Objective:
Designing a gene system which, if used to stably transform a higher dipteran, would result in a transgenic strain of flies in which embryonic female lethality could be chemically induced and fitness levels of sterilized males could be improved to enhance the success of the screwworm eradication program.

Approach:
The horn fly, Haematobia irritans, would be used as the model insect to develop the proper genetic resources and reagents to both demonstrate feasibility in a higher dipteran and also provide the capability to investigate horn fly control through genetic means. A tetracycline-controlled gene system forms the basis of the strategy. In general, a promoter from a highly expressed tightly regulated female-specific gene (such as a yolk protein gene) is used to drive the expression of the coding region for a tetracycline-controlled transactivator (tTA). In the absence of tetracycline, the tTA product is able to bind to and activate a gene promoter, tetO, which is constructed such as to drive expression of a lethal gene, of which there are several available. The presence of tetracycline prohibits tTA from binding to tetO, preventing the production of the lethality-inducing gene product. In practice, one would produce insects in the presence of tetracycline to build up numbers as needed for production requirements. At the point where female insects are no longer necessary, tetracycline is omitted from the growth media, thus inducing female lethality. To optimize savings from the insect rearing process, the female lethality should occur in the embryonic stage prior to larval feeding. It would be optimal to use gene promoters from the organism of interest. Construction of the transgenic strain such that a dominant female lethal gene is present in the homozygous state makes sterilization of males an option rather than a requirement. Following mating with a wild female and Mendelian genetic transmission, the transgenic males will pass the female lethal gene in one copy to each of their offspring. Males would hatch as expected; however, the absence of tetracycline in their native environment will kill all females, eventually resulting in population control by eradication of available females. In the case of the screwworm, the surviving males would not be an acceptable option and the sterilization by irradiation would still be required. However, females require a higher level of radiation to effect sterilization. As the transgenic strain would not contain any females at this stage in production, lower radiation levels could be utilized, resulting in smaller radiation-induced fitness losses in the released sterile male flies.