GENETICALLY ENGINEERED, EMBRYO-SPECIFIC LETHALITY FOR INSECT PEST MANAGEMENT

Many insects compete with us humans for agricultural resources, feed on us, or act as disease vectors. Current control efforts rely mostly on the use of insecticides. The use of chemicals to either repel or kill insect pests is the oldest and most commonly used method of pest control. However, with the massive use of insecticides four major problems have arisen. First, many pests have developed resistance to one or several of the chemicals. Second, the non-specific action of these chemicals results in the destruction of beneficial animals, which has frequently led to ecological backlash phenomena with the rise of insecticide-resistant pests in large numbers. Third, the costs of developing new chemical products to overcome the problem of insecticide resistance are escalating. Fourth, the potential health hazard of many pesticides is a general threat to human welfare. Thus, novel and improved strategies are necessary to combat insect pests. Genetic control based on the sterile insect technique (SIT) uses the release of mass-reared and sterilized insects to cause infertile matings that reduce the pest population level. The concept of fighting insect pests by a repeated inundation with sterile individuals of its own kind was already promoted in the first half of last century. 1 The power of the technique lies in the simplicity of the biological principle on which it is founded and the lack of negative ecological effects following its application. Due to its species specificity, SIT is considered an environmentally friendly alternative to insecticides and has been successfully employed in area-wide approaches to suppress or eradicate pest insects like 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 several tephritid fruit fly species in various regions of the world. Additional programs indicate that these approaches would be valuable for a much more widespread use, but conventional sterilization by ionizing radiation decreases the competitiveness of sterilized insects. Thus, high quantities are required to inundate the pest population. This is a problem especially in lepidopteran pest species, where even highly irradiated males can still produce viable progeny. Moreover, SIT releases often require only males, but both sexes are needed for the rearing process. However, not only is it expensive to rear large numbers of potentially ‘useless’ females, but it is detrimental to release any females, sterile or not, in the case of species that sting fruit with ovipositors or transmit diseases by bitingdomestic animals and humans. In addition, for the Mediterranean fruit fly (medfly), Ceratitis capitata, male-only releases have been shown to increase effectiveness of the SIT. Recent advances in insect transgenesis 2 have encouraged the idea of transgenically manipulating pest insects in a way that will improve SIT approaches and widen its applicability. At least three different traits could be transgenetically introduced into insect strains to improve their use in the SIT: first, a marker gene could enable discrimination of released and naturally occurring insects; second, a female-specific lethality gene could allow for efficient genetic sexing; and third, a gene that causes lethality after transmission to the progeny could replace the irradiation procedure. Transgenic marking. Discrimination between released sterile and wild insects is critical for monitoring the effectiveness of an ongoing SIT program. Currently, released insects are labeled with a fluorescent dye powder, which is expensive, labor intensive, and error-prone. The transgenic introduction of a fluorescent transformation marker, which does not compromise survival or fitness, would enable the identification of released insects in a simple way. Sexing strains. In the medfly, separation of undesirable females has so far been based on genetic sexing strains that cannot be transferred to other species. Recently, transgene-based methods for sex-separation that are based on the female-specific expression of a conditional dominant lethal gene have been examined in the model insect