Blow flies (family Calliphoridae) can be serious and dangerous pests. The new world screwworm fly (Cochliomyia hominivorax) as the latin name says can be a man-eater (hominivorax). While many blow flies prefer to lay eggs on dead and decaying animals, the screwworm fly lays eggs in the open wounds of live animals, including humans. The eggs hatch and the larvae feed on the living flesh of the host. They screw their pointed bodies head first into the flesh where posterior pointing bristles make it difficult to extract them. They are a serious threat to cattle but much less so to humans with adequate hygiene and medical care. The sheep blow fly, Lucilia cuprina, has similar habits and can be serious pests of sheep in Australia and New Zealand. If infestations are high enough, animals can be eaten alive.
Controlling blow flies using genetic technologies – specifically the Sterile Insect Technique (SIT) – has been hugely successful in controlling and locally eradicating the new world screwworm. The sheep blowfly is currently not controlled using SIT.
SIT involves rearing large numbers of the pest species followed by their sterilization and release into the pest population. Wild females mating with sterile males will lay no eggs, resulting in a decline of the population. Release sterile insects often enough and you can cause local populations of the pest insect to go extinct.
SIT works particularly well when the pest population is at relatively low density at which time insecticide applications tend to be less effective. Released sterile males are effectively smart bombs seeking out wild females.
Releasing sterile females is a waste of time and resources and has no positive impact on the SIT and potentially can have devastating consequences. Eliminating females early in the mass rearing process can lead to significant increases in the SIT’s efficacy and efficiency.
Li et al. (2014) describe their successful effort to use insect genetic technologies to genetically modify the sheep blowfly so that all females die under well-defined laboratory conditions, resulting in only males. You rear and release fewer insects as well as accruing other benefits.
The general strategy employed by Li et al (2014) has been well known in the field in insect molecular biology – construct a conditionally expressed ‘lethal gene’ such that under permissive conditions both sexes live and under restrictive conditions only females die. However, assembling functioning systems in target species can be challenging since many of the necessary components may need to be species specific. For example, sex-specificity is achieved using a well-known sex-specific splicing event involving an well-known intron. In females splicing of this intron occurs and in males it does not. Li et al isolated this intron from the tra gene of the new world screwworm fly, Cochliomyia hominivorax. They achieved conditional expression using an off-the-shelf tet-off system. The tet-off system was attached to a heat shock promoter the authors isolated from the sheep blow fly, Lucilia cuprina. So in the presence of tetracycline the heat shock promoter is ‘off”. Remove tetracycline and the heat shock promoter is ‘on’ and will cause the expression of a ‘lethal’ gene. The lethal gene is the tetracycline transcriptional activator (tTA), which at high concentrations in cells, causes lethality for unknown reasons.
The assembled transgenes were introduced into the germ line of Lucilia cuprina using a piggyBac-transposon-based gene vector.
The authors created almost a dozen lines and some had 100% penetrance of female lethality under restrictive conditions (all the female died when they were suppose to). That is pretty encouraging.
The system Li et al (2014) have assembled, while functional in L. cuprina, may also be functional in other blowflies, such as C. hominivorax for which there is a long-standing and extensive SIT program operated by the U.S., Mexico and Central America.
One technically interesting observation made by Li et al (2014) was that the heat shock promoter (hsp70) they isolated from L. cuprina was so active in driving the expression of DsRed that the authors did not need to use a fluorescent light to see the pigment. Indeed those larvae were a beautiful cherry-red under typical white-light.
This work is a nice example of using a well-established genetic design and customizing it by using species-specific components so that a highly effective system for manipulating mass-reared insects is created.
Transgenic sexing system for genetic control of the Australian sheep blow fly Lucilia cuprina
Fang Li E, Holly A. Wantuch, Rebecca J. Linger, Esther J. Belikoff, Maxwell J. Scott
Insect Biochemistry and Molecular Biology Volume 51, August 2014, Pages 80–88