An early female lethal system of the New World screwworm, Cochliomyia hominivorax, for biotechnology-enhanced SIT

Authors: CONCHA, CAROLINA - Smithsonian Tropical Research; YAN, YING - Justus-Liebig University; Arp, Alex; QUILARQUE, EVELIN - North Carolina State University; SAGEL, AGUSTIN - Us Embassy, Panama; Perez De Leon, Adalberto - Beto; MCMILLAN, W. OWEN - Smithsonian Tropical Research; Skoda, Steven; SCOTT, MAXELL - North Carolina State University

Submitted to: BMC Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/24/2020
Publication Date: 12/18/2020
Citation: Concha, C., Yan, Y., Arp, A.P., Quilarque, E., Sagel, A., Perez De Leon, A.A., Mcmillan, W., Skoda, S.R., Scott, M.J. 2020. An early female lethal system of the New World screwworm, Cochliomyia hominivorax, for biotechnology-enhanced SIT. BMC Genetics. 21. https://doi.org/10.1186/s12863-020-00948-x.
DOI: https://doi.org/10.1186/s12863-020-00948-x

Interpretive Summary: New World screwworm (NWS) has been eradicated from North and Central America with mass releases of sterilized flies, a method called sterile insect technique (SIT). Current sterile releases are male and female, reducing the efficiency of the releases and increasing costs associated with mass rearing. Previous work has been completed creating a transgenic conditional male-only strain of NWS, though this strain was not larval lethal and did not confer any cost savings for mass rearing. In this study a new two-component conditional male-only transgenic strain was developed that is active in the embryo, thus eliminating the need to feed female larvae. In this system the primary component is activated by an embryo specific promoter (bottleneck) and produces tetracycline repressible transcription factor (tTA) which acts as an on switch for the second component and is inactivated in the presence of tetracycline. The second component contains a promoter activated by tTA driving gene (hid) that induces cell which has been modified to only function in females. This system was effective at selectively killing females in the embyro and early instars in the absence of tetracycline. Fly strains with this system had fitness equal to production strains. The early female lethal strains described here could be selected by the NWS Control Program for testing at large scale in the production facility to enhance the efficiency of the NWS eradication program.

Technical Abstract: Background: The New World Screwworm fly (NWS), Cochliomyia hominivorax, is an ectoparasite of warm-blooded animals and a major pest of livestock in parts of South America and the Caribbean where it remains endemic. This insect is considered a high-consequence foreign animal pest in North and Central America where it was eradicated using the Sterile Insect Technique (SIT). The Commission for the Eradication and Prevention of Screwworms (COPEG) is managed cooperatively between the governments of the United States and Panama to prevent the northward spread of NWS from infested countries in South America. This is accomplished by maintaining a permanent barrier through the release of millions of sterile male and female flies in the border between Panama and Colombia. Our research team demonstrated the utility of biotechnology-enhanced approaches for SIT by developing a male-only strain of the NWS. The strain carried a single component tetracycline repressible female lethal system where females died at late larval/pupal stages. The control program could be further improved by removing females during embryonic development, which would yield significant savings in the production process for mass rearing by reducing larval diet costs. In this study, we developed a male-only strain of NWS where females died at the embryo or first instar larvae stage. Results: Our early female lethal strains carry a two-component system consisting of the Lucilia sericata bottleneck gene promoter driving expression of the tTA gene (Lsbnk-tTA, driver) and a L. sericata tetO-hid gene cassette (Lshid, effector). Insertion of the sex-specifically spliced intron of the C. hominivorax transformer gene within the Lshid gene ensures that only females die when insects are reared in the absence of tetracycline. We also developed an “All-in-one” strain containing both the driver and effector cassettes in a single construct. In several double homozygous two-component strains and in one All-in-one strain raised without tetracycline, we found that female lethality occurs at the embryonic and/or first instar larval stages. Laboratory evaluation for phenotypes that are relevant for mass rearing in a production facility revealed that most strains had fitness characteristics similar to the wild type J06 strain that is currently reared for release in the permanent barrier. Testing of an “All in one” strain under mass rearing conditions showed that the strain maintained the fitness characteristics observed in the laboratory and produced at least equal numbers of males than J06, with no females, in the large 20 liter trays used for mass production. Conclusions: The early female lethal strains described here could be selected by the NWS Control Program for testing at large scale in the production facility to enhance the efficiency of the NWS eradication program.