Perspective on the combined use of an independent transgenic sexing and a multifactorial reproductive sterility system to avoid resistance development against transgenic Sterile Insect Technique approaches

The Q-system: A Versatile Repressible Binary Expression System

Binary expression systems are useful genetic tools for experimentally labeling or manipulating the function of defined cells. The Q-system is a repressible binary expression system that consists of a transcription factor QF (and the recently improved QF2/QF2^w), the inhibitor QS, a QUAS-geneX effector, and a drug that inhibits QS (quinic acid). The Q-system can be used alone or in combination with other binary expression systems, such as GAL4/UAS and LexA/LexAop. In this review chapter, we discuss the past, present, and future of the Q-system for applications in Drosophila and other organisms. We discuss the in vivo application of the Q-system for transgenic labeling, the modular nature of QF that allows chimeric or split transcriptional activators to be developed, its temporal control by quinic acid, new methods to generate QF2 reagents, intersectional expression labeling, and its recent adoption into many emerging experimental species.

Genetic sexing strains for the population suppression of the mosquito vector Aedes aegypti

Aedes aegypti is the primary vector of arthropod-borne viruses including dengue, chikungunya and Zika. Vector population control methods are reviving to impede disease transmission. An efficient sex separation for male-only releases is crucial for area-wide mosquito population suppression strategies. Here, we report on the construction of two genetic sexing strains using red- and white-eye colour mutations as selectable markers. Quality control analysis showed that the Red-eye genetic sexing strains (GSS) is better and more genetically stable than the White-eye GSS. The introduction of an irradiation-induced inversion (Inv35) increases genetic stability and reduces the probability of female contamination of the male release batches. Bi-weekly releases of irradiated males of both the Red-eye GSS and the Red-eye GSS/Inv35 fully suppressed target laboratory cage populations within six and nine weeks, respectively. An image analysis algorithm allowing sex determination based on eye colour identification at the pupal stage was developed. The next step is to automate the Red-eye-based genetic sexing and validate it in pilot trials prior to its integration in large-scale population suppression programmes.

This article is part of the theme issue ‘Novel control strategies for mosquito-borne diseases’.

Improvement on the genetic engineering of an invasive agricultural pest insect, the cherry vinegar fly, Drosophila suzukii

Background

The invasive fly Drosophila suzukii has become an established fruit pest in Europe, the USA, and South America with no effective and safe pest management. Genetic engineering enables the development of transgene-based novel genetic control strategies against insect pests and disease vectors. This, however, requires the establishment of reliable germline transformation techniques. Previous studies have shown that D. suzukii is amenable to transgenesis using the transposon-based vectors piggyBac and Minos, site-specific recombination (lox/Cre), and CRISPR/Cas9 genome editing.

Results

We experienced differences in the usability of piggyBac-based germline transformation in different strains of D. suzukii: we obtained no transgenic lines in a US strain, a single rare transgenic line in an Italian strain, but observed a reliable transformation rate of 2.5 to 11% in a strain from the French Alps. This difference in efficiency was confirmed by comparative examination of these three strains. In addition, we used an attP landing site line to successfully established φC31-integrase-mediated plasmid integration at a rate of 10% and generated landing site lines with two attP sequences to effectively perform φC31-Recombinase Mediated Cassette Exchange (φC31-RMCE) with 11% efficiency. Moreover, we isolated and used the endogenous regulatory regions of Ds nanos to express φC31 integrase maternally to generate self-docking lines for φC31-RMCE. Besides, we isolated the promoter/enhancer of Ds serendipity α to drive the heterologous tetracycline-controlled transactivator (tTA) during early embryonic development and generated a testes-specific tTA driver line using the endogenous beta-2-tubulin (β2t) promoter/enhancer.

Conclusion

Our results provide evidence that the D. suzukii strain AM derived from the French Alps is more suitable for piggyBac germline transformation than other strains. We demonstrated the feasibility of using φC31-RMCE in the cherry vinegar fly and generated a set of lines that can be used for highly efficient integration of larger constructs. The φC31-based integration will facilitate modification and stabilization of previously generated transgenic lines that carry at least one attP site in the transgene construction. An early embryo-specific and a spermatogenesis-specific driver line were generated for future use of the binary expression system tet-off to engineer tissue- and stage-specific effector gene expression for genetic pest control strategies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12863-020-00940-5.

