Structure of hermes integrations in the germline of the yellow fever mosquito, Aedes aegypti

Functional analysis of the ABCs of eye color in Helicoverpa armigera with CRISPR/Cas9-induced mutations

Many insect pigments are localized in subcellular pigment granules, and transport of pigment precursors from the cytoplasm is accomplished by ABC proteins. Drosophila melanogaster has three half-transporter genes (white, scarlet, and brown, all affecting eye pigments) and Bombyx mori has a fourth (ok). The White, Brown, Scarlet and Ok proteins each have one transmembrane and one cytoplasmic domain and they heterodimerize to form functional transporters with different substrate specificities. We used CRISPR/Cas9 to create somatic and germ-line knockout mutations of these four genes in the noctuid moth Helicoverpa armigera. Somatic knockouts of white block pigmentation of the egg, first instar larva and adult eye, but germ-line knockouts of white are recessive lethal in the embryo. Knockouts of scarlet are viable and produce pigmentless first instar larvae and yellow adult eyes lacking xanthommatin. Knockouts of brown show no phenotypic effects on viability or pigmentation. Knockouts of ok are viable and produce translucent larval cuticle and black eyes. CRISPR/Cas9-induced mutations are a useful tool for analyzing how essential and non-essential genes interact to produce the diversity of insect pigmentation patterns found in nature.

Transposable elements as tools for reshaping the genome: it is a huge world after all!

Transposable elements (TEs) are discrete pieces of DNA that can move from one site to another within genomes and sometime between genomes. They are found in all major branches of life. Because of their wide distribution and considerable diversity, they are a considerable source of genomic variation and as such, they constitute powerful drivers of genome evolution. Moreover, it is becoming clear that the epigenetic regulation of certain genes is derived from defense mechanisms against the activity of ancestral transposable elements. TEs now tend to be viewed as natural molecular tools that can reshape the genome, which challenges the idea that TEs are natural tools used to answer biological questions. In the first part of this chapter, we review the classification and distribution of TEs, and look at how they have contributed to the structural and transcriptional reshaping of genomes. In the second part, we describe methodological innovations that have modified their contribution as molecular tools.

Insect transgenesis: current applications and future prospects

The ability to manipulate the genomes of many insects has become a practical reality over the past 15 years. This has been led by the identification of several useful transposon vector systems that have allowed the identification and development of generalized, species-specific, and tissue-specific promoter systems for controlled expression of gene products upon introduction into insect genomes. Armed with these capabilities, researchers have made significant strides in both fundamental and applied transgenics in key model systems such as Bombyx mori, Tribolium casteneum, Aedes aegypti, and Anopheles stephensi. Limitations of transposon systems were identified, and alternative tools were developed, thus significantly increasing the potential for applied transgenics for control of both agricultural and medical insect pests. The next 10 years promise to be an exciting time of transitioning from the laboratory to the field, from basic research to applied control, during which the full potential of gene manipulation in insect systems will ultimately be realized.

Mosquito transgenic technologies to reduce Plasmodium transmission

The ability to introduce genetic constructs of choice into the genome of Anopheles mosquitoes provides a valuable tool to study the molecular interactions between the Plasmodium parasite and its insect host. In the long term, this technology could potentially offer new ways to control vector-borne diseases through the suppression of target mosquito populations or through the introgression of traits that preclude pathogen transmission. Here, we describe in detail protocols for the generation of transgenic Anopheles gambiae mosquitoes based on germ-line transformation using either modified transposable elements or the site-specific PhiC31 recombinase.

