Oxitec’s self-limiting approach to controlling the Aedes aegypti mosquito uses modern biotechnology and advanced genetics to provide effective, safe, and sustainable control. Oxitec insects are engineered to contain a self-limiting gene that causes their offspring to die, but the Oxitec insects can live and reproduce normally when they are fed a diet containing an antidote. They also contain a heritable, fluorescent marker to distinguish them from native pest insects and to help scientists with the management of pest control programmes. The following sections describe how these genes work.
How the Self-Limiting Gene Works
Oxitec genetic control works by inserting a gene into the target organism, which prevents the insect from surviving to adulthood. The self-limiting gene, tTAV (tetracycline repressible transactivator variant), is a gene variant that has been optimised to only work in insect cells. In the wild, offspring that contain the self-limiting gene make a non-toxic protein that ties up the cell’s machinery so its other genes aren’t expressed and the insect dies. The proteins produced are non-toxic in the insects, so if any animals eat them it would be the same as eating a wild insect – they will be digested in just the same way that all other insects are digested.
But how do we produce insects if they die? There’s an antidote given to the insects in the rearing facility that acts like a switch to turn off the tTAV gene preventing the tTAV protein from working. This antidote, tetracycline, an antibiotic, binds to the tTAV protein and disables it. So in the presence of the antidote, the Oxitec insects are able to survive and reproduce in the rearing facility, but when the males are released into the wild, their offspring can’t access the antibiotic in the quantities needed to survive, so they die before reaching adulthood.
The self-limiting gene comprises tetO (tetracycline Operator) sites, which bind tTAV (tetracycline repressible Trans-Activating factor Variant) protein, a promoter and coding sequence for tTAV. Without the tetracycline antidote, tTAV protein is produced, which simultaneously binds to transcriptional machinery and tetO sites, thereby enhancing the expression of the self-limiting gene. This positive feedback system produces large amounts of tTAV, which binds to more and more transcriptional machinery, without the need to bind to the tetO, eventually making the transcriptional machinery unavailable for other essential gene expression. The inhibition of essential gene expression leads to cell death and the death of the insect before it reaches adulthood. When tetracycline is added to the larval aquatic diet, it binds and inactivates tTAV, switching off the positive feedback system. The transcriptional machinery is not depleted as only a small amount of tTAV is produced, which does not affect normal cell function, and the insects survive to reproduce naturally in the production facility.
Markers and Monitoring
All of Oxitec’s strains contain a heritable, fluorescent marker to distinguish them from native pest insects and to help scientists with the management of pest control programmes.
Effective monitoring is the key to all insect pest management programmes. Understanding the dynamics of pest populations makes it easier to make the right treatment at the right time – maximising effectiveness. It is important to be able to accurately determine whether insects caught in traps are the offspring of released self-limiting ones or fertile wild ones. Wrongly identifying a wild insect as self-limiting means an infestation may be missed, while misidentification of a self-limiting insect as wild means that resources will be wasted dealing with a non-existent infestation.
In other insect control programmes such as the sterile insect technique, insects are marked with fluorescent dusts or food dye which are not always easily identified on insects recovered from field traps and may fade with age, and in the case of any incomplete sterility do not pass on to the next generation. Oxitec’s strains contain genes that make fluorescent proteins that glow when viewed under certain filters.