Germline transformation of the spotted wing drosophilid, Drosophila suzukii, with a piggyBac transposon vector

Validation of heat-inducible Ixodes scapularis HSP70 and tick-specific 3xP3 promoters in ISE6 cells

Ixodes scapularis is an important vector of many pathogens, including the causative agent of Lyme disease. The gene function studies in I. scapularis and other ticks are hampered by the lack of genetic tools, including an inducible promoter for temporal control over transgene-encoding protein or double-stranded RNA. We characterized an intergenic sequence upstream of a heat shock protein 70 (HSP70) gene that can drive Renilla luciferase and mCherry expression in the I. scapularis cell line ISE6 (IsHSP70). In another construct, we replaced the Drosophila melanogaster minimal HSP70 promoter of the 3xP3 promoter with a minimal portion of IsHSP70 promoter and generated an I. scapularis-specific 3xP3 (Is3xP3) promoter. Both IsHSP70 and Is3xP3 have a heat-inducible expression of mCherry fluorescence in ISE6 cells with an approximately 10-fold increase in the percentage of fluorescent cells upon 2 h heat shock. These promoters described will be valuable tools for gene function studies.

Protocol for genetic engineering in Drosophila suzukii using microinjection

The spotted wing Drosophila (Drosophila suzukii Matsumura) is recognized globally as a significant economic pest. Here, we present a protocol for genetic engineering in D. suzukii using microinjection. We describe steps for genetic engineering techniques, including transposon-mediated germline transformation, recombinase-mediated genome targeting, and CRISPR-mediated gene editing. This protocol can significantly expand the toolkit for functional genomics and genetic control studies of this pest. For complete details on the use and execution of this protocol, please refer to Schetelig and Handler,1 Schetelig et al.2 Yan et al.,3 and Yan et al.4.

Arthropod promoters for genetic control of disease vectors

Vector-borne diseases (VBDs) impose devastating effects on human health and a heavy financial burden. Malaria, Lyme disease, and dengue fever are just a few examples of VBDs that cause severe illnesses. The current strategies to control VBDs consist mainly of environmental modification and chemical use, and to a small extent, genetic approaches. The genetic approaches, including transgenesis/genome modification and gene-drive technologies, provide the basis for developing new tools for VBD prevention by suppressing vector populations or reducing their capacity to transmit pathogens. The regulatory elements such as promoters are required for a robust sex-, tissue-, and stage-specific transgene expression. As discussed in this review, information on the regulatory elements is available for mosquito vectors but is scant for other vectors.

Validation of a heat-inducible Ixodes scapularis HSP70 promoter and developing a tick-specific 3xP3 promoter sequence in ISE6 cells

Ixodes scapularis is an important vector of many pathogens, including the causative agent of Lyme disease, tick-borne encephalitis, and anaplasmosis. The study of gene function in I. scapularis and other ticks has been hampered by the lack of genetic tools, such as an inducible promoter to permit temporal control over transgenes encoding protein or double-stranded RNA expression. Studies of vector-pathogen relationships would also benefit from the capability to activate anti-pathogen genes at different times during pathogen infection and dissemination. We have characterized an intergenic sequence upstream of the heat shock protein 70 (HSP70) gene that can drive Renilla luciferase expression and mCherry fluorescence in the I. scapularis cell line ISE6. In another construct, we replaced the Drosophila melanogaster minimal HSP70 promoter in the synthetic 3xP3 promoter with a minimal portion of the I. scapularis HSP70 promoter and generated an I. scapularis specific 3xP3 (Is3xP3) promoter. Both promoter constructs, IsHSP70 and Is3xP3, allow for heat-inducible expression of mCherry fluorescence in ISE6 cells with an approximately 10-fold increase in the percentage of fluorescent positive cells upon exposure to a 2 h heat shock. These promoters described here will be valuable tools for gene function studies and temporal control of gene expression, including anti-pathogen genes.