Mobility properties of the Hermes transposable element in transgenic lines of Aedes aegypti

The Hermes transposable element has been used to genetically transform a wide range of insect species, including the mosquito, Aedes aegypti, a vector of several important human pathogens. Hermes integrations into the mosquito germline are characterized by the non-canonical integration of the transposon and flanking plasmid and, once integrated, Hermes is stable in the presence of its transposase. In an effort to improve the post-integration mobility of Hermes in the germline of Ae. aegypti, a transgenic helper Mos1 construct expressing Hermes transposase under the control of a testis-specific promoter was crossed to a separate transgenic strain containing a target Hermes transposon. In less than 1% of the approximately 1,500 progeny from jumpstarter lines analyzed, evidence of putative Hermes germline remobilizations were detected. These recovered transposition events occur through an aberrant mechanism and provide insight into the non-canonical cut-and-paste transposition of Hermes in the germ line of Ae. aegypti.

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.

Towards the genetic control of insect vectors: An overview

Insects are responsible for the transmission of major infectious diseases. Recent advances in insect genomics and transformation technology provide new strategies for the control of insect borne pathogen transmission and insect pest management. One such strategy is the genetic modification of insects with genes that block pathogen development. Another is to suppress insect populations by releasing either sterile males or males carrying female-specific dominant lethal genes into the environment. Although significant progress has been made in the laboratory, further research is needed to extend these approaches to the field. These insect control strategies offer several advantages over conventional insecticide-based strategies. However, the release of genetically modified insects into the environment should proceed with great caution, after ensuring its safety, and acceptance by the target populations.

Post-integration stability of piggyBac in Aedes aegypti

The post-integration activity of piggyBac transposable element gene vectors in Aedes aegypti mosquitoes was tested under a variety of conditions. The embryos from five independent transgenic lines of Ae. aegypti, each with a single integrated non-autonomous piggyBac transposable element gene vector, were injected with plasmids containing the piggyBac transposase open-reading frame under the regulatory control of the Drosophila melanogaster hsp70 promoter. No evidence for somatic remobilization was detected in the subsequent adults whereas somatic remobilization was readily detected when similar lines of transgenic D. melanogaster were injected with the same piggyBac transposase-expressing plasmid. Ae. aegypti heterozygotes of piggyBac reporter-containing transgenes and piggyBac transposase-expressing transgenes showed no evidence of somatic and germ-line remobilization based on phenotypic and molecular detection methods. The post-integration mobility properties of piggyBac in Ae. aegypti enhance the utility of this gene vector for certain applications, particularly those where any level of vector remobilization is unacceptable.

nanos gene control DNA mediates developmentally regulated transposition in the yellow fever mosquito Aedes aegypti

Transposable elements (TEs) are proposed as a basis for developing drive systems to spread pathogen resistance genes through vector mosquito populations. The use of transcriptional and translational control DNA elements from genes expressed specifically in the insect germ line to mediate transposition offers possibilities for mitigating some of the concerns about transgene behavior in the target vector species and eliminating effects on nontarget organisms. Here, we describe the successful use of the promoter and untranslated regions from the nanos (nos) orthologous gene of the yellow fever mosquito, Aedes aegypti, to control sex- and tissue-specific expression of exogenously derived mariner MosI transposase-encoding DNA. Transgenic mosquitoes expressed transposase mRNA in abundance near or equal to the endogenous nos transcript and exclusively in the female germ cells. In addition, MosI mRNA was deposited in developing oocytes and localized and maintained at the posterior pole during early embryonic development. Importantly, four of five transgenic lines examined were capable of mobilizing a second MosI transgene into the mosquito genome, indicating that functional transposase was being produced. Thus, the nos control sequences show promise as part of a TE-based gene drive system.

Development and characterization of a double subgenomic chikungunya virus infectious clone to express heterologous genes in Aedes aegypti mosquitoes

Three full-length infectious cDNA clones based on the alphavirus chikungunya (CHIKV) were developed and characterized in vitro and in vivo. The full-length clone retained the viral phenotypes of CHIKV in both cell culture and in mosquitoes and should be a valuable tool for the study of virus interactions in an epidemiologically significant natural vector, Aedes aegypti. Two additional infectious clones were constructed that express green fluorescent protein (EGFP) in the midgut, salivary glands, and nervous tissue of Aedes aegypti mosquitoes following oral infection. The two constructs differed in the placement of the subgenomic promoter and the gene encoding EGFP. Viruses derived from the pCHIKic EGFP constructs (5' CHIKV EGFP and 3' CHIKV EGFP) expressed EGFP in 100% of the Ae. aegypti mosquitoes tested on days 7 and 14 post infection (p.i.). The 5' CHIKV EGFP disseminated to 90% of the salivary glands and nervous tissue by day 14 p.i. Dissemination rates of this new viral vector exceeds those of previous systems, thus expanding the repertoire and potential for gene expression studies on this important vector species.

