Fluorescent transformation markers for insect transgenesis

Production of fosmid genomic libraries optimized for liquid culture recombineering and cross-species transgenesis

Genomic DNA libraries are a valuable source of large constructs that can contain all the regulatory elements necessary for recapitulating wild-type gene expression when introduced into animal genomes as a transgene. Such clones can be directly used in complementation studies. In combination with recombineering manipulation, the tagged genomic clones can serve as faithful in vivo gene activity reporters that enable studies of tissue specificity of gene expression, subcellular protein localization, and affinity purification of complexes. We present a detailed protocol for generating an unbiased genomic library in a custom pFlyFos vector that is optimized for liquid culture recombineering manipulation and site-specific transgenesis of fosmid-size loci across different Drosophila species. The cross-species properties of the library can be used, for example, to establish the specificity of RNAi phenotypes or to selectively introgress specific genomic loci among different Drosophila species making it an ideal tool for experimental evolutionary studies. The FlyFos system can be easily adapted to other organisms.

Transgene-mediated suppression of dengue viruses in the salivary glands of the yellow fever mosquito, Aedes aegypti

Controlled sex-, stage- and tissue-specific expression of antipathogen effector molecules is important for genetic engineering strategies to control mosquito-borne diseases. Adult female salivary glands are involved in pathogen transmission to human hosts and are target sites for expression of antipathogen effector molecules. The Aedes aegypti 30K a and 30K b genes are expressed exclusively in adult female salivary glands and are transcribed divergently from start sites separated by 263 nucleotides. The intergenic, 5'- and 3'-end untranslated regions of both genes are sufficient to express simultaneously two different transgene products in the distal-lateral lobes of the female salivary glands. An antidengue effector gene, membranes no protein (Mnp), driven by the 30K b promoter, expresses an inverted-repeat RNA with sequences derived from the premembrane protein-encoding region of the dengue virus serotype 2 genome and reduces significantly the prevalence and mean intensities of viral infection in mosquito salivary glands and saliva.

Recombination technologies for enhanced transgene stability in bioengineered insects

Transposon-based vectors currently provide the most suitable gene transfer systems for insect germ-line transformation and are used for molecular improvement of the Sterile Insect Technique. However, the long time stability of genome-integrated transposon constructs depends on the absence of transposase activity that could remobilize the transposon-embedded transgenes. To achieve transgene stability transposon vectors are usually non-autonomous, lacking a functional transposase gene, and chosen so that endogenous or related transposon activities are not present in the host. Nevertheless, the non-autonomous transposon-embedded transgenes could become unstable by the unintended presence of a mobilizing transposase that may have been undetected or subsequently entered the host species by horizontal gene transfer. Since the field release of transgenic insects will present environmental concerns relating to large populations and high mobility, it will be important to ensure that transgene constructs are stably integrated for maintaining strain integrity and eliminating the possibility for unintentional transfer into the genome of another organism. Here we review efficient methods to delete or rearrange terminal repeat sequences of transposons necessary for their mobility, subsequent to their initial genomic integration. These procedures should prevent transposase-mediated remobilization of the transgenes, ensuring their genomic stability.

Activation of Akt signaling reduces the prevalence and intensity of malaria parasite infection and lifespan in Anopheles stephensi mosquitoes

Malaria (Plasmodium spp.) kills nearly one million people annually and this number will likely increase as drug and insecticide resistance reduces the effectiveness of current control strategies. The most important human malaria parasite, Plasmodium falciparum, undergoes a complex developmental cycle in the mosquito that takes approximately two weeks and begins with the invasion of the mosquito midgut. Here, we demonstrate that increased Akt signaling in the mosquito midgut disrupts parasite development and concurrently reduces the duration that mosquitoes are infective to humans. Specifically, we found that increased Akt signaling in the midgut of heterozygous Anopheles stephensi reduced the number of infected mosquitoes by 60-99%. Of those mosquitoes that were infected, we observed a 75-99% reduction in parasite load. In homozygous mosquitoes with increased Akt signaling parasite infection was completely blocked. The increase in midgut-specific Akt signaling also led to an 18-20% reduction in the average mosquito lifespan. Thus, activation of Akt signaling reduced the number of infected mosquitoes, the number of malaria parasites per infected mosquito, and the duration of mosquito infectivity.

Functionality of the GAL4/UAS system in Tribolium requires the use of endogenous core promoters

Background

The red flour beetle Tribolium castaneum has developed into an insect model system second only to Drosophila. Moreover, as a coleopteran it represents the most species-rich metazoan taxon which also includes many pest species. The genetic toolbox for Tribolium research has expanded in the past years but spatio-temporally controlled misexpression of genes has not been possible so far.

Results

Here we report the establishment of the GAL4/UAS binary expression system in Tribolium castaneum. Both GAL4Δ and GAL4VP16 driven by the endogenous heat shock inducible promoter of the Tribolium hsp68 gene are efficient in activating reporter gene expression under the control of the Upstream Activating Sequence (UAS). UAS driven ubiquitous tGFP fluorescence was observed in embryos within four hours after activation while in-situ hybridization against tGFP revealed expression already after two hours. The response is quick in relation to the duration of embryonic development in Tribolium - 72 hours with segmentation being completed after 24 hours - which makes the study of early embryonic processes possible using this system. By comparing the efficiency of constructs based on Tribolium, Drosophila, and artificial core promoters, respectively, we find that the use of endogenous core promoters is essential for high-level expression of transgenic constructs.

Conclusions

With the established GAL4/UAS binary expression system, ectopic misexpression approaches are now feasible in Tribolium. Our results support the contention that high-level transgene expression usually requires endogenous regulatory sequences, including endogenous core promoters in Tribolium and probably also other model systems.

piggyBac-like elements in the pink bollworm, Pectinophora gossypiella

A transgenic line of the pink bollworm, Pectinophora gossypiella, a key lepidopteran cotton pest, was generated previously using the piggyBac transposon IFP2 from Trichoplusia ni. Here we identified an endogenous piggyBac-like element (PLE), designated as PgPLE1, in the pink bollworm. A putatively intact copy of PgPLE1 (PgPLE1.1) presents the canonical features of PLE: inverted terminal repeats with three C/G residues at the extreme ends, inverted subterminal repeats, TTAA target site and an open reading frame encoding transposase with 68% similarity to IFP2. Vectorette PCR revealed large variation in the insertion sites of PgPLE1 amongst worldwide populations, indicating the potential mobility of PgPLE1. The PgPLE1 was undetectable in the genome of Pectinophora endema, implying the recent invasion of PgPLE1 after the divergence of these two closely related species.

