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 tetracycline-regulated gene promoter, tetO, controls the expression of the coding region for a protein, tTA, which is lethal in insects when produced in high amounts. Within the coding region of tTA, an intronic sequence from the transformer gene (part of the sex determination pathway in insects) which contains a stop codon in the correct reading frame has been inserted. Males are not able to splice out this intronic sequence and the intronic stop codon prevents the synthesis of tTA, allowing males to develop normally. Females have been shown to splice out this intronic sequence and can thus produce functional (and consequently lethal) tTA protein. When present, tetracycline tightly binds to tetO and prevents tetO promoter activity, thus preventing the synthesis of tTA and allowing females to develop. In the absence of tetracycline, tTA is produced in females, resulting in death of all female individuals. 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 might not be an acceptable option and the sterilization by irradiation might 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.