Mutant Mos1 mariner transposons are hyperactive in Aedes aegypti

The development of genetic strategies to control the spread of mosquito-borne diseases through the use of class II transposons has been hampered by suboptimal rates of transformation and the absence of post-integration mobility for all transposons evaluated to date. Two Mos1 mariner transposase mutants were produced by the site-directed mutagenesis of amino acids, E137 and E264, to K and R, respectively. The effects of these mutations on the transpositional activities of Mos1-derived transposon constructs were evaluated by interplasmid transposition assays in Escherichia coli and Aedes aegypti. The transpositional activities of two Mos1 transposons, one with imperfect wild type inverted terminal repeats (ITRs) and another that contained two perfectly matched 3' ITRs, were increased when the mutant transposases were supplied in trans in E. coli. The use of the perfect repeat transposon with wild type transposase did not result in an increase in transposition frequency in Ae. aegypti. However, an improvement in the integrity of the transposition process did occur, as evidenced by a lower rate of recombination events in which the transgene was transferred. An increase in the transpositional activity of the perfect repeat transposon was observed in the mosquito in the presence of either mutant transposase, and in the case of the E264R transposase, the observed increase in transposition frequency was also accompanied by a further improvement in the integrity of transposition. We discuss the possible contributions of these mutant residues to the transposition of the perfect repeat Mos1 transposon, the implications of these results with respect to the molecular evolution of Mos1, and the potential uses of the perfect repeat transposon and mutant transposases for the improvement of Mos1 mediated germ line transformation of Ae. aegypti.

Gambol and Tc1 are two distinct families of DD34E transposons: analysis of the Anopheles gambiae genome expands the diversity of the IS630-Tc1-mariner superfamily

Tc1 is a family of DNA transposons found in diverse organisms including vertebrates, invertebrates and fungi. Tc1 belongs to the IS630-Tc1-mariner superfamily, which is characterized by common 'TA' target site and conserved D(Asp)DE(Glu) or DDD catalytic triad. All functional Tc1-like transposons contain a transposase with a DD34E catalytic triad. We conducted a systematic analysis of DD34E transposons in the African malaria mosquito, Anopheles gambiae, using a reiterative and exhaustive search program. In addition to previously described Tc1-like elements, we uncovered 26 new DD34E transposons including a novel family that we named gambol. Designation of family status to gambol is based on phylogenetic analyses of transposase sequences that showed gambol and Tc1 transposons as distinct clades that were separated by mariner and other families of the IS630-Tc1-mariner superfamily. The distinction between Tc1 and gambol is also consistent with the unique TIRs in gambol elements and the presence of a 'W[I/L/V]DEDC' signature near their N-termini. This signature is predicted as part of the 'RED' domain, a component of the 'PAI' and 'RED' DNA binding domains in Tc1 and possibly mariner. Although gambol appears to be related to a few DD34E transposons from cyanobacteria and fungi, no gambol has been reported in any other insects or animals thus far. Several gambol and Tc1 elements have intact ORFs and different genomic copies with high sequence identity, which suggests that they may have been recently active.

piggyBac internal sequences are necessary for efficient transformation of target genomes