Forward genetics in Tribolium castaneum: opening new avenues of research in arthropod biology

A recent paper in BMC Biology reports the first large-scale insertional mutagenesis screen in a non-drosophilid insect, the red flour beetle Tribolium castaneum. This screen marks the beginning of a non-biased, 'forward genetics' approach to the study of genetic mechanisms operating in Tribolium.

See research article http://biomedcentral.com/1741-7007/7/73

Large-scale insertional mutagenesis of a coleopteran stored grain pest, the red flour beetle Tribolium castaneum, identifies embryonic lethal mutations and enhancer traps

Background

Given its sequenced genome and efficient systemic RNA interference response, the red flour beetle Tribolium castaneum is a model organism well suited for reverse genetics. Even so, there is a pressing need for forward genetic analysis to escape the bias inherent in candidate gene approaches.

Results

To produce easy-to-maintain insertional mutations and to obtain fluorescent marker lines to aid phenotypic analysis, we undertook a large-scale transposon mutagenesis screen. In this screen, we produced more than 6,500 new piggyBac insertions. Of these, 421 proved to be recessive lethal, 75 were semi-lethal, and eight indicated recessive sterility, while 505 showed new enhancer-trap patterns. Insertion junctions were determined for 403 lines and often appeared to be located within transcription units. Insertion sites appeared to be randomly distributed throughout the genome, with the exception of a preference for reinsertion near the donor site.

Conclusion

A large collection of enhancer-trap and embryonic lethal beetle lines has been made available to the research community and will foster investigations into diverse fields of insect biology, pest control, and evolution. Because the genetic elements used in this screen are species-nonspecific, and because the crossing scheme does not depend on balancer chromosomes, the methods presented herein should be broadly applicable for many insect species.

Transgenic silkworms (Bombyx mori) produce recombinant spider dragline silk in cocoons

Spider dragline silk is a unique fibrous protein with a combination of tensile strength and elasticity, but the isolation of large amounts of silk from spiders is not feasible. In this study, we generated germline-transgenic silkworms (Bombyx mori) that spun cocoons containing recombinant spider silk. A piggyBac-based transformation vector was constructed that carried spider dragline silk (MaSp1) cDNA driven by the sericin 1 promoter. Silkworm eggs were injected with the vector, producing transgenic silkworms displaying DsRed fluorescence in their eyes. Genotyping analysis confirmed the integration of the MaSp1 gene into the genome of the transgenic silkworms, and silk protein analysis revealed its expression and secretion in the cocoon. Compared with wild-type silk, the recombinant silk displayed a higher tensile strength and elasticity. The results indicate the potential for producing recombinant spider silk in transgenic B. mori.

The Anopheles gambiae salivary protein gSG6: an anopheline-specific protein with a blood-feeding role

The Anopheles gambiae salivary gland protein 6 (gSG6) is a small protein specifically found in the salivary glands of adult female mosquitoes. We report here the expression of a recombinant form of the protein and we show that in vivo gSG6 is expressed in distal-lateral lobes and is secreted with the saliva while the female mosquito probes for feeding. Injection of gSG6 dsRNA into adult A. gambiae females results in decreased gSG6 protein levels, increased probing time and reduced blood feeding ability. gSG6 orthologs have been found so far only in the salivary glands of Anopheles stephensi and Anopheles funestus, both members of the Cellia subgenus. We report here the gSG6 sequence from five additional anophelines, four species of the A. gambiae complex and Anopheles freeborni, a member of the subgenus Anopheles. We conclude that gSG6 plays some essential blood feeding role and was recruited in the anopheline subfamily most probably after the separation of the lineage which gave origin to Cellia and Anopheles subgenera.

Development and utilization of transgenic New World screwworm, Cochliomyia hominivorax

The New World screwworm (NWS), Cochliomyia hominivorax (Coquerel) (Diptera: Calliphoridae), was the first insect to be effectively controlled using the sterile insect technique (SIT). Recent efforts to improve SIT control of this species have centred on the development of genetically transformed strains using the piggyBac transposon vector system. Eight transgenic strains were produced incorporating an enhanced green fluorescent protein (EGFP) marker gene under polyubiquitin regulation that has the potential for use as a genetic marking system for released males. The transgenic strains were genetically and phenotypically characterized, including for life fitness parameters and mating competitiveness. These characteristics were unique for each strain and thus some strains were deemed suitable for incorporation into SIT eradication programmes. The strain with the best attributes is designated 'CLAY'. Four of the strains, including CLAY, have been successfully cryopreserved so that their original characteristics should be unchanged when further evaluation is required. With the demonstration of efficient germ-line transformation in NWS, allowing production of fit and competitive transformants, it is now possible to consider further transgenic strain development to improve SIT that are currently being tested in other dipteran species. This includes strains that allow genetic marking with fluorescent proteins, genetic sexing by female lethality, male-specific fluorescent sorting and male sterility by testis-specific lethality. The SIT may also be improved upon by new strategies resulting in lethality of offspring of released insects using conditional lethal systems based upon temperature-dependent or dietary tetracycline regulation of lethal gene expression. Both the creation of new NWS transgenic strains and the ecological safety of their release will be enhanced by new vector systems that allow specific genomic targeting of vector constructs and their subsequent immobilization, ensuring transgene and strain stability.

Analysis of apyrase 5' upstream region validates improved Anopheles gambiae transformation technique

Background

Genetic transformation of the malaria mosquito Anopheles gambiae has been successfully achieved in recent years, and represents a potentially powerful tool for researchers. Tissue-, stage- and sex-specific promoters are essential requirements to support the development of new applications for the transformation technique and potential malaria control strategies. During the Plasmodium lifecycle in the invertebrate host, four major mosquito cell types are involved in interactions with the parasite: hemocytes and fat body cells, which provide humoral and cellular components of the innate immune response, midgut and salivary glands representing the epithelial barriers traversed by the parasite during its lifecycle in the mosquito.

