Our long-term goal is to produce modified mosquitoes that are fertile and that can pass on to their progeny a change in their genes that is self-sustaining. We are currently investigating two options: 1) fertile males carrying a gene that will spread through the mosquito population and as it does so causes a male bias sex ratio; 2) fertile males carrying a gene that will spread through the mosquito population and as it does so causes females that inherit the gene from both parents to be sterile. Both approaches would lead to a reduction in the malaria mosquito population. While these approaches are different, they may not be mutually exclusive.
The initial steps of our development pathway are the same as for the sterile males and the self-limiting male bias. But if our tests and evaluations advance sufficiently, and we feel confident that we have a viable technology that could be used to fight malaria, then our development pathway will be different from the previous phases.
Because we need to assume that the genetic modification could persist and spread in the environment in Africa, we need to think differently about the stages of research that takes place in the region where these mosquitoes are native.
As we are pioneering the developmental pathway for self-sustaining modified mosquitoes there are many consultation and evaluation steps that we need to take and these may also change over time. These currently will include evaluation by the World Health Organisation (WHO) Vector Control Advisory Group and other groups at the WHO (e.g the vector control pre-qualification team) before and after release, as well as comprehensive independent ecological and socio-economic risk assessments, community and stakeholder support.
What Are Our Approaches
Biasing the Sex Ratio
This strategy relies on altering the sex ratio in malaria mosquito populations to decrease the number of female malaria mosquitoes relative to males. Only female Anopheles gambiae transmit the disease, and a reduction in the number of females limits reproduction and the future population size, therefore reducing the number of vectors for malaria
The approach is based around the sex-determining chromosomes (XY for males, and XX for females) and relies on the fact that female offspring require two functional X chromosomes– one from each parent – in order for female offspring to be produced.
Target Malaria researchers have used nuclease enzymes (image 1) to identify and cut through several key sites on the X chromosome (image 2) in the sperm of male Anopheles gambiae which leads to a fragmentation of this chromosome (image 3). When these males reproduce, they can still pass on a functional Y chromosome to their offspring, but they cannot pass on a functional X chromosome due to its fragmentation (image 4). This results in a bias toward male (XY) offspring.
Work published by our team in June 2014 has shown that we can successfully distort the sex ratio of a laboratory population, as over 95% of the offspring produced by modified Anopheles gambiae were male, with only 5% being female. By comparison, under normal circumstances, researchers would expect a 50:50 split between male and females, meaning that our modification reduces the number of females produced by 10-fold.
Mathematical models indicate that the approach could be highly effective in reducing mosquito numbers. Because the modification is intended to be carried on the Y chromosome, it would also be self-sustaining. The male offspring of modified males would remain fertile and every single one of them would have the sex-distorting nuclease gene on their Y chromosome, allowing them to continue the trend of producing mostly male offspring.
Biasing the Sex Ratio
Focusing on Mosquito Female Fertility
This strategy focuses on using nucleases to knock out genes that are key to fertility in female Anopheles gambiae mosquitoes. The approach could significantly reduce the prevalence of malaria because the number and productivity of females in a population determines future population size, and female Anopheles gambiae are highly effective vectors for the disease.
In order to knock out female fertility genes, the nucleases are designed to identify the specified genes (image 1) and cut through them (image 2). When this stretch of DNA is repaired, the nuclease gene is copied and inserted into the cut site (image 3), interrupting the original gene and preventing it from working properly (image 4).
A female that has one copy of this fertility gene disrupted will be able to reproduce normally, but when both copies within her chromosomes are disrupted, the female cannot produce viable offspring.
We have designed these nucleases so that they are only active in the cells of the mosquito that make the sperm and the eggs. Due to the preferential copying mechanism of these nuclease genes in the sperm and eggs, an individual initially containing only one copy of the gene will transmit it to many more offspring than normal.
As fertility genes are fully disrupted in females that inherit two copies of the nuclease gene, this should lead to an overall reduction in the population.
Reducing Female Fertility