Improved piggyBac Transformation with Capped Transposase mRNA in Pest Insects

Creating transgenic insects is a key technology in insect genetics and molecular biology. A widely used instrument in insect transgenesis is the piggyBac transposase, resulting in essentially random genomic integrations. In contrast, site-specific recombinases allow the targeted integration of the transgene construct into a specific genomic target site. Both strategies, however, often face limitations due to low transgenesis efficiencies. We aimed to enhance transgenesis efficiencies by utilizing capped mRNA as a source of transposase or recombinase instead of a helper plasmid. A systematic comparison of transgenesis efficiencies in Aedes mosquitoes, as models for hard-to-transform insects, showed that suppling piggyBac transposase as mRNA increased the average transformation efficiency in Aedes aegypti from less than 5% with the plasmid source to about 50% with mRNA. Similar high activity was observed in Ae. albopictus with pBac mRNA. No efficiency differences between plasmid and mRNA were observed in recombination experiments. Furthermore, a hyperactive version of piggyBac transposase delivered as a plasmid did not improve the transformation efficiency in Ae. aegypti or the agricultural pest Drosophila suzukii. We believe that the use of mRNA has strong potential for enhancing piggyBac transformation efficiencies in other mosquitoes and important agricultural pests, such as tephritids.

Sequence and expression analysis of the spermatogenesis-specific gene cognates, wampa and Prosα6T, in Drosophila suzukii

The sterile insect technique (SIT) is a highly effective biologically-based method for the population suppression of highly invasive insect pests of medical and agricultural importance. The efficacy of SIT could be significantly enhanced, however, by improved methods of male sterilization that avoid the fitness costs of irradiation. An alternative sterilization method is possible by gene-editing that targets genes essential for sperm maturation and motility, rendering them nonfunctional, similar to the CRISPR-Cas9 targeting of β2-tubulin in the genetic model system, Drosophila melanogaster. However, since genetic strategies for sterility are susceptible to breakdown or resistance in mass-reared populations, alternative targets for sterility are important for redundancy or strain replacement. Here we have identified and characterized the sequence and transcriptional expression of two genes in a Florida strain of Drosophila suzukii, that are cognates of the D. melanogaster spermatocyte-specific genes wampa and Prosalpha6T. Wampa encodes a coiled-coil dynein subunit required for axonemal assembly, and the proteasome subunit gene, Prosalpha6T, is required for spermatid individualization and nuclear maturation. The reading frames of these genes differed from their NCBI database entries derived from a D. suzukii California strain by 44 and 8 nucleotide substitutions/polymorphisms, respectively, though all substitutions were synonymous resulting in identical peptide sequences. Expression of both genes is predominant in the male testis, and they share similar transcriptional profiles in adult males with β2-tubulin. Their amino acid sequences are highly conserved in dipteran species, including pest species subject to SIT control, supporting their potential use in targeted male sterilization strategies.

Genome editing efficiency of four Drosophila suzukii endogenous U6 promoters

The invasive spotted-wing drosophila, Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) has caused serious economic losses to the fruit industry. The conventional control methods have many limitations and genetic engineering technologies such as CRISPR/Cas9-mediated gene drive are promising approaches. In the CRISPR/Cas9 system, the transcriptional regulatory elements play an important role in the activities of gRNA. Thus, in order to improve the genome editing efficiency of the CRISPR/Cas9 system in D. suzukii, we cloned and tested four endogenous U6 promoters to drive mutagenesis of the white gene. Our results showed that all the four promoters could be used with variable efficiency. The promoter DsU6-3 had the highest genome editing efficiency among the four DsU6 promoters. Compared with the DsU6-3 promoter, the DmU6:3 promoter showed lower efficiency to drive mutagenesis in D. suzukii. These findings expand the range of promoters available to express gRNAs in D. suzukii, facilitating the basic and applied research on this important pest.

A transgenic female killing system for the genetic control of Drosophila suzukii

The spotted wing Drosophila (Drosophila suzukii) is an invasive pest of soft-skinned fruit crops. It is rapidly transmitted in Europe and North America, causing widespread agricultural losses. Genetic control strategies such as the sterile insect technique (SIT) have been proposed as environment-friendly and species-restricted approaches for this pest. However, females are inefficient agents in SIT programs. Here we report a conditional female-killing (FK) strategy based on the tetracycline-off system. We assembled sixteen genetic constructs for testing in vitro and in vivo. Twenty-four independent transgenic strains of D. suzukii were generated and tested for female-specific lethality. The strongest FK effect in the absence of tetracycline was achieved by the construct containing D. suzukii nullo promoter for early gene expression, D. suzukii pro-apoptotic gene hidAla4 for lethality, and the transformer gene intron from the Mediterranean fruit fly Ceratitis capitata for female-specific splicing. One strain carrying this construct eliminated 100% of the female offspring during embryogenesis and produced only males. However, homozygous females from these FK strains were not viable on a tetracycline-supplemented diet, possibly due to the basal expression of hidAla4. Potential improvements to the gene constructs and the use of such FK strains in an SIT program are discussed.