Findings

We have analyzed the upstream regulatory sequence of the An. gambiae salivary gland-specific apyrase (AgApy) gene in transgenic An. gambiae using a piggyBac transposable element vector marked by a 3xP3 promoter:DsRed gene fusion. Efficient germ-line transformation in An. gambiae mosquitoes was obtained and several integration events in at least three different G₀ families were detected. LacZ reporter gene expression was analyzed in three transgenic lines/groups, and in only one group was tissue-specific expression restricted to salivary glands.

Conclusion

Our data describe an efficient genetic transformation of An. gambiae embryos. However, expression from the selected region of the AgApy promoter is weak and position effects may mask tissue- and stage- specific activity in transgenic mosquitoes.

A new versatile gene-trap vector for insect transgenics

A new piggyBac-based gene-trap vector, pB-GT1, was constructed. pB-GT1 contains three marker genes, dsRed, Gal4, and EGFP. dsRed is under the control of the constitutive 3xP3 promoter, which induces dsRed expression wherever the vector is inserted in the host genome. The Gal4 sequence has no promoter but is preceded by the splice acceptor site so that it can be transcribed as a transcript fused with the host exon 5' to the insertion site. EGFP is driven by the constitutive ie+hr promoter but lacks a poly(A)(+) signal sequence, and thus the EGFP expression is detectable only when its transcript is fused with the host exon 3' downstream of the insertion. By the microinjection of the vector into fertilized eggs, we obtained transgenic Drosophila with a single copy of pB-GT1, which was inserted into the first intron of the ovo gene. The female flies of this transgenic line are sterile, indicating that the insertion inactivated the ovo gene, generating a new allele of this locus, ovo(pB-GT1). RT-PCR analysis demonstrated that an ovo-Gal4-fusion transcript is produced in ovo(pB-GT1) flies. The fact that UAS-EGFP reporter expression was detected in ovo(pB-GT1) germ cells in a pattern similar to that reported for wild-type ovo indicates that functional Gal4 is expressed via pB-GT1, recapitulating the endogenous expression pattern of the trapped gene. pB-GT1 is thus useful in insect genomics for the efficient assignment of functions of individual genes.

Transgene expression and silencing in a tick cell line: A model system for functional tick genomics

The genome project of the black legged tick, Ixodes scapularis, provides sequence data for testing gene function and regulation in this important pathogen vector. We tested Sleeping Beauty (SB), a Tc1/mariner group transposable element, and cationic lipid-based transfection reagents for delivery and genomic integration of transgenes into I. scapularis cell line ISE6. Plasmid DNA and dsRNA were effectively transfected into ISE6 cells and they were successfully transformed to express a red fluorescent protein (DsRed2) and a selectable marker, neomycin phosphotransferase (NEO). Frequency of transformation was estimated as 1 transformant per 5000-10,000 cells and cultures were incubated for 2-3 months in medium containing the neomycin analog G418 in order to isolate transformants. Genomic integration of the DsRed2 transgene was confirmed by inverse PCR and sequencing that demonstrated a TA nucleotide pair inserted between SB inverted/direct repeat sequences and tick genomic sequences, indicating that insertion of the DsRed2 gene into the tick cell genome occurred through the activity of SB transposase. RNAi using dsRNA transcribed from the DsRed2 gene silenced expression of red fluorescent protein in transformed ISE6 cells. SB transposition in cell line ISE6 provides an effective means to explore the functional genomics of I. scapularis.

Transgenesis upgrades for Drosophila melanogaster

Drosophila melanogaster is a highly attractive model system for the study of numerous biological questions pertaining to development, genetics, cell biology, neuroscience and disease. Until recently, our ability to manipulate flies genetically relied heavily on the transposon-mediated integration of DNA into fly embryos. However, in recent years significant improvements have been made to the transgenic techniques available in this organism, particularly with respect to integrating DNA at specific sites in the genome. These new approaches will greatly facilitate the structure-function analyses of Drosophila genes, will enhance the ease and speed with which flies can be manipulated, and should advance our understanding of biological processes during normal development and disease.

Gene expression studies in mosquitoes

Research on gene expression in mosquitoes is motivated by both basic and applied interests. Studies of genes involved in hematophagy, reproduction, olfaction, and immune responses reveal an exquisite confluence of biological adaptations that result in these highly-successful life forms. The requirement of female mosquitoes for a bloodmeal for propagation has been exploited by a wide diversity of viral, protozoan and metazoan pathogens as part of their life cycles. Identifying genes involved in host-seeking, blood feeding and digestion, reproduction, insecticide resistance and susceptibility/refractoriness to pathogen development is expected to provide the bases for the development of novel methods to control mosquito-borne diseases. Advances in mosquito transgenesis technologies, the availability of whole genome sequence information, mass sequencing and analyses of transcriptomes and RNAi techniques will assist development of these tools as well as deepen the understanding of the underlying genetic components for biological phenomena characteristic of these insect species.

Structural characterization of the medfly hsp83 gene and functional analysis of its proximal promoter region in vivo by germ-line transformation

In order to define the regulatory elements responsible for the expression of the medfly hsp83 (Cchsp83) gene, we determined the sequence of a genomic region of the gene that included 3,536 bp upstream of the transcription initiation site, the first untranslated exon of 144 bp, a 275-bp intron, and 516 bp of the second coding exon. Structural analysis of the 5' flanking region revealed the presence of a typical TATA box, 28 bp upstream of the transcription start site, and seven putative heat shock elements (HSEs) further upstream. The 5' untranslated region of the Cchsp83 mRNA was found to contain extensive secondary structure in the first 126 nucleotides. We carried out deletion functional analysis of the proximal promoter region (-380/+139) in vivo by germ line transformation using the lacZ as a reporter gene. We found that sequences in the -380/-86 region are essential for the constitutive expression of the Cchsp83 gene. Under normal conditions, the -380/+139 region was able to drive significant levels of transgene expression in all developmental stages of the medfly as well as in the ovaries and testis. In most stages, the temporal expression pattern of the reporter gene was similar to the respective pattern of the endogenous Cchsp83 gene. Although the -380/+139 promoter region contained two putative HSEs, it was found unable to confer any heat-induced expression in the reporter gene.