A previously reported piggyBac minimal sequence cartridge, which is capable of efficient transposition in embryo interplasmid transposition assays, failed to produce transformants at a significant frequency in Drosophila melanogaster compared with full-length or less extensive internal deletion constructs. We have re-examined the importance of these internal domain (ID) sequences for germline transformation using a PCR strategy that effectively adds increasing lengths of ID sequences to each terminus. A series of these piggyBac ID synthetic deletion plasmids containing the 3xP3-ECFP marker gene are compared for germline transformation of D. melanogaster. Our analyses identify a minimal sequence configuration that is sufficient for movement of piggyBac vectored sequences from plasmids into the insect genome. Southern hybridizations confirm the presence of the piggyBac transposon sequences, and insertion site analyses confirm these integrations target TTAA sites. The results verify that ID sequences adjacent to the 5' and 3' terminal repeat domains are crucial for effective germline transformation with piggyBac even though they are not required for excision or interplasmid transposition. Using this information we reconstructed an inverted repeat cartridge, ITR1.1k, and a minimal piggyBac transposon vector, pXL-BacII-ECFP, each of which contains these identified ID sequences in addition to the terminal repeat configuration previously described as essential for mobility. We confirm in independent experiments that these new minimal constructs yield transformation frequencies similar to the control piggyBac vector. Sequencing analyses of our constructs verify the position and the source of a point mutation within the 3' internal repeat sequence of our vectors that has no apparent effect on transformation efficiency.

Flight muscle-specific expression of act88F: GFP in transgenic Culex quinquefasciatus Say (Diptera: Culicidae)

A strategy to engineer a strain of Culex mosquitoes refractory to filarial transmission is described. A requirement for success of the strategy is identification of a flight muscle-specific promoter that functions in the mosquito. A GFP marker gene under the control of the promoter region of the Drosophila melanogaster act88F gene was inserted into the genome of Culex quinquefasciatus. Transformation was confirmed by Mendelian genetics. Hybridization of a genomic Southern blot to a radiolabeled probe verified that the entire donor plasmid integrated into the mosquito genome. GFP expression in the transgenic mosquitoes was restricted to the flight muscles.

Mosquito transposable elements

The completion of the genome assembly for the African malaria mosquito, Anopheles gambiae, and continuing genomic efforts for the yellow fever mosquito, Aedes aegypti, have allowed the use of bioinformatics tools to identify and characterize a diverse array of transposable elements (TEs) in these and other mosquito genomes. An overview of the types and number of both RNA-mediated and DNA-mediated TEs that are found in mosquito genomes is presented. A number of novel and interesting TEs from these species are discussed in more detail. These findings have significant implications for our understanding of mosquito genome evolution and for future modifications of natural mosquito populations through the use of TE-mediated genetic transformation.

Bactrocera tryoni and closely related pest tephritids--molecular analysis and prospects for transgenic control strategies

Bactrocera tryoni is a serious pest of horticulture in eastern Australia. Here we review molecular data relevant to pest status and development of a transformation system for this species. The development of transformation vectors for non-drosophilid insects has opened the door to the possibility of improving the sterile insect technique (SIT), by genetically engineering factory strains of pest insects to produce male-only broods. Transposition assays indicate that all five of the vectors currently used for transformation in non-drosophilid species have the potential to be useful as transformation vectors in B. tryoni. Evidence of cross mobilization of hobo by an endogenous Homer element emphasises the necessity to understand the endogenous transposons within a species. The sex-specific doublesex and yolk protein genes have been characterized with a view to engineering a female-specific lethal gene or modifying gene expression through RNA interference (RNAi). Data are presented which indicate the potential of RNAi to modify the sex ratio of resultant broods. An understanding of how pest status is determined and maintained is being addressed through the characterization of genes of the circadian clock that enable the fly to adapt to environmental cues. Such an understanding will be useful in the future to the effective delivery of sophisticated pest control measures.

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.