Characterization of the Drosophila suzukii β2-tubulin gene and the utilization of its promoter to monitor sex separation and insemination

The Drosophila melanogaster β2-tubulin gene (Dm-β2t) controls the function of microtubules in the testis and sperm, and has been evaluated for use in biocontrol strategies based on the sterile insect technique, including sexing and the induction of male sterility. The spotted-wing Drosophila (Drosophila suzukii) is native to eastern Asia but has spread globally as an invasive pest of fruit crops, so biocontrol strategies are urgently required for this species. We therefore isolated the β2tubulin ortholog Ds-β2t from the USA laboratory strain of D. suzukii and confirmed the presence of functional motifs by aligning orthologs from multiple insects. The developmental expression profile of Ds-β2t was determined by RT-PCR using gene-specific primers and was similar to that of Dm-β2t. We then isolated the Ds-β2t promoter and used it to generate transgenic strains expressing a testis-specific fluorescent protein starting from the thirdinstar larvae. Efficient sexing was achieved based on fluorescence detection, and the transgenic males showed a similar survival rate to wild-type males. Fluorescence imaging and PCR were also used to confirm the insemination of wild-type females by transgenic males. We therefore confirm that D. suzukii strains expressing fluorescent markers under the control of the Ds-β2t promoter can be used for sexing and the confirmation of mating, and we discuss the wider potential of the Ds-β2t promoter in the context of genetic control strategies for D. suzukii.

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.

Identification and characterization of four Drosophila suzukii cellularization genes and their promoters

Background

The spotted-wing Drosophila (Drosophila suzukii) is a widespread invasive pest that causes severe economic damage to fruit crops. The early development of D. suzukii is similar to that of other Drosophilids, but the roles of individual genes must be confirmed experimentally. Cellularization genes coordinate the onset of cell division as soon as the invagination of membranes starts around the nuclei in the syncytial blastoderm. The promoters of these genes have been used in genetic pest-control systems to express transgenes that confer embryonic lethality. Such systems could be helpful in sterile insect technique applications to ensure that sterility (bi-sex embryonic lethality) or sexing (female-specific embryonic lethality) can be achieved during mass rearing. The activity of cellularization gene promoters during embryogenesis controls the timing and dose of the lethal gene product.

Results

Here, we report the isolation of the D. suzukii cellularization genes nullo, serendipity-α, bottleneck and slow-as-molasses from a laboratory strain. Conserved motifs were identified by comparing the encoded proteins with orthologs from other Drosophilids. Expression profiling confirmed that all four are zygotic genes that are strongly expressed at the early blastoderm stage. The 5′ flanking regions from these cellularization genes were isolated, incorporated into piggyBac vectors and compared in vitro for the promoter activities. The Dsnullo promoter showed the highest activity in the cell culture assays using D. melanogaster S2 cells.

Conclusions

The similarities in the gene coding and 5′ flanking sequence as well as in the expression pattern of the four cellularization genes between D. melanogaster and D. suzukii, suggest that conserved functions may be involved in both species. The high expression level at the early blastoderm stage of the four cellularization genes were confirmed, thus their promoters can be considered in embryonic lethality systems. While the Dsnullo promoter could be a suitable candidate, all reported promoters here are subject to further in vivo analyses before constructing potential pest control systems.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12863-020-00939-y.

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.