The DNA transposon Minos as a tool for transgenesis and functional genomic analysis in vertebrates and invertebrates

Transposons are powerful tools for conducting genetic manipulation and functional studies in organisms that are of scientific, economic, or medical interest. Minos, a member of the Tc1/mariner family of DNA transposons, exhibits a low insertional bias and transposes with high frequency in vertebrates and invertebrates. Its use as a tool for transgenesis and genome analysis of rather different animal species is described.

Comparative analysis of binary expression systems for directed gene expression in transgenic insects

Binary expression systems are of key interest to functional gene analysis by over- or misexpression. The application of such systems in diverse organisms would allow the study of many biological problems not addressable in model organisms. Here we report a set of constructs and an effective kinetic approach to quantitatively compare a series of diverse binary expression systems based on GAL4/UAS, LexA/(LL)(4) and tetracycline-controlled tTA/TRE. By the use of these constructs, we could show that in Drosophila melanogaster the yeast-derived GAL4/UAS systems are more effective in activating responder gene expression than the bacterial-derived LexA/(LL)(4) and tTA/TRE systems. The constructs are embedded in broad-range piggyBac-based transposon vectors and the transactivators are driven by the widely applicable 3xP3 promoter. These constructs should therefore be transferable to evaluate the functionality of binary expression systems in non-model insect species.

Temporal and spatial control of transgene expression using laser induction of the hsp70 promoter

Background

Precise temporal and spatial regulation of transgene expression is a critical tool to investigate gene function in developing organisms. The most commonly used technique to achieve tight control of transgene expression, however, requires the use of specific DNA enhancers that are difficult to characterize in non-model organisms. Here, we sought to eliminate the need for this type of sequence-based gene regulation and to open the field of functional genetics to a broader range of organisms.

Results

We have developed a new laser mediated method to heat shock groups of cells that provides precise spatio-temporal control of gene expression without requiring knowledge of specific enhancer sequences. We tested our laser-system in a transgenic line of Bicyclus anynana butterflies containing the EGFP reporter gene attached to the heat sensitive hsp70 promoter of Drosophila melanogaster. Whole organismal heat shocks demonstrated that this Drosophila promoter can drive gene expression in butterflies, and the subsequent laser heat shocks showed that it was possible to activate cell-specific gene expression in very precise patterns on developing pupal wings.

Conclusion

This laser-mediated gene expression system will enable functional genetic investigations, i.e., the ectopic expression of genes and their knock-down in targeted groups of cells in model and non-model organisms with little or no available regulatory data, as long as a compatible heat-shock promoter is used and the target tissue is accessible to a laser beam. This technique will also be useful in evolutionary developmental biology as it will enable the study of the evolution of gene function across a variety of organisms.

Germ-line transformation and RNAi of the ladybird beetle, Harmonia axyridis

To elucidate the molecular mechanisms underlying the tremendous diversity of insect wing colour patterns, it is imperative to identify and functionally characterize the genes involved in this developmental process. Here we report the first successful germ-line transformation using the transposable element vector piggyBac in the ladybird beetle Harmonia axyridis, which demonstrates typical genetic polymorphism in its wing colour patterns. The transformation efficiency by piggyBac was 3.7% per fertile G(0). We investigated the effectiveness of RNAi in Harmonia by injecting EGFP (enhanced green fluorescent protein) dsRNA into early transgenic EGFP-expressing embryos and observed substantial reduction of EGFP fluorescence in 87.2% of hatched larvae. Application of these new genetic tools to non-model insects such as Harmonia will facilitate the broad understanding of developmental mechanisms and evolutionary processes that are inaccessible using established model systems.

Insulated piggyBac vectors for insect transgenesis

Background

Germ-line transformation of insects is now a widely used method for analyzing gene function and for the development of genetically modified strains suitable for pest control programs. The most widely used transposable element for the germ-line transformation of insects is piggyBac. The site of integration of the transgene can influence gene expression due to the effects of nearby transcription enhancers or silent heterochromatic regions. Position effects can be minimized by flanking a transgene with insulator elements. The scs/scs' and gypsy insulators from Drosophila melanogaster as well as the chicken β-globin HS4 insulator function in both Drosophila and mammalian cells.

Results

To minimize position effects we have created a set of piggyBac transformation vectors that contain either the scs/scs', gypsy or chicken β-globin HS4 insulators. The vectors contain either fluorescent protein or eye color marker genes and have been successfully used for germ-line transformation of Drosophila melanogaster. A set of the scs/scs' vectors contains the coral reef fluorescent protein marker genes AmCyan, ZsGreen and DsRed that have not been optimized for translation in human cells. These marker genes are controlled by a combined GMR-3xP3 enhancer/promoter that gives particularly strong expression in the eyes. This is also the first report of the use of the ZsGreen and AmCyan reef fluorescent proteins as transformation markers in insects.

Conclusion

The insulated piggyBac vectors should protect transgenes against position effects and thus facilitate fine control of gene expression in a wide spectrum of insect species. These vectors may also be used for transgenesis in other invertebrate species.

Go with the glow: fluorescent proteins to light transgenic organisms

Once a biological novelty known for their role in bioluminescence, fluorescent proteins (FPs) from marine invertebrates have revolutionized the life sciences. Organisms from all kingdoms have been transformed with the Aequorea victoria green fluorescent protein (GFP), and biotechnology has been advanced by the use of FPs. This article reviews the current uses of FPs in whole transgenic organisms and genomics and looks beyond GFP to the complete color palette and spectral properties afforded by FPs from other marine organisms. Coupled with electronic devices for visualizing and quantifying FPs, recently cloned FP genes might be useful for the ecological monitoring of transgenic organisms in the environment. Therefore, this review also addresses the in vivo labeling of organisms with an emphasis on plants.

Mosquito transgenesis: what is the fitness cost?

The generation of transgenic mosquitoes with a minimal fitness load is a prerequisite for the success of strategies for controlling mosquito-borne diseases using transgenic insects. It is important to assemble as much information as possible on this subject because realistic estimates of transgene fitness costs are essential for modeling and planning release strategies. Transgenic mosquitoes must have minimal fitness costs, because such costs would reduce the effectiveness of the genetic drive mechanisms that are used to introduce the transgenes into field mosquito populations. Several factors affect fitness of transgenic mosquitoes, including the potential negative effect of transgene products and insertional mutagenesis. Studies to assess fitness of transgenic mosquitoes in the field (as opposed to the laboratory) are still needed.

