Biological approaches to insect pest management offer alternatives to pesticidal control. In area-wide control programs that cover entire regions, the sterile insect technique (SIT) can be used to successfully suppress economically important pest species by the mass release of sterilized pest organisms. However, conventional sterilization by ionizing radiation reduces insect fitness, which can result in reduced competitiveness of the sterilized insects. Scientists recently report a transgene-based, dominant embryonic lethality system that allows for generation of large quantities of competitive but sterile insects without the need of irradiation. The system involves the ectopic expression of a hyperactive pro-apoptotic gene that causes embryo-specific lethality when driven by the tetracycline-controlled transactivator (tTA) under the regulation of a cellularization gene enhancer-promoter. The researchers have successfully tested this system in Drosophila melanogaster. The embryonic lethality can be suppressed maternally, which will allow it to be combined with transgenic female-specific lethality systems to raise only vigorous but sterile males.
Many insects heavily damage crops and forests or transmit deadly diseases to animals and humans. Current control efforts mostly rely on the use of insecticides, but chemical control can have adverse consequences and the costs of developing new chemical products to circumvent insecticide resistance are increasing. In genetic control based on SIT, mass-reared, sterile insects are released into the field, resulting in infertile matings and thereby reducing the pest population. In SIT programs the terms 'sterility' or 'sterile insect' do not usually indicate that the individuals generate no sperm or eggs, but rather refer to the transmission of dominant lethal mutations that kill the progeny. Because of its species specificity, SIT is considered an ecologically safe procedure and has been successfully used in area-wide approaches to suppress or eradicate in entire regions pest insects such as the pink bollworm Pectinophora gossypiella in California, the tsetse fly Glossina austeni in Zanzibar, the New World screwworm Cochliomyia hominivorax in North and Central America, and various tephritid fruit fly species in different parts of several continents.
For the Mediterranean fruit fly (medfly) Ceratitis capitata, male-only releases increase effectiveness of the SIT. Separation of undesirable females has been based on genetic sexing strains. However, recent advances in insect transgenesis have promoted the development of transgene-based methods for sex separation that are based on the female-specific expression of a conditional dominant lethal gene. Such systems have been examined in the model insect D. melanogaster and might be transferable to other insect pest species.
Knipling stated in 1955 that in addition to the requirement for mass rearing and sexing, SIT also necessitates that "sterilization methods must produce sterility without serious adverse effects on the mating behavior or length of life of the males" and that "if females of a species mate more frequently, the sperms from sterile males must be produced in essentially the same number and compete with sperms from fertile males". Conventional sterilization is based on ionizing radiation that causes chromosome fragmentation. Chromosome fragments without centromeres will not be transmitted correctly to the progeny. However, radiation also has adverse effects on viability and sperm quality, which results in reduced overall competitiveness of the sterilized individuals. Certain insects with holocentric chromosomes, such as many lepidopteran pest species, do not possess chromosomes with defined centromeres. Instead the whole chromosome has centromeric properties, which allow fragmented chromosomes to be inherited correctly. Therefore, sterilization of these insect pests requires very high doses of radiation, which often greatly impair fitness. Thus large quantities of sterilized insects are required to inundate the pest population, which results in high operational costs.
This team of scientists describe now a transgenic approach to cause sterility without interfering with the adult phase of the insect life cycle or with gametogenesis. The sterility is based on the transmission of a transgene combination that causes dominant embryo-specific lethality in the progeny. This allows for the generation of vigorous and potent sterile insects, with males being able to transfer competitive sperm. For the effector gene, which will cause organismal lethality, we chose the pro-apoptotic gene head involution defective (hid) (also known as Wrinkled (W)). This gene induces cell death when expressed ectopically. To avoid downregulation of HID by Ras signaling pathways, we used the phosphoacceptor-site mutant allele hidAla5. To limit the effect of the transgenes to the embryonic stage, we used enhancer-promoters of genes that are expressed at high levels but are specific to the blastoderm stage. In D. melanogaster, the genes serendipity (sry) andnullo encode structural components of the microfilament network that are specifically required for blastoderm cellularization. To establish conditional embryonic lethality, we used a suppressible binary expression system based on tTA.