CRISPR/Cas9 mediated disruption of the white gene leads to pigmentation deficiency and copulation failure in Drosophila suzukii

The Spotted-wing Drosophila (Drosophila suzukii) is a devastating invasive pest of fruit crops. In D. melanogaster, the white (w) gene was associated with pigmentation and mating behavior, which are also important aspects to understand the invasion biology as well as to develop control strategies for D. suzukii. Here, we show that the generation of D. suzukii white-eyed mutants by CRISPR/Cas9 mutagenesis of the w gene resulted in the complete failure of copulation when w^- males were individually paired with w^- females in small circular arenas (diameter 0.7 cm) for 24 h. Further analysis showed that the mating defect was associated with w^- males and could not be rectified by two years of inbreeding by crossing sibling w^- females with w⁺ males, dim red illumination, male-female sexual training, changing to large arenas (diameter 3.5 cm), or different sex ratios. Profound pigmentation deficiency was detected in the compound eyes, ocelli, Malpighian tubules and testis sheaths in the w^- flies. Specifically, testis imaging showed that w^- males failed to deposit any pigments into pigment cells of the testis sheath, and produced smaller sperms and less seminal fluid compared to those from wildtype males. Together these observations suggest that the w gene plays an essential role in the regulation of sexual behavior and reproduction in D. suzukii. The similarities and differences in w gene function between D. suzukii and D. melanogaster in the context of pigmentation and mating behavior are discussed.

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.

A Cross-Sectional Survey of Biosafety Professionals Regarding Genetically Modified Insects

BACKGROUND

Genetic technologies such as gene editing and gene drive create challenges for existing frameworks used to assess risk and make regulatory determinations by governments and institutions. Insect genetic technologies including transgenics, gene editing, and gene drive may be particularly challenging because of the large and increasing number of insect species being genetically modified and the degree of familiarity with these organisms and technologies by biosafety officials charged with making containment decisions.

METHODS

An anonymous online survey of biosafety professionals was distributed to the membership of ABSA International, a global society of biosafety professionals, to investigate their perspectives on their preparedness to meet these new challenges.

RESULTS

Existing guidance used to make containment decisions for nongenetically modified insects was widely seen as adequate, and most respondents thought the available guidance for making containment decisions for genetically modified insects with and without gene drives was inadequate. Most respondents reported having less confidence in their decisions concerning containment of genetically modified insects compared to decisions involving genetically modified microbes, (noninsect) animals, and plants.

CONCLUSIONS

These results reveal a need for additional support for biosafety professionals to improve the quality of and confidence in containment decisions regarding genetically modified insects with and without gene drive. These needs might be addressed by increasing training, updating existing guidance, creating new guidance, and creating a third-party accreditation entity to support institutions. Sixty percent of the respondents said they either would or might use a voluntary third-party accreditation service to support insect containment decisions.

Identification and functional analysis of promoters of heat-shock genes from the fall armyworm, Spodoptera frugiperda

The functional information on heat-shock proteins (Hsp) and heat-shock promoters from an important agricultural insect pest, Spodoptera frugiperda, is still lacking. We conducted a genome-wide identification of Hsp genes and identified a total of 21 genes belonging to four major insect Hsp families (small heat-shock proteins, Hsp60, Hsp70, and Hsp90) in S. frugiperda. Expression of most of S. frugiperda (SfHsp) genes could be detected in Sf9 cells, embryos and larval tissues of S. frugiperda. The heat-inducible activity of heat-shock promoters from several SfHsp genes was tested in Sf9 cells and embryos. The promoter of SfHsp70D showed the high constitutive activity in cell line and embryos, while the activity of SfHsp20.15 and SfHsp20.71 promoters was most dramatically induced in Sf9 cells and embryos. In embryos, the heat-induced activity of SfHsp20.71 and SfHsp70D promoters outperformed commercially used ie1 and ie2 promoters. The heat-induced activity of SfHsp70D and SfHsp19.07 promoters were more robust than ie2 promoter in Sf9 cells. These SfHsp promoters with high basal activity or with heat-induced activity from low basal activity, could be used in S. frugiperda or other lepidopteran insects for many applications including transgenesis and genome editing.

Genomic targeting by recombinase-mediated cassette exchange in the spotted wing drosophila, Drosophila suzukii

Drosophila suzukii is a significant pest of stone and small fruits. The genome of this species has been sequenced and manipulated by transposon-mediated transformation and CRISPR/Cas9 gene editing. These technologies open a variety of possibilities for functional genomics and genetic modifications that might improve biologically based population control strategies. Both of these approaches, however, would benefit from genome targeting that would avoid position effects and insertional mutations associated with random transposon vector insertions, and the limited DNA fragment insertion size allowed by gene editing. Here, we describe an efficient recombinase-mediated cassette exchange (RMCE) system for D. suzukii in which heterospecific lox recombination sites were integrated into the genome by transposon-mediated transformation and subsequently targeted for double recombination by a donor vector in the presence of Cre recombinase. Three loxN/lox2272 landing site lines have previously been created in D. suzukii, and quantitative PCR determined that polyubiquitin-regulated enhanced green fluorescent protein expression is least susceptible to position effect suppression in the 443_M26m1 line. We presume that RMCE target sites may also be inserted more specifically into the genome by homology-directed repair gene editing, thereby avoiding position effects and mutations, while eliminating restrictions on the size of donor constructs for subsequent insertion.