Site-specific genomic targeting in Drosophila

Transcriptional regulation of transgenes depends upon genomic localization in higher eukaryotes. For the applied use of transgenic organisms as producers of pharmaceutically relevant proteins or as pest population control agents, a method to make transgene expression predictable is highly desirable. A targeting method that allows precise cassette replacement comprising solely genes of interest (without extraneous donor vector sequences) would be highly advantageous for insects and other multicellular organisms. In this report, we describe a method for transgene targeting to predefined chromosomal sites in Drosophila by using a transposon vector that, once integrated in the germ line, acts as an acceptor site for donor vectors. To make recombinational insertions irreversible, a FLP recombinase-mediated cassette exchange strategy was used, and to enhance donor-target pairing, a homing sequence from the linotte locus was used. Site-specific recombinants were screened by interconvertible eye fluorescence marker phenotypes yielding, on average, targeted insertions at a frequency of 23%. The cassette exchange system provides for repetitive integrations into the same locus, allowing comparative analysis of true transgenic alleles. Furthermore, this method was used to stabilize a targeted transgene by the postintegration excision of putatively mobile transposon sequences. The genomic targeting and stabilization strategy described for Drosophila should be applicable to other insects, specifically for the goals of optimizing heterologous protein expression and enhancing ecological safety of transgenic strains intended for release in biocontrol programs.

Precocious metamorphosis in transgenic silkworms overexpressing juvenile hormone esterase

Insect growth and development are intricately regulated by the titers of juvenile hormones (JHs) and ecdysteroids (and/or their metabolites) in the insect hemolymph. Hydrolysis of the methyl ester of JH by a JH-specific esterase (JHE) is a key pathway for the degradation of JH. Here, we generate transgenic silkworm strains that overexpress JHE by using the binary GAL4/UAS system. Overexpression of JHE from the embryonic stage resulted in larval-pupal metamorphosis after the third stadium, two stadia earlier than that observed in wild-type insects. This precocious metamorphosis suggests that JHs are not critical for normal development of embryo or larva before the second molt in Lepidoptera (moths and butterflies). Our transgenic approach allowed us to dissect the function of key physiological events that occur from embryogenesis. Until now, these types of studies were possible only in later larval stadia by using physical techniques such as allatectomy or the application of JH analogues. We believe that our system will allow further pioneering studies in insect physiology.

Functional analysis of the chicken delta1-crystallin enhancer activity in Drosophila reveals remarkable evolutionary conservation between chicken and fly

Functional conservation of enhancers among evolutionarily diverged organisms is a powerful way to identify basic regulatory circuits and key developmental regulators. This is especially applicable to Crystallin genes. Despite unexpected heterogeneity and diversity in their DNA sequences, many studies have revealed that most of the Crystallin genes are regulated by a relatively small set of developmentally important transcription factors. The chicken delta1-crystallin is one of the best-characterized Crystallin genes. Its lens-specific regulation is governed by a 30 bp long DC5 fragment present in the third intron of the gene. DC5 contains PAX6 and SOX2 binding sites, and its activity depends on the cooperative binding of these two transcription factors. To test the idea that Pax6 and Sox2, together with the DC5 enhancer, could form a basic regulatory circuit functional in distantly related animals, we introduced the DC5 fragment into Drosophila and studied its activation pattern and regulation. The results show that the DC5 enhancer is not only active in the compound eye but, remarkably, is specifically active in those cells responsible for Crystallin secretion in Drosophila, i.e. the cone cells. However, regulation of the DC5 enhancer is carried out not by Pax6, but by Pax2 (D-Pax2; shaven--FlyBase) in combination with the Sox2 homologue SoxN. Both proteins (D-PAX2 and SOXN) bind cooperatively to the DC5 fragment and activate the enhancer synergistically. As PAX6 and PAX2 proteins derive from the same ancestor, we propose that during evolution Pax6 function in vertebrate lens development was retained by Pax2 in Drosophila.

Establishing genetic transformation for comparative developmental studies in the crustacean Parhyale hawaiensis

The amphipod crustacean Parhyale hawaiensis has been put forward as an attractive organism for evolutionary developmental comparisons, and considerable effort is being invested in isolating developmental genes and studying their expression patterns in this species. The scope of these studies could be significantly expanded by establishing means for genetic manipulation that would enable direct studies of gene functions to be carried out in this species. Here, we report the use of the Minos transposable element for the genetic transformation of P. hawaiensis. Transformed amphipods can be obtained from approximately 30% of surviving individuals injected with both a Minos element carrying the 3xP3-DsRed fluorescent marker and with mRNA encoding the Minos transposase. Integral copies of the transposon are inserted into the host genome and are stably inherited through successive generations. We have used reporter constructs to identify a muscle-specific regulatory element from Parhyale, demonstrating that this transformation vector can be used to test the activity of cis-regulatory elements in this species. The relatively high efficiency of this transgenic methodology opens new opportunities for the direct study of cis-regulatory elements and gene functions in Parhyale, allowing functional studies to be carried out beyond previously established model systems in insects.

Efficient transformation of the beetle Tribolium castaneum using the Minos transposable element: quantitative and qualitative analysis of genomic integration events

Genetic transformation in insects holds great promise as a tool for genetic manipulation in species of particular scientific, economic, or medical interest. A number of transposable elements have been tested recently as potential vectors for transformation in a range of insects. Minos is one of the most promising elements because it appears to be active in diverse species and has the capacity to carry large inserts. We report here the use of the Minos element as a transformation vector in the red flour beetle Tribolium castaneum (Coleoptera), an important species for comparative developmental and pest management studies. Transgenic G(1) beetles were recovered from 32.4% of fertile G(0)'s injected with a plasmid carrying a 3xP3-EGFP-marked transposon and in vitro synthesized mRNA encoding the Minos transposase. This transformation efficiency is 2.8-fold higher than that observed when using a plasmid helper. Molecular and genetic analyses show that several independent insertions can be recovered from a single injected parent, but that the majority of transformed individuals carry single Minos insertions. These results establish Minos as one of the most efficient vectors for genetic transformation in insects. In combination with piggyBac-based transgenesis, our work allows the introduction of sophisticated multicomponent genetic tools in Tribolium.