Transgenic planarian lines obtained by electroporation using transposon-derived vectors and an eye-specific GFP marker

To generate transgenic planarians we used a set of versatile vectors for animal transgenesis based on the promiscuous transposons, mariner, Hermes and piggyBac, and a universal enhanced GFP (EGFP) marker system with three Pax6 dimeric binding sites, the 3xP3-EGFP developed by Berghammer et al. [Berghammer, A. J., Klinger, M. & Wimmer, E. A. (1999) Nature 402, 370-371]. This marker is expressed specifically in the eyes of various arthropod taxa. Upon microinjection into the parenchyma of adult planarians and subsequent electroporation, these vectors transpose efficiently into the planarian genome. One of the cell types transformed are the totipotent "neoblast" stem cells present in the adults, representing 30% of total cells. The neoblast represents a unique cell type with the capacity to proliferate and to differentiate into all somatic cell types as well as into germ cells. All three transposon vectors have high transformation efficiency, but only Hermes and piggyBac show stable integration. The mariner vector is frequently lost presumably because of the presence of active mariner-type transposons in the genome of the Girardia tigrina. Transformed animals are mosaics containing both transformed and untransformed neoblasts. These differentiate to form EGFP-positive and -negative photoreceptor cells. Such mosaicism is maintained through several cycles of regeneration induced by decapitation or asexual reproduction. Transformed neoblasts also contribute to the germ line, and can give rise to pure transgenic planarian lines in which EGFP is expressed in all photoreceptor cells after sexual reproduction. The presence of the transgenes was confirmed by PCR, plasmid rescue assay, inverse PCR, and Southern blotting. Our results with the 3xP3-EGFP marker confirm the presence of Pax6 activity in the differentiated photoreceptor cells of planarian eyes. Transgenesis will be an important tool to dissect developmental molecular mechanisms in planarian regeneration, development and stem cell biology, and may also be an entry point to analyze the biology of parasitic Platyhelminthes.

Gene vector and transposable element behavior in mosquitoes

The development of efficient germ-line transformation technologies for mosquitoes has increased the ability of entomologists to find, isolate and analyze genes. The utility of the currently available systems will be determined by a number of factors including the behavior of the gene vectors during the initial integration event and their behavior after chromosomal integration. Post-integration behavior will determine whether the transposable elements being employed currently as primary gene vectors will be useful as gene-tagging and enhancer-trapping agents. The post-integration behavior of existing insect vectors has not been extensively examined. Mos1 is useful as a primary germ-line transformation vector in insects but is inefficiently remobilized in Drosophila melanogaster and Aedes aegypti. Hermes transforms D. melanogaster efficiently and can be remobilized in this species. This element is also useful for creating transgenic A. aegypti, but its mode of integration in mosquitoes results in the insertion of flanking plasmid DNA. Hermes can be remobilized in the soma of A. aegypti and transposes using a common cut-and-paste mechanism; however, the element does not remobilize in the germ line. piggyBac can be used to create transgenic mosquitoes and occasionally integrates using a mechanism other than a simple cut-and-paste mechanism. Preliminary data suggest that remobilization is infrequent. Minos also functions in mosquitoes and, like the other gene vectors,appears to remobilize inefficiently following integration. These results have implications for future gene vector development efforts and applications.

Nuclear localization of the Hermes transposase depends on basic amino acid residues at the N-terminus of the protein

For the Hermes transposable element to be mobilized in its eukaryotic host, the transposase, encoded by the element, must make contact with its DNA. After synthesis in the cytoplasm, the transposase has to be actively imported into the nucleus because its size of 70.1 kDa prevents passive diffusion through the nuclear pore. Studies in vitro using transient expression of a Hermes-EGFP fusion protein in Drosophila melanogaster Schneider 2 cells showed the transposase was located predominantly in the nucleus. In silico sequence analysis, however, did not reveal any nuclear localization signal (NLS). To identify the sequence(s) responsible for localization of Hermes transposase in the nucleus, truncated or mutated forms of the transposase were examined for their influence on sub-cellular localization of marker proteins fused to the transposase. Using the same expression system and a GFP-GUS fusion double marker, residues 1-110 were recognized as sufficient, and residues 1-32 as necessary, for nuclear localization. Amino acid K25 greatly facilitated nuclear localization, indicating that at least this basic amino acid plays a significant role in this process. This sequence overlaps the proposed DNA binding region of the Hermes transposase and is not necessarily conserved in all members of the hAT transposable element family.

