Genetic approaches to controlling pest insects

SPEAKER: The advent of modern molecular biology has unlocked a range of new tools for the development of new genetic methods to control insect populations and the diseases they transmit. Genetic tools have the potential to provide new species specific and environmentally friendly approaches for mosquito control. The sterile insect technique has proven a highly successful method to control insect pests, but cannot be applied to all insects. Mainly because the fitness effects from irradiation are too high, and sex sorting to remove females is too complicated and expensive. Initially, classic genetic selection was used to develop strains for sex sorting in order to reduce costs. A strain of anopheles albimanus was used in El Salvador in the 1970s.

In this strain, males were resistant to insecticide, but females were not. This allowed the selective removal of females by treating the eggs with an insecticide, which greatly improved the rearing capacity and drastically reduced the effort of sex sorting. This facilitated the mass rearing of sufficient numbers of sterile males that suppression of the local population could ultimately be achieved. In the med fly, a significant pest of fruits, temperature sensitive lethal strains were developed, where males are resistant to elevated temperatures. Thus, resulting in a killing of females and a removing of the need for sex sorting. These strains were developed in the 1990s and are still in use today.

They were also incompatible strains developed to remove the need for irradiation by using chromosomal inversions to induce sterility. However, these genetic traits had several limitations. They were generally hard to produce. They tended to be unstable, and they tended to have fitness effects that required careful quality control. There are a few different methods to introduce DNA into an organism, but the main one used for insects has been the micro injection of DNA into the developing embryo. This is a simple method in principle. The desired gene or genes are contained within a transposable element, such as Piggyback, Hermes, or the CRISPR-Cas9 system. They are injected into young embryos approximately one to three hours old.

The aim is to transpose the genes into the germline of the developing embryo that will eventually become the ovaries or the testes. Any survivors of the injection process across a wild type and then the progeny are screened for the fluorescent marker. Any expressing the fluorescent marker are crossed with wild type to secure a line for further molecular and phenotypic analysis. The development of fluorescent markers initially isolated from jellyfish and corals allow for easy identification of transgenic lines. They come in a variety of colours that can be used to track several separate gene insertions if required. The fluorescent marker can also be used to track the transgenic insects and the spread of their genes in the field.

There are currently two main approaches to the genetic control of insects, population modification, also known as population replacement, where a gene or genes are spread that renders the population refractory to disease, or population suppression to reduce the total population numbers, and therefore, the risk of disease transmission. Population suppression can involve the release of males that are either sterile or carry a genetic trait that kills the offspring. This type of approach usually requires the continued release of large numbers of males in excess of the number of wild males locally. Depending on the insect to be controlled, it can be challenging and sometimes impractical to release such large numbers of insects.

However, one type of genetic control approach being developed relies on gene drive systems to spread traits into a population rapidly, even starting from a very low initial release frequency. So what is a gene drive? Gene drives increase the likelihood that a modified gene will be inherited by its offspring. Normally, genes have a 50/50 chance of being inherited, but gene drive systems can significantly increase that chance. In some cases, greater than 99% of the offspring will inherit the gene drive. Over the course of several generations, a gene drive carrying a selected trait could spread rapidly within the species, even starting from a very low frequency at the point of release.

Another possibility with gene drive approach is that of population replacement or population modification, whereby the gene drive is used to spread a trait that makes insect resistant to the diseases they transmit. Gene drive systems use a range of different mechanisms to drive themselves into a population. This slide shows some of the systems that have been developed or proposed as mosquito gene drives. There is not enough time to go into the details of each system, but each is a way of achieving the same aim to spread desirable genes into a population to prevent the spread of disease.

Of the gene drive systems that have been built and tested to date, those that depend on the CRISPR-Cas9 system to cut and copy from one chromosome to another have had the most success. Gene drives are generally at the research stage, but the hope is that they will soon be ready for testing on the semi-field and then field conditions. However, there are still some significant technical, regulatory, and public acceptance hurdles to address. This section will cover population suppression tools. The two most advanced examples of population suppression using genetic approaches are from Oxitec and Target Malaria. Oxitec has developed two different population reduction systems for insect pests of agricultural and public health importance.

The first generation system kills both males and females and has been proving in numerous field trials over the past decade, achieving over 90% reduction in mosquito populations when compared to untreated control sites. The second generation system works in a similar way, but only kills females. It has several advantages. Males do not require sex sorting from females significantly reducing rearing costs. There is the potential for egg release systems to be developed that remove the requirement for large scale adult production facilities. This system can also potentially restore a target populations susceptibility to insecticides. Both systems are self limiting and do not persist in the environment.

Target malaria have developed a CRISPR based gene drive approach that targets a gene essential for female reproduction. In this case, the gene is called double sex. As the gene drive spreads through the population, it prevents normal development of females, such that they can no longer blood feed, transmit disease, or produce eggs. Laboratory and small cage experiments have shown proof of principle that this system can function as a population reduction tool, driving itself into a population and crushing that population. This slide shows the current status of different control methods. Note that wolbachia is not considered a genetically modified approach and is covered in detail in other parts of this course.

Generally, population reduction approaches have been more extensively tested and are closer to becoming tools for the control of insects. There are still some significant challenges to overcome before most of these tools will be widely accepted and deployed. Most of the genetic control tools have been proven in the laboratory and/or the field. Few people would argue that these tools are not technically feasible in concept. However, there remains challenges around proving performance in the field, cost of deployment, stability of gene drive systems, development of resistance, et cetera. But these are successfully being addressed in several systems, de-risking these challenges. Regulatory approval is required for the testing and use of genetically modified organisms.

This presents some significant challenges as some countries have banned the development and use of genetically modified organisms. Others do not have a regulatory system in place. And those that do are mainly developed around transgenic crops, not for public health pests. Genetic control tools that are designed for population replacement present a significant challenge as they can potentially cross borders and could require the approval of a large number of countries. However, there has been significant regulatory assessment and approval successes in recent years, especially with population reduction methods. These are generating knowledge and guidance for a regulatory assessment of GM tools to follow. One of the most challenging areas is the community acceptance of genetically modified organisms.

This is a complex and challenging area that we do not have time to go into. However, Oxitec, Target Malaria, and other groups are investing significant time and resources into educating the community about genetic control as this will be extremely important for acceptance and use of these tools. In summary, this brief introduction to genetic control techniques for pest insects has shown that these tools have a place in an integrated vector management strategy. There are challenges as with any new technology, but these are promising and potentially powerful genetic control tools, which are available now. Time will tell if they are accepted as mainstream insect pest control methods.