Isolation of white gene orthologue of the sawfly, Athalia rosae (Hymenoptera) and its functional analysis using RNA interference

We isolated and characterized the white gene orthologue of the sawfly, Athalia rosae (Hymenoptera). The A. rosae white (Ar white) cDNA cloned was 2058-bp long encoding 685 amino acids in a single open reading frame (ORF). Comparison of the cDNA sequence with the genomic DNA sequence revealed that the ORF was derived from 11 exons. Ar white was a single copy gene as evidenced by genomic Southern blotting and its cytological localization on the metaphase chromosomes. The deduced amino acid sequence aligned well with known insect white orthologous gene products sharing conserved regions such as the ATP-binding motif and the six transmembrane-spanning segments. Expression of Ar white was detected at embryonic and pupal stages by Northern blotting. In situ hybridization detected the embryonic expression in a pair of the lateral tips of protocephalic placodes from where optic organs are formed. Ar white function was examined using double-stranded RNA (dsRNA)-mediated interference. The synthesized dsRNA targeting Ar white transcripts caused a decrease in the level of the original mRNAs, and resulted in the white phenocopy in the embryonic eye pigmentation when microinjected into eggs from wild-type females. The effects occurred in a dose-dependent manner.

The genetics and genomics of the silkworm, Bombyx mori

We review progress in applying molecular genetic and genomic technologies to studies in the domesticated silkworm, Bombyx mori, highlighting its use as a model for Lepidoptera, and in sericulture and biotechnology. Dense molecular linkage maps are being integrated with classical linkage maps for positional cloning and marker-assisted selection. Classical mutations have been identified by a candidate gene approach. Cytogenetic and sequence analyses show that the W chromosome is composed largely of nested full-length long terminal repeat retrotransposons. Z-chromosome-linked sequences show a lack of dosage compensation. The downstream sex differentiation mechanism has been studied via the silkworm homolog of doublesex. Expressed sequence tagged databases have been used to discover Lepidoptera-specific genes, provide evidence for horizontal gene transfer, and construct microarrays. Physical maps using large-fragment bacterial artificial chromosome libraries have been constructed, and whole-genome shotgun sequencing is underway. Germline transformation and transient expression systems are well established and available for functional studies, high-level protein expression, and gene silencing via RNA interference.

SVZa neural stem cells differentiate into distinct lineages in response to BMP4

Neural stem cells (NSCs) reside in the anterior portion of the forebrain subventricular zone (SVZa) and generate the progenitors which will differentiate into neurons, and via a tangential migratory pathway, known as the rostral migratory stream (RMS), migrate to the olfactory bulbs (OB). Bone morphogenetic proteins (BMPs) play significant roles in neural development at different stages and locations, but their roles have not been determined in the SVZa. To explore possible roles of BMPs in SVZa NSCs, BMP4 at various concentrations were tested for their capacity to induce SVZa NSCs. The expression of BMP4 was also examined in living cells using a reportor vector, in which the BMP4 promotor was conjugated with red fluorescent protein (RFP). In the meantime, the differentiation of SVZa NSCs was dynamically monitored by using reportor vectors of the Nestin enhancer and the promoters of TH and GFAP. In the OB, high expression of BMP4 was found using both promoter activity analysis and in situ hybridization. However, low BMP4 expression was found in the RMS and only moderate expression of BMP4 was displayed in the SVZa. The results also demonstrated that low concentrations (1-5 ng/ml) of BMP4 promoted the proliferation of SVZa NSCs but high concentrations (10-100 ng/ml) of BMP4 inhibited this proliferation. BMP4 enhanced neuron commitment before 4 days but inhibited it after 4 days. As the antagonist of BMP4, Noggin almost completely blocked all these BMP4 responses. Thus, our findings indicate that BMP4 promotes the exit from the cell cycle and triggers the differentiation of neuron progenitors in the OB. BMP4 also promotes the proliferation of the committed neuron progenitors in the RMS, but in the SVZa, BMP4 may facilitate the commitment of NSCs into astrocytes.

Post-integration stabilization of a transposon vector by terminal sequence deletion in Drosophila melanogaster

Germline transformation systems for nearly 20 insect species have been derived from transposable elements, allowing the development of transgenic insects for basic and applied studies. These systems use a defective nonautonomous vector that results in stable vector integrations after the disappearance of transiently provided transposase helper plasmid, which is essential to maintain true breeding lines and consistent transgene expression that would otherwise be lost after vector remobilization. The risk of remobilization by an unintended transposase source has so far not been a concern for laboratory studies, but the prospective use of millions of transgenic insects in biocontrol programs will likely increase the risk, therefore making this a critical issue for the ecological safety of field release programs. Here we describe an efficient method that deletes a terminal repeat sequence of a transposon vector after genomic integration. This procedure prevents transposase-mediated remobilization of the other terminal sequence and associated genes, ensuring their genomic stability.

Insect transgenesis and its potential role in agriculture and human health

The ability to genetically engineer insects other than Drosophila melanogaster has further extended modern genetic techniques into important insect pest species ranging from fruit fly pests of horticulture to mosquito vectors of human disease. In only a relatively short period of time, a range of transgenes have been inserted into more than 10 insect pest species. Genetic transformation of these pest species has proven to be a very important laboratory tool in analyzing gene function and effects on phenotype however the full extension of this technology into the field is yet to be realized. Here we briefly review the development of transgenic technology in pest insect species and discuss the challenges that remain in this applied area of insect genetics and entomology.