Building a transgenic sexing strain for genetic control of the Australian sheep blow fly Lucilia cuprina using two lethal effectors

Background

The sterile insect technique (SIT) has been successfully used in many pest management programs worldwide. Some SIT programs release both sexes due to the lack of genetic sexing strains or efficient sex separation methods but sterile females are ineffective control agents. Transgenic sexing strains (TSS) using the tetracycline-off control system have been developed in a variety of insect pests, from which females die by either of two commonly used lethal effectors: overexpression of the transcription factor tetracycline transactivator (tTA) or ectopic expression of a proapoptotic gene, such as head involution defective (hid). The lethality from tTA overexpression is thought to be due to “transcriptional squelching”, while hid causes lethality by induction of apoptosis. This study aims to create and characterize a TSS of Lucilia cuprina, which is a major pest of sheep, by combining both lethal effectors in a single transgenic strain.

Results

Here a stable TSS of L. cuprina (DH6) that carries two lethal effectors was successfully generated, by crossing FL3#2 which carries a female-specific tTA overexpression cassette, with EF1#12 which carries a tTA-regulated Lshid^Ala2 cassette. Females with one copy of the FL3#2 transgene are viable but up to 99.8% of homozygous females die at the pupal stage when raised on diet that lacks tetracycline. Additionally, the female lethality of FL3#2 was partially repressed by supplying tetracycline to the parental generation. With an additional Lshid^Ala2 effector, the female lethality of DH6 is 100% dominant and cannot be repressed by maternal tetracycline. DH6 females die at the late-larval stage. Several fitness parameters important for mass rearing such as hatching rate, adult emergence and sex ratio were comparable to those of the wild type strain.

Conclusions

Compared to the parental FL3#2 strain, the DH6 strain shows stronger female lethality and lethality occurs at an earlier stage of development. The combination of two tTA-dependent lethal effectors could improve strain stability under mass rearing and could reduce the risk of resistance in the field if fertile males are released. Our approach could be easily adapted for other pest species for an efficient, safe and sustainable genetic control program.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12863-020-00947-y.

Genetic engineering of sex chromosomes for batch cultivation of non-transgenic, sex-sorted males

The field performance of Sterile Insect Technique (SIT) is improved by sex-sorting and releasing only sterile males. This can be accomplished by resource-intensive separation of males from females by morphology. Alternatively, sex-ratio biasing genetic constructs can be used to selectively remove one sex without the need for manual or automated sorting, but the resulting genetically engineered (GE) control agents would be subject to additional governmental regulation. Here we describe and demonstrate a genetic method for the batch production of non-GE males. This method could be applied to generate the heterogametic sex (XY, or WZ) in any organism with chromosomal sex determination. We observed up to 100% sex-selection with batch cultures of more than 103 individuals. Using a stringent transgene detection assay, we demonstrate the potential of mass production of transgene free males.

Functional characterization of the Drosophila suzukii pro-apoptotic genes reaper, head involution defective and grim

Apoptosis is a fundamental process for the elimination of damaged or unwanted cells, and is a key aspect of development. It is triggered by pro-apoptotic genes responding to the intrinsic pathway that senses cell stress or the extrinsic pathway that responds to signals from other cells. The disruption of these genes can therefore lead to developmental defects and disease. Pro-apoptotic genes have been studied in detail in the fruit fly Drosophila melanogaster, a widely-used developmental model. However, little is known about the corresponding genes in its relative D. suzukii, a pest of soft fruit crops that originates from Asia but is now an invasive species in many other regions. The characterization of D. suzukii pro-apoptotic genes could lead to the development of transgenic sexing strains for pest management. Here, we describe the isolation and characterization of the pro-apoptotic genes reaper (Dsrpr), head involution defective (Dshid) and grim (Dsgrim) from a laboratory strain of D. suzukii. We determined their expression profiles during development, revealing that all three genes are expressed throughout development but Dsrpr is expressed most strongly, especially at the pupal stage. Functional analysis was carried out by expressing single genes or pairs (linked by a 2A peptide) in S2 cell death assays, indicating that Dsgrim and Dshid are more potent pro-apoptotic genes than Dsrpr, and the lethality can be significantly enhanced by co-expression of two genes. Therefore, the binary or multiple expression of different pro-apoptotic genes can be considered to build an efficient transgenic sexing system in D. suzukii.