Genetic transformation of mosquitoes: a quest for malaria control

Malaria inflicts an enormous toll in human lives and this burden is increasing. Present means to fight the disease, such as drugs and insecticides, are insufficient. Moreover, an effective vaccine has not yet been developed. This review examines an alternative strategy for malaria control, namely the genetic modification of mosquitoes to make them inefficient vectors for the parasite. The article summarises progress made toward the development of transposable element vectors for germ line transformation and the search for mosquito markers of transformation. Also reviewed is the search for anti-malarial effector genes whose products can inhibit development of the parasite in the mosquito with minimal fitness burden. While much progress has been made, much work remains to be done. Future research directions are discussed.

Germline transformants spreading out to many insect species

The past 5 years have witnessed significant advances in our ability to introduce genes into the genomes of insects of medical and agricultural importance. A number of transposable elements now exist that are proving to be sufficiently robust to allow genetic transformation of species within three orders of insects. In particular all of these transposable elements can be used genetically to transform mosquitoes. These developments, together with the use of suitable genes as genetic markers, have enabled several genes and promoters to be transferred between insect species and their effects on the phenotype of the transgenic insect determined. Within a very short period of time, insights into the function of insect promoters in homologous and heterologous insect species are being gained. Furthermore, strategies aimed at ameliorating the harmful effects of pest insects, such as their ability to vector human pathogens, are now being tested in the pest insects themselves. We review the progress that has been made in the development of transgenic technology in pest insect species and conclude that the repertoire of transposable element-based genetic tools, long available to Drosophila geneticists, can now be applied to other insect species. In addition, it is likely that these developments will lead to the generation of pest insects that display a significantly reduced ability to transmit pathogens in the near future.

Engineering mosquito resistance to malaria parasites: the avian malaria model

Genetic approaches to controlling the transmission of mosquito-borne diseases are being developed to augment the available chemical control practices and environmental manipulation methods. Much progress has been made in laboratory-based research that seeks to develop antipathogen or antivector effector genes and methods for genetically manipulating host vector strains. Research is summarized here in the development of a malaria-resistant phenotype using as a model system the avian parasite, Plasmodium gallinaceum, and the mosquito, Aedes aegypti. Robust transformation technology based on a number of transposable elements, the identification of promoter regions derived from endogenous mosquito genes, and the development of single-chain antibodies as effector genes have made it possible to produce malaria-resistant mosquitoes. Future challenges include discovery of methods for spreading antiparasite genes through mosquito populations, determining the threshold levels below which parasite intensities of infection must be held, and defining the circumstances in which a genetic control strategy would be employed in the field.

Developing arbovirus resistance in mosquitoes

Diseases caused by arthropod-borne viruses are increasingly significant public health problems, and novel methods are needed to control pathogen transmission. The hypothesis underlying the research described here is that genetic manipulation of Aedes aegypti mosquitoes can profoundly and permanently reduce their competence to transmit dengue viruses to human hosts. Recent key findings now allow us to test the genetic control hypothesis. We have identified viral genome-derived RNA segments that can be expressed in mosquito midguts and salivary glands to ablate homologous virus replication and transmission. We have demonstrated that both transient and heritable expression of virus-derived effector RNAs in cultured mosquito cells can silence virus replication, and have characterized the mechanism of RNA-mediated resistance. We are now developing virus-resistant mosquito lines by transformation with transposable elements that express effector RNAs from mosquito-active promoters.

Genetic engineering in insects of agricultural importance

The past five years have witnessed the extension of genetic transformation techniques into 11 insect species covering four orders within the Insecta. While the robustness of these transformation systems can be improved, there is now a highly likely probability that transformation of a given insect species will ensue, provided transposable element-containing plasmid DNA can be effectively delivered to the embryo or some other life stage. These developments have shifted emphasis to concerns of transgene stability and the regulation of the rearing and release of these transgenic insects. They have also led to some elegant demonstrations of genetic sexing mechanisms in Drosophila melanogaster with the expectation that similar systems be extended into pest insect species. These developments and issues are discussed in this short review.