Understanding and improving transgene stability and expression in insects for SIT and conditional lethal release programs

Genetically transformed insect pests provide significant opportunities to create strains for improved sterile insect technique and new strategies based on conditional lethality. A major concern for programs that rely on the release of transgenic insects is the stability of the transgene, and maintenance of consistent expression of genes of interest within the transgene. Transgene instability would influence the integrity of the transformant strain upon which the effectiveness of the biological control program depends. Loss or intra-genomic transgene movement would result in strain attributes important to the program being lost or diminished, and the mass-release of such insects could significantly exacerbate the insect pest problem. Instability resulting in intra-genomic movement may also be a prelude to inter-genomic transgene movement between species resulting in ecological risks. This is less of a concern for short-term releases, where transgenic insects are not expected to survive in the environment beyond two or three generations. Transgene movement may occur, however, into infectious agents during mass-rearing, and the potential for movement after release is a possibility for programs using many millions of insects. The primary methods of addressing potential transgene instability relate to an understanding of the vector system used for gene transfer, the potential for its mobilization by the same or a related vector system, and methods required to identify transformants and determine if unexpected transgene movement has occurred. Methods also exist for preventing transposon-mediated mobilization, by deleting or rearranging vector sequences required for transposition using recombination systems. Stability of transgene expression is also a critical concern, especially in terms of potential epigenetic interactions with host genomes resulting in gene silencing that have been observed in plants and fungi, and it must be determined if this or related phenomena can occur in insects.

The ecdysone regulatory cascade and ovarian development in lepidopteran insects: insights from the silkmoth paradigm

The developing ovariole of the silkmoth Bombyx mori represents an excellent model for studies on the changes that occur in gene expression during the execution of long-term developmental programs. All stages of follicle development, which differ from each other by 2-2.5 h of developmental time, are organized in a single array and can be isolated simultaneously for physiological, biochemical and gene expression studies from a single animal. Recently, significant progress has been made toward the understanding of the hormonal regulation and autonomous implementation of the developmental program that governs follicular cell differentiation during oogenesis in Bombyx. In this review, the developmental career of the ovarian follicle during pharate adult development is discussed in view of the new physiological, biochemical and gene expression data that have recently accumulated. An overview is presented of the changes in expression of structural and regulatory genes and their hormonal regulation in the developing follicle during the transitions among the broad developmental periods of previtellogenesis, vitellogenesis and choriogenesis. Ovarian development in silkmoth pharate adults is induced by 20-hydroxy-ecdysone (20E) through the activation of a regulatory cascade similar to the one observed during Drosophila development. The transition from previtellogenesis to vitellogenesis corresponds to a late response to the hormone and is characterized by the induction of the expression of a unique isoform of the nuclear receptor BmHR3 and the follicular cell-specific yolk protein ESP. The transition from early to middle and late vitellogenesis and choriogenesis, on the other hand, is regulated by positively and negatively acting intra- and extra-ovarian factors. In vitro cultures of developing ovarioles reveal the requirement for the presence of an as yet unidentified growth factor(s) in the hemolymph, while the follicle developmental arrest that is observed after treatment with the ecdysone agonist tebufenozide indicates the requirement for a decline in the level of 20E. The initiation of choriogenesis is characterized by the transcriptional activation of the gene BmGATAbeta that encodes GATA transcription factors, and the chorion genes in the follicular cells. Furthermore, modulation of the activity of BmGATAbeta at the posttranscriptional level is crucial for the stage-specific activation of chorion genes during late choriogenesis. The developing ovariole of the silkmoth is, therefore, emerging as an important model for the study of insect oogenesis and the action of the steroid hormone 20E at the molecular level.

Targeted Gene Expression Using the GAL4/UAS System in the Silkworm Bombyx mori

The silkworm Bombyx mori is one of the most well-studied insects in terms of both genetics and physiology and is recognized as the model lepidopteran insect. To develop an efficient system for analyzing gene function in the silkworm, we investigated the feasibility of using the GAL4/UAS system in conjunction with piggyBac vector-mediated germ-line transformation for targeted gene expression. To drive the GAL4 gene, we used two endogenous promoters that originated from the B. mori actin A3 (BmA3) and fibroin light-chain (FiL) genes and the artificial promoter 3xP3. GFP was used as the reporter. In initial tests of the function of the GAL4/UAS system, we generated transgenic animals that carried the UAS-GFP construct plus either BmA3-GAL4 or 3xP3-GAL4. GFP fluorescence was observed in the tissues of GFP-positive animals, in which both promoters drove GAL4 gene expression. Animals that possessed only the GAL4 gene or UAS-GFP construct did not show GFP fluorescence. In addition, as a further test of the ability of the GAL4/UAS system to drive tissue-specific expression we constructed FiL-GAL4 lines with 3xP3-CFP as the transformation marker. FiL-GAL4 × UAS-GFP crosses showed GFP expression in the posterior silk gland, in which the endogenous FiL gene is normally expressed. These results show that the GAL4/UAS system is applicable to B. mori and emphasize the potential of this system for controlled analyses of B. mori gene function.

piggyBac-based insertional mutagenesis in the presence of stably integrated P elements in Drosophila

P element-mediated mutagenesis has been used to disrupt an estimated 25% of genes essential for Drosophila adult viability. Mutation of all genes in the fly genome, however, poses a problem, because P elements show significant hotspots of integration. In addition, advanced screening scenarios often require the use of P element-based tools like the generation of germ-line mosaics using FLP recombinase-mediated recombination or gene misexpression using the UAS/Gal4 system. These techniques are P element-based and can therefore not be combined with the use of P elements as mutagenic agents. To circumvent these limitations, we have developed an insertional mutagenesis system using non-P element transposons. An enhanced yellow fluorescent protein-marked piggyBac-based mutator element was mobilized by a piggyBac specific transposase source expressed from a Hermes-based jump-starter transposon marked with enhanced cyan fluorescent protein. In a pilot screen, we have generated 798 piggyBac insertions on FRT bearing third chromosomes of which 9% have sustained a putatively piggyBac-related lethal hit. The FRTs present on the target chromosome remained stably integrated during the screen and could subsequently be used to generate germ-line clones associated with maternal and zygotic phenotypes. PCR-based analysis of insertion loci shows that 57% of the insertions are in genes for which no P element insertions have been reported. Our data demonstrate the potential of this technique to facilitate the quest for saturation mutagenesis of the Drosophila genome. The system is Drosophila nonspecific and potentially applicable in a broad spectrum of nonmodel organisms.