piggyBac- and phiC31 integrase-mediated transgenesis in Drosophila prolongata

The development of transgenesis systems in non-model organisms provides a powerful tool for molecular analysis and contributes to the understanding of phenomena that are not observed in model organisms. Drosophila prolongata is a fruit fly that has unique morphology and behavior not found in other Drosophila species including D. melanogaster. In this study, we developed a phiC31 integrase-mediated transgenesis system for D. prolongata. First, using piggyBac-mediated transgenesis, 37 homozygous attP strains were established. These strains were further transformed with the nosP-Cas9 vector, which was originally designed for phiC31-mediated transgenesis in D. melanogaster. The transformation rate varied from 0% to 3.4%. Nine strains with a high transformation rate of above 2.0% were established, which will serve as host strains in future transformation experiments in D. prolongata. Our results demonstrate that genetic tools developed for D. melanogaster are applicable to D. prolongata with minimal modifications.

Synthetically engineered Medea gene drive system in the worldwide crop pest Drosophila suzukii

Synthetic gene drive systems possess enormous potential to replace, alter, or suppress wild populations of significant disease vectors and crop pests; however, their utility in diverse populations remains to be demonstrated. Here, we report the creation of a synthetic Medea gene drive system in a major worldwide crop pest, Drosophila suzukii We demonstrate that this drive system, based on an engineered maternal "toxin" coupled with a linked embryonic "antidote," is capable of biasing Mendelian inheritance rates with up to 100% efficiency. However, we find that drive resistance, resulting from naturally occurring genetic variation and associated fitness costs, can be selected for and hinder the spread of such a drive. Despite this, our results suggest that this gene drive could maintain itself at high frequencies in a wild population and spread to fixation if either its fitness costs or toxin resistance were reduced, providing a clear path forward for developing future such systems in this pest.

Synthetically engineered Medea gene drive system in the worldwide crop pest Drosophila suzukii

Here we describe a fully functional gene drive system constructed in a major worldwide crop pest, Drosophila suzukii. This system is composed of a synthetic Medea drive with a maternal miRNA “toxin” and a zygotic “antidote,” and we demonstrate that it can bias inheritance with 100% efficiency and can persist in a population given high release frequencies. We discuss how such a system may be used to suppress D. suzukii populations or render them harmless to target crops.

Synthetic gene drive systems possess enormous potential to replace, alter, or suppress wild populations of significant disease vectors and crop pests; however, their utility in diverse populations remains to be demonstrated. Here, we report the creation of a synthetic Medea gene drive system in a major worldwide crop pest, Drosophila suzukii. We demonstrate that this drive system, based on an engineered maternal “toxin” coupled with a linked embryonic “antidote,” is capable of biasing Mendelian inheritance rates with up to 100% efficiency. However, we find that drive resistance, resulting from naturally occurring genetic variation and associated fitness costs, can be selected for and hinder the spread of such a drive. Despite this, our results suggest that this gene drive could maintain itself at high frequencies in a wild population and spread to fixation if either its fitness costs or toxin resistance were reduced, providing a clear path forward for developing future such systems in this pest.

A universal vector concept for a direct genotyping of transgenic organisms and a systematic creation of homozygous lines

Diploid transgenic organisms are either hemi- or homozygous. Genetic assays are, therefore, required to identify the genotype. Our AGameOfClones vector concept uses two clearly distinguishable transformation markers embedded in interweaved, but incompatible Lox site pairs. Cre-mediated recombination leads to hemizygous individuals that carry only one marker. In the following generation, heterozygous descendants are identified by the presence of both markers and produce homozygous progeny that are selected by the lack of one marker. We prove our concept in Tribolium castaneum by systematically creating multiple functional homozygous transgenic lines suitable for long-term fluorescence live imaging. Our approach saves resources and simplifies transgenic organism handling. Since the concept relies on the universal Cre-Lox system, it is expected to work in all diploid model organisms, for example, insects, zebrafish, rodents and plants. With appropriate adaptions, it can be used in knock-out assays to preselect homozygous individuals and thus minimize the number of wasted animals.

