Examples Of Insect Genetic Engineering For Insect Population Control

Transgenic technology in nondrosophilid insects has already been applied to examine promoter function and gene expression in transgenic lines of Ae. aegypti and C. capitata. In addition, recent work performed in D. melanogaster illustrates how transgenic approaches may be applied to pest insect control in the foreseeable future. This approach involves using transgenic technology to develop new genetic sexing strains. Although these experiments have been performed in D. melanogaster, the concepts are applicable to any insect species in which transgenic technology has been developed, and the ability to generate and test novel genetic strains in pest insect species should result from such additional experiments.

Both systems were centered on exploiting the tetracycline-controlled transactivator (rTA) gene, which is inactivated in the presence of tetracycline. As a dietary component, tetracycline can readily be administered to Drosophila larvae in measured doses. Both systems consist of two components, which are combined in a single strain when transgenic lines containing each component are crossed. The rTA gene was placed under the control of the enhancer from the yolk protein 1 (yp1) gene of D. melanogaster. This enhancer results in fat-body-and female-specific expression of the yp1 gene. The second component of their system was a proapoptosis gene (head involution defective—hid), the expression of which leads to apoptosis and the death of the organism. The hid gene was placed under the control of the tetracycline operator (tetO), which contains the binding site for the rTA protein. Thus, in females the yp1-rTA gene is induced and, in the absence of tetracycline in the diet, the rTA protein binds to the tetO sequence and so induces the expression of the hid gene. All transgenic females that were raised in the absence of tetracycline and possessed both components of this lethal genetic system died. When tetracycline was added to the diet, the rTA protein was inactivated, and there was no female lethality. Males containing both components were unaffected on either diet.

These experiments clearly demonstrate that transgenic technology can be used to construct efficient genetic sexing strains in at least one species of insect—D. melanogaster. The genes, promoters, and enhancers chosen to do so are predicted to be of generic use in insects. The tetracycline-controlled transactivator system is from bacteria and, given that it functions correctly in Drosophila, will most likely be applicable to all insects in which tetracycline, or its analogues, can be delivered in measured doses. Female-specific enhancers would be expected to exist in nondrosophilids, should the D. melanogaster enhancers not function correctly in these species. Similarly, should conditional lethal alleles of Drosophila genes not function in other species, it should be possible to generate analogous mutants either by established procedures or by employing an RNAi-based approach. The extension of these strategies into nondrosophilid insects requires, in the end, genetic transformation procedures and, as already discussed, several of these now exist for nondrosophilid insect species.

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