Germline transformation of the sawfly, Athalia rosae (Hymenoptera: Symphyta), mediated by a piggyBac-derived vector

A piggyBac construct carrying two green fluorescent protein (GFP)-coding sequences one driven by Bombyx mori actin gene promoter and the other by Drosophila melanogaster heat-shock protein 70 (hsp70) promoter were injected together with a nonautonomous helper plasmid containing an active piggyBac transposase gene into the posterior end of mature unfertilized eggs dissected from the ovaries of Athalia rosae (Hymenoptera: Symphyta). These injected eggs, which developed as haploid male embryos upon artificial activation, were cultured to adulthood. Of 278 injected eggs, 61 grew to G(0) haploid adult males. These G(0) haploid males were individually mated to diploid females. The progeny embryos (G(1) generation) were examined for GFP expression. Four GFP-positive embryos (from three independent G(0) matings) were obtained. Two eclosed as diploid adult G(1) females. Mature unfertilized eggs dissected from the GFP-positive G(1) diploid females were activated artificially, and the resultant embryos were examined for GFP expression, separated and cultured to adulthood (G(2) generation). The G(2) haploid embryos segregated to GFP-positive and -negative individuals. By mating the G(2) adult haploid males individually to diploid females, stocks were established in which the piggyBac construct was stably integrated into the genome, as evidenced by GFP expression and Southern blot hybridization. The piggyBac transposition occurred at its canonical target TTAA sequence. These results, which demonstrate the first successful stable transposon-mediated germline transformation in Hymenoptera, will expand the usefulness of the piggyBac vector.

Innovations: applications of insect transgenesis

The recent establishment of broadly applicable genetic transformation systems will allow the analysis of gene function in diverse insect species. This will increase our understanding of developmental and evolutionary biology. Furthermore, insect transgenesis will provide new strategies for insect pest management and methods to impair the transmission of pathogens by human disease vectors. However, these powerful techniques must be applied with great care to avoid harm to our environment.

piggyBac-based insertional mutagenesis and enhancer detection as a tool for functional insect genomics

Transposon mutagenesis provides a fundamental tool for functional genomics. Here we present a non-species-specific, combined enhancer detection and binary expression system based on the transposable element piggyBac: For the different components of this insertional mutagenesis system, we used widely applicable transposons and distinguishable broad-range transformation markers, which should enable this system to be operational in nonmodel arthropods. In a pilot screen in Drosophila melanogaster, piggyBac mutator elements on the X chromosome were mobilized in males by a Hermes-based jumpstarter element providing piggyBac transposase activity under control of the alpha1-tubulin promoter. As primary reporters in the piggyBac mutator elements, we employed the heterologous transactivators GAL4delta or tTA. To identify larval and adult enhancer detectors, strains carrying UASp-EYFP or TRE-EYFP as secondary reporter elements were used. Tissue-specific enhancer activities were readily observed in the GAL4delta/UASp-based systems, but only rarely in the tTA/TRE system. Novel autosomal insertions were recovered with an average jumping rate of 80%. Of these novel insertions, 3.8% showed homozygous lethality, which was reversible by piggyBac excision. Insertions were found in both coding and noncoding regions of characterized genes and also in noncharacterized and non-P-targeted CG-number genes. This indicates that piggyBac will greatly facilitate the intended saturation mutagenesis in Drosophila.

A transgene-based, embryo-specific lethality system for insect pest management

Biological approaches to insect pest management offer alternatives to pesticidal control. In area-wide control programs that cover entire regions, the sterile insect technique (SIT) can be used to successfully suppress economically important pest species by the mass release of sterilized pest organisms. However, conventional sterilization by ionizing radiation reduces insect fitness, which can result in reduced competitiveness of the sterilized insects. Here we report a transgene-based, dominant embryonic lethality system that allows for generation of large quantities of competitive but sterile insects without the need of irradiation. The system involves the ectopic expression of a hyperactive pro-apoptotic gene that causes embryo-specific lethality when driven by the tetracycline-controlled transactivator (tTA) under the regulation of a cellularization gene enhancer-promoter. We have successfully tested this system in Drosophila melanogaster. The embryonic lethality can be suppressed maternally, which will allow it to be combined with transgenic female-specific lethality systems to raise only vigorous but sterile males.

Stable transformation of a Mamestra brassicae (lepidoptera) cell line with the lepidopteran-derived transposon piggyBac

Cabbage moth cells were transfected with the vector pBac[3xP3-EGFPafm] and helper phsp-pBac. Seventeen percent of the transfected cells showed stable EGFP-expression. This indicates successful and stable transformation of M. brassicae cells with a piggyBac-derived vector. Genomic integration of Bac[3xP3-EGFPafm] in stably transformed cells was confirmed by Southern blots and inverse PCR. Since the integrations are stable, and transfection with pBac[3xP3-EGFPafm] alone did not yield in transformations, no cross-reacting transposase activity seems present in M. brassicae cells. Moreover, Southern blotting with a probe for piggyBac transposase indicated the absence of piggyBac-related elements in the genome of Mamestra brassicae. Due to the tissue specificity of the 3xP3-EGFP marker for eye and nervous tissues, it is intriguing that 3xP3-EGFP can successfully be used to identify stably transformed M. brassicae cells of cell line IZD-MB0503, which is hemocyte-derived. Sequence analysis of the insertion sites showed that piggyBac inverted repeats were adjacent to TTAA sequences on both termini in all the clones. The present results are particularly important as they suggest that piggyBac can be used for transgenesis of cabbage moth cells.

Use of the piggyBac transposon for germ-line transformation of insects

Germ-line transformation of insects is now possible with four independent transposable element vector systems. Among these, the TTAA-insertion site specific transposon, piggyBac, discovered in Trichoplusia ni, is one of the most widely used. Transformations have been achieved in a wide variety of dipterans, lepidopterans, and a coleopteran, and for many species, piggyBac transposition was first tested by plasmid-based mobility assays in cell lines and embryos. All plasmid and genomic insertions are consistent with the duplication of a TTAA insertion site, and most germ-line integrations appear to be stable, though this is largely based on stable marker phenotypes. Of the vector systems presently in use for non-drosophilids, piggyBac is the only one not currently associated with a superfamily of transposable elements, though other elements exist that share its TTAA insertion site specificity. While functional piggyBac elements have only been isolated from T. ni, nearly identical elements have been discovered in a dipteran species, Bactrocera dorsalis, and closely related elements exist in another moth species, Spodoptera frugiperda. It appears that piggyBac has recently traversed insect orders by horizontal transmission, possibly mediated by a baculovirus or other viral system. This interspecies movement has important implications for the practical use of piggyBac to create transgenic insect strains for field release.