Genetic breakdown of a Tet-off conditional lethality system for insect population control

Genetically modified conditional lethal strains have been created to improve the control of insect pest populations damaging to human health and agriculture. However, understanding the potential for the genetic breakdown of lethality systems by rare spontaneous mutations, or selection for inherent suppressors, is critical since field release studies are in progress. This knowledge gap was addressed in a Drosophila tetracycline-suppressible embryonic lethality system by analyzing the frequency and structure of primary-site spontaneous mutations and second-site suppressors resulting in heritable survivors from 1.2 million zygotes. Here we report that F₁ survivors due to primary-site deletions and indels occur at a 5.8 × 10⁻⁶ frequency, while survival due to second-site maternal-effect suppressors occur at a ~10⁻⁵ frequency. Survivors due to inherent lethal effector suppressors could result in a resistant field population, and we suggest that this risk may be mitigated by the use of dual redundant, albeit functionally unrelated, lethality systems.

Insect population control using conditional lethal systems could break down due to spontaneous mutations that render the system ineffective. Here the authors analyse the structure and frequency of such mutations in Drosophila and suggest the use of dual lethality systems to mitigate their survival.

Knocking down the sex peptide receptor by dsRNA feeding results in reduced oviposition rate in olive fruit flies

Insect pests can cause crop damage in yield or quality, resulting in profit losses for farmers. The primary approach to control them is still the use of chemical pesticides resulting in significant hazards to the environment and human health. Biological control and the sterile insect technique are alternative strategies to improve agriculture protection. However, both strategies have significant limitations. A newly introduced approach that could be both effective and species-specific is the RNA interference mechanism. One key point for the success of this strategy is the delivery method of double-strand RNA (dsRNA) to the insects. A method of dsRNA delivery to insects with potential use in the field is the oral delivery, feeding the insects engineered microorganisms that produce dsRNA. Here, we present the first protocol for dsRNA feeding using modified bacteria, in the olive fruit fly, the most important insect pest of cultivated olives. We chose to target the sex peptide receptor gene. The sex peptide receptor interacts with the sex peptide, a peptide that is responsible for the postmating behavior in the model organism, Drosophila melanogaster. Feeding the female olive fruit fly with bacteria that produced dsRNA for the sex peptide receptor gene resulted in the development of female insects with significantly lower oviposition rates. Administration of dsRNA producing bacteria in insect diet against target genes that lead to genetic sexing or female-specific lethality could be added in the armory of control methods.

Bicistronic expression and differential localization of proteins in insect cells and Drosophila suzukii using picornaviral 2A peptides

Polycistronic expression systems in insects can be used for applications such as recombinant protein production in cells, enhanced transgenesis methods, and the development of novel pest-control strategies based on the sterile insect technique (SIT). Here we tested the performance of four picornaviral 2A self-cleaving peptides (TaV-2A, DrosCV-2A, FMDV 2A1/31 and FMDV 2A1/32) for the co-expression and differential subcellular targeting of two fluorescent marker proteins in cell lines (Anastrepha suspensa AsE01 and Drosophila melanogaster S2 cells). We found that all four 2A peptides showed comparable activity in cell lines, leading to the production of independent upstream and downstream proteins that were directed to the nucleus or membrane by a C-terminal nuclear localization signal (NLS) on the upstream protein and a poly-lysine/CAAX membrane anchor on the downstream protein. TaV-2A and DrosCV-2A were inserted into piggyBac constructs to create transgenic D. suzukii strains, confirming efficient ribosomal skipping in vivo. Interestingly, we found that the EGFP-CAAX protein was distributed homogeneously in the membrane whereas the DsRed-CAAX protein formed clumps and aggregates that induced extensive membrane blebbing. Accordingly, only flies expressing the DsRed-NLS and EGFP-CAAX proteins could be bred to homozygosity whereas expression of EGFP-NLS and DsRed-CAAX was lethal in the homozygous state. Our results therefore demonstrate that the 2A constructs and two novel targeting motifs are functional in D. suzukii, and that the combination of EGFP-NLS and DsRed-CAAX shows dosage-dependent lethality. These molecular elements could be further used to improve expression systems in insects and generate novel pest control strains.

