WT), and homozygous mutants (M) for the CRISPR/Cas9 interrupted sex lethal (Sxl) gene. Note that the mutants developed an intersexual phenotype with abnormal genitalia and abnormal ovaries. (Graphic: Adapted from Li and Scott 2016 Biochem Biophys Res Commun)

For insect pest control, genes involved in reproduction, development, feeding, and/or immune system are desirable targets. The vision is that fast spreading of the mutations created by CRISPR/Cas9 will skew sex ratios, reduce fitness, or restore susceptibility to insecticides in target populations.

 

In addition to agriculture insect pest control, controlling human and animal disease vectors such as mosquitos using CRISPR/Cas9 based gene drive has been envisioned as well. In fact, considerably more resources and efforts have been committed to this area to fight deadly diseases such as dengue, malaria, and Zika fever.

In practice, designing and constructing a specific CRISPR/Cas9 -based gene drive is a straightforward process. For delivering the gene drive into cells, embryo injection is commonly used. Through a series of selections in subsequent generations, insect carrying a desirable CRISPR/Cas9-based gene drive will be identified and raised. Like many other biological control agents, field trials in a contained environment will be conducted to evaluate both the intended and unintended effects. Before releasing these transgenic insects into the farm field or orchard for pest control, they also must be approved by the government, which will conduct an extensive regulatory review.

CRISPR/Cas9 Control of Insects
What studies have been done toward CRISPR/Cas9-based control of crop insect pests?

So far, several major insect pests have been investigated for the feasibility of pest control using CRISPR/Cas9-based gene drive, including drosophila, moth, beetle, grasshopper, and others. The progress has been encouraging. Below are two examples:

Spotted Wing Drosophila (SWD) (Drosophila suzukii). In a proof-of-concept study for the gene-drive-based control of SWD, a nasty pest for cherry and berry crops, the female specific sex lethal (Sxl) gene was targeted, which is a master regulator in female development. The study finds that if both of their Sxl alleles (copies of the gene) are interrupted by CRISPR/Cas9, the female flies will die in early developmental stages due to abnormal development of their reproductive system (Figure 1).

Figure 2. Male and female larvae of diamondback moth wild type and their abdominal-A gene mutants. The squares and arrows in red indicate the region of the testis in male and the disordered segments in mutants, respectively. (Graphic: Adapted from Huang et al. 2016 Insect Biochem Mol Biol)

 

Since most, if not all, females carrying the CRISPR/Cas9-based gene drive will become homozygous for the interrupted Sxl gene, the number of female flies will decline, which will eventually collapse the population.

Diamondback Moth (Plutella xylostella). The damage of diamondback moth to the cruciferous crops is a pressing problem

worldwide. To explore a genetics-based control of diamondback moth, a moth gene, called abdominal-A that plays a crucial role in determining the identity and functionality of abdominal segments, was targeted by the CRISPR/Cas9 system in another proof-of-concept study. The study demonstrates that disrupting abdominal-A led to a range of inheritable defects, such as abnormal prolegs and malformed segments in both male and female larvae (Figure 2).

Implementation Timeline
How close are CRISPR/Cas9-based gene drives to a practical solution for crop pest control?

As mentioned earlier, rapid progress has been accomplished to make CRISPR/Cas9-based gene drives a solution for crop pest control. From a technical point of view, implementation could occur soon. However, considering the time needed for government approval and for evaluation of unintended effects on related species to minimize the risk of biosafety, it may take years to see commercial application of CRISPR/Cas9-based gene drives for insect pest control.

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