Gene-drive suppression of mosquito populations in large cages as a bridge between lab and field

Scientists from Imperial College London tested gene drive (GD) mosquitoes in large experimental cages and published their findings in the scientific journal, Nature Communications. The GD mosquitoes were exposed to conditions reflecting those expected in natural conditions. The mosquito populations collapsed within a year.

Basics from a previous study

The study tested a gene drive previously described in Anopheles gambiae mosquitoes in 2018 (Kyrou, K. et al., 2018). It is a CRISPR/Cas9-based gene drive targeting the gene doublesex (dsx). This gene plays an important role in embryonic development: it is critical in determining whether mosquitoes develop into females or males. The gene drive modification resulted in the development of non-fertile pseudo-females and normal males. Populations kept in small cages in the laboratory collapsed after about ten generations. The sequence in the dsx gene targeted by the gene drive is essential for the development of females and is ultra-conserved, meaning that very few natural mutations occur in this area of the gene. This is an important feature for the efficiency of the gene drive, i.e. if mutations were to be induced through NHEJ repair activity (after CRISPR/Cas9 cuts the target sequence), resistance to the gene drive could develop and stop the spread of the gene drive. However, no resistance was observed within the target sequence of the gene in the small cage experiments.

Structure of the gene drive construct

The gene drive construct consists of the CRISPR/Cas9 gene scissors, a guide RNA designed to recognize the target sequence in the dsx gene and a red fluorescent marker. After the gene scissors recognize and cut the target sequence in the mosquito’s genome, the GD construct is inserted and disrupts gene function. The red fluorescent marker is used by the scientists to monitor the transfer of the GD construct.

Results of the study

Scientists tested the GD construct in large cages built to mimic ecological conditions (e.g. sunrise and sunset). They were able to show that the mosquitoes in these cages typically exhibited natural behaviour, such as swarming, that they did not exhibit in small cages.

At the beginning of the experiments, low and medium levels of GD mosquitoes were mixed with wild-type mosquito populations in large cages, respectively. The researchers then followed the spread of the gene drive in the wild-type population and observed the fitness of the populations over many generations. They found that the populations in the cages with both low and medium levels of the GD mosquitoes completely collapsed after 245-311 days.

In addition, differences were found between the parameters calculated by computer models and those actually observed: in the large cages, for example, interbreeding between younger and older mosquitoes occurred less frequently than predicted. The scientists can now use the results to improve their computer calculations.

The researchers also tested possible resistance formation at the target sequence at two different times. This involved taking 400 larvae from each of the large cages and analyzing their DNA. It was found that no resistance had developed and that the gene drive could, therefore, spread rapidly in the population.

What is the next step?

The results show that the tested GD construct is a very efficient and global gene drive. This implies that, if released, the GD mosquitoes could spread uncontrollably in nature. From the researchers’ perspective, their experiments represent a bridge between laboratory and field testing. Many scientists and civil society organizations oppose the release of such gene-drive organisms. Although the results of this study show that a global gene drive is technically feasible, this does not mean that feasibility equates to safety. A comprehensive environmental risk assessment of GD mosquitoes to, for example, investigate outcrossing with related wild species, has not yet been conducted.