Development and use of a piggyBac-based jumpstarter system in Drosophila suzukii

Spotted wing drosophila, Drosophila suzukii, is an invasive pest that primarily attacks fresh, soft-skinned fruit. Although others have reported successful integration of marked piggyBac elements into the D. suzukii genome, with a very respectable transgenesis rate of ∼16%, here we take this work a step further by creating D. suzukii jumpstarter strains. These were generated through integration of a fluorescent-marked Minos element carrying a heat shock protein 70-driven piggyBac transposase gene. We demonstrate that there is a dramatic increase in transformation rates when germline transformation is performed in a transposase-expressing background. For example, we achieved transformation rates as high as 80% when microinjecting piggyBac-based plasmids into embryos derived from one of these D. suzukii jumpstarter strains. We also investigate the effect of insert size on transformation efficiency by testing the ability of the most efficient jumpstarter strain to catalyze integration of differently-sized piggyBac elements. Finally, we demonstrate the ability of a jumpstarter strain to remobilize an already-integrated piggyBac element to a new location, demonstrating that our jumpstarter strains could be used in conjunction with a piggyBac-based donor strain for genome-wide mutagenesis of D. suzukii.

Seasonal Reproductive Biology of Drosophila suzukii (Diptera: Drosophilidae) in Temperate Climates

Drosophila suzukii (Matsumura; Diptera: Drosophilidae) is a key pest of sweet cherry and small fruits worldwide. The present studies were designed to describe the reproductive physiology in both sexes, through dissections of their reproductive organs. We extensively dissected female D. suzukii throughout the season from 2013 to 2016 and classified the reproductive status flies based on five recognizable ovarian maturation stages: 1) no ovaries; 2) unripe ovaries 3) ripening eggs in ovarioles; 4) mature eggs in ovarioles; and 5) mature eggs in the abdomen. Development was examined as a function of calendar days as well as degree-days (DD). Results obtained from winter collections revealed that females collected from November to March contained a lower percentage of mature eggs than females collected from April to September. These data suggest that environmental conditions during the dormant period induce reproductive diapause. Oogenesis likely increased with an increase in mean monthly temperatures and DD. The first overwintered females with mature eggs were dissected as early as 21 February 2014 in Trento (7 DD). Additionally, we found that a low proportion of males (less than 50%) had sperm in their testes between January and March, yet during the same period females already have sperm stored in their spermathecal. Ivy berries was an alternative but unfavorable non-crop host during the late dormant period, as evidenced by emergence of smaller adults when compared to individuals emerging from cherry fruits. This study showed that D. suzukii females have great potential for oviposition early in the season, posing a risk to early season maturing crop hosts.

Reduced Drosophila suzukii Infestation in Berries Using Deterrent Compounds and Laminate Polymer Flakes

Drosophila suzukii (Matsumura) is a recent invasive pest of soft fruits in North and South America and Europe. Control relies on frequent applications of synthetic insecticides. Additional tactics are needed for development of an effective integrated pest management program. Study objectives were to evaluate the repellency and oviposition deterrent capability of compounds in plant essential oils and the effect of select compounds on infestation rates in strawberries, using laminate polymer flakes as a carrier. Of 14 compounds from 5 essential oils, thymol was the most repellent to adult D. suzukii males and females for up to 24 h in the laboratory. Citronellol, geraniol and menthol were moderately repellent. In a choice assay, thymol on cotton wicks adjacent to ripe raspberries reduced female fly landings and larval infestation levels. In a no-choice assay, thymol reduced female fly landings by 60%, larval infestation by 50% and increased fly mortality compared to controls. Neither citronellol alone nor a blend of four repellent compounds was as effective as thymol alone at reducing fly landing, larval infestation, or increasing fly mortality. In a choice assay using polymer flakes, larval infestation was greater in raspberries near untreated flakes than in raspberries near flakes treated with thymol or peppermint oil. In the field, thymol and peppermint flakes reduced larval infestation levels by 25% in strawberries at 4, but not 7, days after application, compared to untreated flakes. With future improvements in application strategies, deterrent compounds may have a role in improving the management of D. suzukii.

