Enhancing the stability and ecological safety of mass-reared transgenic strains for field release by redundant conditional lethality systems

Authors: Handler, Alfred - Al

Submitted to: Insect Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/5/2015
Publication Date: 6/18/2015
Citation: Handler, A.M. 2015. Enhancing the stability and ecological safety of mass-reared transgenic strains for field release by redundant conditional lethality systems. Insect Science. doi: 10.1111/1744-7919.12245.

Interpretive Summary: The creation of transgenic strains of economically important insects for the development of more effective biological control programs, the sterile insect technique in particular, is a major goal of the USDA, Agriculture Research Service, Center for Medical, Beterinary and Agricultural Entomology laboratory in Gainesville, Florida. Development of this methodology and strategies to effectively and safely utilize transgenic insects for biological control will depend upon new conditional lethal vectors for genetic sexing and male sterility, and strategies that ensure that lethal revertants do not survive in the field. Tetracycline-suppressible (Tet-off) lethality specific to females has resulted in strains that can be used for genetic-sexing, and Tet-off spermatocyte-specific lethality may result in the surviving males being sterile. Non-specific Tet-off lethality has also been achieved whereby all the progeny of males that carry this system are expected to die in the field in the absence of tetracycline. While these systems can be highly effective, all genetic-based transgenic systems for control are subject to mutations that can disrupt or modify the system, potentially, resulting in transgenic individuals that can survive in the field. In some cases, such reversion may result in these survivors and their progeny becoming resistant to the mechanism of control. Here we discuss an evaluation of the potential risk of lethal revertants occurring during mass rearing, and testing this risk in the model-insect, Drosophila melanogaster, under large-scale laboratory rearing. If this risk is significant, we then propose the use off a secondary conditional lethal system, based on a temperature-sensitive lethal putation, that acts by an independent mechanism later in development. By preventing the survival of transgenic revertants in the field, we expect the use of transgenic strains to improve biologically-based population control to be more effective and environmentally safe.

Technical Abstract: Advances in the genetic manipulation of agriculturally important insects now allows the development of genetic sexing and male sterility systems for more highly efficient biologically-based population control programs, most notably SIT, in fruit pests throughout the world. Potentially, these conditional lethal systems will be functional together in the same transgenic strains so that they will be viable and fertile on a tetracycline-containing diet, but female-lethal and male sterile in tet-free conditions. This would allow their most efficacious use as a unified system for sterile male-only production on Tet-free diet that would provide a model for other pest species controlled by SIT. A critical consideration for the field release of transgenic insect strains is a determination of the frequency and evaluation of the genetic and biological basis of lethality revertant survival. This will provide critical fundamental knowledge necessary to evaluate the genetic stability of the lethality system, its environmental safety, and provide the basis for modifications ensuring optimal efficacy. For Tet-off lethal survival determinations, development of large-scale screening protocols will also allow the testing of these modifications, and in the short-term, test the ability of existing conditional lethal systems to fully suppress propagation of rare Tet-off survivors. If the dominant temperature-sensitive lethal mutation (DTS7) proves efficient for secondary lethality in D. melanogaster, it may provide the safeguard (and quantitative data) needed to support the release of sexing/sterility strains, and potentially, the release of uni-sex lethality strains as a form of genetic sterility (since progeny of released males would fail to propagate). Should the DTS7 Prosß21 cognate prove effective for redundant lethality, its high level of structural and functional conservation should allow host-specific cognates to be created and utilized in a wide range of insect species.