Genetic Variation and Potential for Resistance Development to the tTA Overexpression Lethal System in Insects

Release of insect pests carrying the dominant lethal tetracycline transactivator (tTA) overexpression system has been proposed as a means for population suppression. High levels of the tTA transcription factor are thought to be toxic due to either transcriptional squelching or interference with protein ubiquitination. Here we utilized the Drosophila melanogaster Genetic Reference Panel (DGRP) to examine the influence of genetic variation on the efficacy of a female-specific tTA overexpression system. The level of female lethality between DGRP lines varied from 11 to 97% with a broad sense heritability of 0.89. A genome-wide association analysis identified 192 allelic variants associated with high or low lethality (P < 10^-5), although none were significant when corrected for multiple testing. 151 of the variants fell within 108 genes that were associated with several biological processes including transcription and protein ubiquitination. In four lines with high female lethality, tTA RNA levels were similar or higher than in the parental tTA overexpression strain. In two lines with low lethality, tTA levels were about two fold lower than in the parental strain. However, in two other lines with low lethality, tTA levels were similar or approximately 30% lower. RNAseq analysis identified genes that were up or downregulated in the four low female lethal lines compared to the four high lethal lines. For example, genes associated with RNA processing and rRNA maturation were significantly upregulated in low lethal lines. Our data suggest that standing genetic variation in an insect population could provide multiple mechanisms for resistance to the tTA overexpression system.

Improvement and use of CRISPR/Cas9 to engineer a sperm-marking strain for the invasive fruit pest Drosophila suzukii

BACKGROUND

The invasive fruit pest Drosophila suzukii was reported for the first time in Europe and the USA in 2008 and has spread since then. The adoption of type II clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) as a tool for genome manipulation provides new ways to develop novel biotechnologically-based pest control approaches. Stage or tissue-specifically expressed genes are of particular importance in the field of insect biotechnology. The enhancer/promoter of the spermatogenesis-specific beta-2-tubulin (β2t) gene was used to drive the expression of fluorescent proteins or effector molecules in testes of agricultural pests and disease vectors for sexing, monitoring, and reproductive biology studies. Here, we demonstrate an improvement to CRISPR/Cas-based genome editing in D. suzukii and establish a sperm-marking system.

RESULTS

To improve genome editing, we isolated and tested the D. suzukii endogenous promoters of the small nuclear RNA gene U6 to drive the expression of a guide RNA and the Ds heat shock protein 70 promoter to express Cas9. For comparison, we used recombinant Cas9 protein and in vitro transcribed gRNA as a preformed ribonucleoprotein. We demonstrate the homology-dependent repair (HDR)-based genome editing efficiency by applying a previously established transgenic line that expresses DsRed ubiquitously as a target platform. In addition, we isolated the Ds_β2t gene and used its promoter to drive the expression of a red fluorescence protein in the sperm. A transgenic sperm-marking strain was then established by the improved HDR-based genome editing.

CONCLUSION

The deployment of the endogenous promoters of the D. suzukii U6 and hsp70 genes to drive the expression of gRNA and Cas9, respectively, enabled the effective application of helper plasmid co-injections instead of preformed ribonucleoproteins used in previous reports for HDR-based genome editing. The sperm-marking system should help to monitor the success of pest control campaigns in the context of the Sterile Insect Technique and provides a tool for basic research in reproductive biology of this invasive pest. Furthermore, the promoter of the β2t gene can be used in developing novel transgenic pest control approaches and the CRISPR/Cas9 system as an additional tool for the modification of previously established transgenes.

Extensive Genetic Differentiation between Homomorphic Sex Chromosomes in the Mosquito Vector, Aedes aegypti