Temperature-dependent sex-reversal by a transformer-2 gene-edited mutation in the spotted wing drosophila, Drosophila suzukii

Female to male sex reversal was achieved in an emerging agricultural insect pest, Drosophila suzukii, by creating a temperature-sensitive point mutation in the sex-determination gene, transformer-2 (tra-2), using CRISPR/Cas9 (clustered regularly interspaced palindromic repeats/CRISPR-associated) homology-directed repair gene-editing. Ds-tra-2^ts2 mutants developed as normal fertile XX and XY adults at permissive temperatures below 20 °C, but at higher restrictive temperatures (26 to 29 °C) chromosomal XX females developed as sterile intersexuals with a predominant male phenotype, while XY males developed with normal morphology, but were sterile. The temperature-dependent function of the Ds-TRA-2^ts2 protein was also evident by the up- and down-regulation of female-specific Ds-Yolk protein 1 (Ds-Yp1) gene expression by temperature shifts during adulthood. This study confirmed the temperature-dependent function of a gene-edited mutation and provides a new method for the more general creation of conditional mutations for functional genomic analysis in insects, and other organisms. Furthermore, it provides a temperature-dependent system for creating sterile male populations useful for enhancing the efficacy of biologically-based programs, such as the sterile insect technique (SIT), to control D. suzukii and other insect pest species of agricultural and medical importance.

Cre/lox-Recombinase-Mediated Cassette Exchange for Reversible Site-Specific Genomic Targeting of the Disease Vector, Aedes aegypti

Site-specific genome modification (SSM) is an important tool for mosquito functional genomics and comparative gene expression studies, which contribute to a better understanding of mosquito biology and are thus a key to finding new strategies to eliminate vector-borne diseases. Moreover, it allows for the creation of advanced transgenic strains for vector control programs. SSM circumvents the drawbacks of transposon-mediated transgenesis, where random transgene integration into the host genome results in insertional mutagenesis and variable position effects. We applied the Cre/lox recombinase-mediated cassette exchange (RMCE) system to Aedes aegypti, the vector of dengue, chikungunya, and Zika viruses. In this context we created four target site lines for RMCE and evaluated their fitness costs. Cre-RMCE is functional in a two-step mechanism and with good efficiency in Ae. aegypti. The advantages of Cre-RMCE over existing site-specific modification systems for Ae. aegypti, phiC31-RMCE and CRISPR, originate in the preservation of the recombination sites, which 1) allows successive modifications and rapid expansion or adaptation of existing systems by repeated targeting of the same site; and 2) provides reversibility, thus allowing the excision of undesired sequences. Thereby, Cre-RMCE complements existing genomic modification tools, adding flexibility and versatility to vector genome targeting.

Insect transformation with piggyBac: getting the number of injections just right

The insertion of exogenous genetic cargo into insects using transposable elements is a powerful research tool with potential applications in meeting food security and public health challenges facing humanity. piggyBac is the transposable element most commonly utilized for insect germline transformation. The described efficiency of this process is variable in the published literature, and a comprehensive review of transformation efficiency in insects is lacking. This study compared and contrasted all available published data with a comprehensive data set provided by a biotechnology group specializing in insect transformation. Based on analysis of these data, with particular focus on the more complete observational data from the biotechnology group, we designed a decision tool to aid researchers' decision-making when using piggyBac to transform insects by microinjection. A combination of statistical techniques was used to define appropriate summary statistics of piggyBac transformation efficiency by species and insect order. Publication bias was assessed by comparing the data sets. The bias was assessed using strategies co-opted from the medical literature. The work culminated in building the Goldilocks decision tool, a Markov-Chain Monte-Carlo simulation operated via a graphical interface and providing guidance on best practice for those seeking to transform insects using piggyBac.