Mechanisms and evolutionary dynamics of sex-determination systems are of particular interest in insect vectors of human pathogens like mosquitoes because novel control strategies aim to convert pathogen-transmitting females into nonbiting males, or rely on accurate sexing for the release of sterile males. In Aedes aegypti, the main vector of dengue and Zika viruses, sex determination is governed by a dominant male-determining locus, previously thought to reside within a small, nonrecombining, sex-determining region (SDR) of an otherwise homomorphic sex chromosome. Here, we provide evidence that sex chromosomes in Ae. aegypti are genetically differentiated between males and females over a region much larger than the SDR. Our linkage mapping intercrosses failed to detect recombination between X and Y chromosomes over a 123-Mbp region (40% of their physical length) containing the SDR. This region of reduced male recombination overlapped with a smaller 63-Mbp region (20% of the physical length of the sex chromosomes) displaying high male–female genetic differentiation in unrelated wild populations from Brazil and Australia and in a reference laboratory strain originating from Africa. In addition, the sex-differentiated genomic region was associated with a significant excess of male-to-female heterozygosity and contained a small cluster of loci consistent with Y-specific null alleles. We demonstrate that genetic differentiation between sex chromosomes is sufficient to assign individuals to their correct sex with high accuracy. We also show how data on allele frequency differences between sexes can be used to estimate linkage disequilibrium between loci and the sex-determining locus. Our discovery of large-scale genetic differentiation between sex chromosomes in Ae. aegypti lays a new foundation for mapping and population genomic studies, as well as for mosquito control strategies targeting the sex-determination pathway.

Towards the genetic control of invasive species

Invasive species remain one of the greatest threats to global biodiversity. Their control would be enhanced through the development of more effective and sustainable pest management strategies. Recently, a novel form of genetic pest management (GPM) has been developed in which the mating behaviour of insect pests is exploited to introduce genetically engineered DNA sequences into wild conspecific populations. These 'transgenes' work in one or more ways to reduce the damage caused by a particular pest, for example reducing its density, or its ability to vector disease. Although currently being developed for use against economically important insect pests, these technologies would be highly appropriate for application against invasive species that threaten biodiversity. Importantly, these technologies have begun to advance in scope beyond insects to vertebrates, which include some of the world's worst invasives. Here we review the current state of this rapidly progressing field and, using an established set of eradication criteria, discuss the characteristics which make GPM technologies suitable for application against invasive pests.

More than one rabbit out of the hat: Radiation, transgenic and symbiont-based approaches for sustainable management of mosquito and tsetse fly populations

Mosquitoes (Diptera: Culicidae) and tsetse flies (Diptera: Glossinidae) are bloodsucking vectors of human and animal pathogens. Mosquito-borne diseases (malaria, filariasis, dengue, zika, and chikungunya) cause severe mortality and morbidity annually, and tsetse fly-borne diseases (African trypanosomes causing sleeping sickness in humans and nagana in livestock) cost Sub-Saharan Africa an estimated US$ 4750 million annually. Current reliance on insecticides for vector control is unsustainable: due to increasing insecticide resistance and growing concerns about health and environmental impacts of chemical control there is a growing need for novel, effective and safe biologically-based methods that are more sustainable. The integration of the sterile insect technique has proven successful to manage crop pests and disease vectors, particularly tsetse flies, and is likely to prove effective against mosquito vectors, particularly once sex-separation methods are improved. Transgenic and symbiont-based approaches are in development, and more advanced in (particularly Aedes) mosquitoes than in tsetse flies; however, issues around stability, sustainability and biosecurity have to be addressed, especially when considering population replacement approaches. Regulatory issues and those relating to intellectual property and economic cost of application must also be overcome. Standardised methods to assess insect quality are required to compare and predict efficacy of the different approaches. Different combinations of these three approaches could be integrated to maximise their benefits, and all have the potential to be used in tsetse and mosquito area-wide integrated pest management programmes.

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

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 the Sterile Insect Technique (SIT), for both plant and animal insect pests. Tetracycline-suppressible (Tet-off) conditional lethal systems may function together so that transgenic strains will be viable and fertile on a tetracycline-containing diet, but female-lethal and male sterile in tetracycline-free conditions. This would allow their most efficacious use in a unified system for sterile male-only production for SIT. A critical consideration for the field release of such transgenic insect strains, however, is a determination of the frequency and genetic basis of lethality revertant survival. This will provide knowledge essential to evaluating 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 should also allow the testing of these modifications, and test the ability of other conditional lethal systems to fully suppress propagation of rare Tet-off survivors. If a dominant temperature sensitive (DTS) pupal lethality system proves efficient for secondary lethality in Drosophila, it may provide the safeguard needed to support the release of sexing/sterility strains, and potentially, the release of unisex lethality strains as a form of genetic male sterility. Should the DTS Prosβ2(1) mutation prove effective for redundant lethality, its high level of structural and functional conservation should allow host-specific cognates to be created for a wide range of insect species.