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.

piggyBac Transposon

The piggyBac transposon was originally isolated from the cabbage looper moth, Trichoplusia ni, in the 1980s. Despite its early discovery and dissimilarity to the other DNA transposon families, the piggyBac transposon was not recognized as a member of a large transposon superfamily for a long time. Initially, the piggyBac transposon was thought to be a rare transposon. This view, however, has now been completely revised as a number of fully sequenced genomes have revealed the presence of piggyBac-like repetitive elements. The isolation of active copies of the piggyBac-like elements from several distinct species further supported this revision. This includes the first isolation of an active mammalian DNA transposon identified in the bat genome. To date, the piggyBac transposon has been deeply characterized and it represents a number of unique characteristics. In general, all members of the piggyBac superfamily use TTAA as their integration target sites. In addition, the piggyBac transposon shows precise excision, i.e., restoring the sequence to its preintegration state, and can transpose in a variety of organisms such as yeasts, malaria parasites, insects, mammals, and even in plants. Biochemical analysis of the chemical steps of transposition revealed that piggyBac does not require DNA synthesis during the actual transposition event. The broad host range has attracted researchers from many different fields, and the piggyBac transposon is currently the most widely used transposon system for genetic manipulations.

CRISPR/Cas9-mediated mutagenesis of the white and Sex lethal loci in the invasive pest, Drosophila suzukii

Drosophila suzukii (commonly called spotted wing Drosophila) is an invasive pest of soft-skinned fruit (e.g. blueberries, strawberries). A high quality reference genome sequence is available but functional genomic tools, such as used in Drosophila melanogaster, remain to be developed. In this study we have used the CRISPR/Cas9 system to introduce site-specific mutations in the D. suzukii white (w) and Sex lethal (Sxl) genes. Hemizygous males with w mutations develop white eyes and the mutant genes are transmissible to the next generation. Somatic mosaic females that carry mutations in the Sxl gene develop abnormal genitalia and reproductive tissue. The D. suzukii Sxl gene could be an excellent target for a Cas9-mediated gene drive to suppress populations of this highly destructive pest.

Morphological Markers for Cryopreservation in the Embryonic Development of Drosophila suzukii (Diptera: Drosophilidae)

Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) is an invasive pest recently reported in Europe whose spread into new areas has caused severe economic damage to many agricultural crops. There are serious concerns about the currently available chemical insecticides because of their low efficacy in controlling the species and their environmental impact; so, several studies have focused on environmentally safe strategies. The sterile insect technique (SIT), which requires colony maintenance in laboratory and production of large numbers of live animals, can be utilized in pest management programs and could be integrated with other control strategies if the potential risks associated with the rearing and maintenance of the insect line under laboratory conditions are given sufficient attention. In this regard, the ability to cryobiologically preserve such stocks would be of substantial value. Important prerequisites for long-term cryopreservation are determination of the embryonic stages, identification of specific embryonic stages, and knowledge of development time. This paper describes the main visible markers for the different stages of embryonic development and determines the timing of development at 25°C. D. suzukii embryogenesis lasts 23-25 h at 25°C and can be divided into 17 stages defined by specific morphological markers. The point at which 50% of embryos are at Stage 14 and 50% are at Stage 15, the most tolerant stages for cryopreservation treatment, as ascertained for Drosophila melanogaster Meigen in prior studies, is reached in 14-15 h. The efficiency of this procedure might be impaired by the retention of eggs in the oviducts, making it impossible to determine the stage of embryonic development for ∼25% of laid eggs.

A nucleolus-predominant piggyBac transposase, NP-mPB, mediates elevated transposition efficiency in mammalian cells

PiggyBac is a prevalent transposon system used to deliver transgenes and functionally explore the mammalian untouched genomic territory. The important features of piggyBac transposon are the relatively low insertion site preference and the ability of seamless removal from genome, which allow its potential uses in functional genomics and regenerative medicine. Efforts to increase its transposition efficiency in mammals were made through engineering the corresponding transposase (PBase) codon usage to enhance its expression level and through screening for mutant PBase variants with increased enzyme activity. To improve the safety for its potential use in regenerative medicine applications, site-specific transposition was achieved by using engineered zinc finger- and Gal4-fused PBases. An excision-prone PBase variant has also been successfully developed. Here we describe the construction of a nucleolus-predominant PBase, NP-mPB, by adding a nucleolus-predominant (NP) signal peptide from HIV-1 TAT protein to a mammalian codon-optimized PBase (mPB). Although there is a predominant fraction of the NP-mPB-tGFP fusion proteins concentrated in the nucleoli, an insertion site preference toward nucleolar organizer regions is not detected. Instead a 3–4 fold increase in piggyBac transposition efficiency is reproducibly observed in mouse and human cells.