Germ-line transformation of the South American malaria vector, Anopheles albimanus, with a piggyBac/EGFP transposon vector is routine and highly efficient

Stable and efficient germ-line transformation was achieved in the South American malaria vector, Anopheles albimanus, using a piggyBac vector marked with an enhanced green fluorescent protein gene regulated by the Drosophila melanogaster polyubiquitin promoter. Transgenic mosquitoes were identified from four independent experiments at frequencies ranging from 20 to 43% per fertile G0. Fluorescence was observable throughout the body of larvae and pupae, and abdominal segments of adults. Transgenic lines analysed by Southern hybridization had one to six germ-line integrations, with most lines having three or more integrations. Hybridized transposon vector fragments and insertion site sequences were consistent with precise piggyBac-mediated integrations, although this was not verified for all lines. The piggyBac/PUbnlsEGFP vector appears to be a robust transformation system for this anopheline species, in contrast to the use of a piggyBac vector in An. gambiae. Further tests are needed to determine if differences in anopheline transformation efficiency are due to the marker systems or to organismal or cellular factors specific to the species.

Transposable element-mediated transgenesis in insects beyond Drosophila

In the last few years, cases of transformation involving insects other than Dipterans have been reported. Although transgenics have been created only in a few species, transposable element vectors may be successfully developed in most insect forms in the near future. The major remaining problems revolving round transformation in wide-ranging species of insects are mainly related to methods of DNA delivery. Transposable element-mediated gene transfer in non-Drosophila insects is reviewed. In addition, the current status of honeybee transformation will be explained as an example of an insect transgenic system that faces substantial obstacles to the creation of germ-line transformants.

Development and applications of transgenesis in the yellow fever mosquito, Aedes aegypti

Transgenesis technology has been developed for the yellow fever mosquito, Aedes aegypti. Successful integration of exogenous DNA into the germline of this mosquito has been achieved with the class II transposable elements, Hermes, mariner and piggyBac. A number of marker genes, including the cinnabar(+) gene of Drosophila melanogaster, and fluorescent protein genes, can be used to monitor the insertion of these elements. The availability of multiple elements and marker genes provides a powerful set of tools to investigate basic biological properties of this vector insect, as well as the materials for developing novel, genetics-based, control strategies for the transmission of disease.

Germ line transformation of the yellow fever mosquito, Aedes aegypti, mediated by transpositional insertion of a piggyBac vector

Mosquito-vectored diseases such as yellow fever and dengue fever continue to have a substantial impact on human populations world-wide. Novel strategies for control of these mosquito vectored diseases can arise through the development of reliable systems for genetic manipulation of the insect vector. A piggyBac vector marked with the Drosophila melanogaster cinnabar (cn) gene was used to transform the white-eyed khw strain of Aedes aegypti. Microinjection of preblastoderm embryos resulted in four families of cinnabar transformed insects. An overall transformation frequency of 4%, with a range of 0% to as high as 13% for individual experiments, was achieved when using a heat-shock induced transposase providing helper plasmid. Southern hybridizations indicated multiple insertion events in three of four transgenic lines, while the presence of duplicated target TTAA sites at either ends of individual insertions confirmed characteristic piggyBac transposition events in these three transgenic lines. The transgenic phenotype has remained stable for more than twenty generations. The transformations effected using the piggyBac element establish the potential of this element as a germ-line transformation vector for Aedine mosquitoes.

Efficient transformation of the yellow fever mosquito Aedes aegypti using the piggyBac transposable element vector pBac[3xP3-EGFP afm]

We report efficient germ-line transformation in the yellow fever mosquito Aedes aegypti accomplished using the piggyBac transposable element vector pBac[3xP3-EGFP afm]. Two transgenic lines were established and characterized; each contained the Vg-Defensin A transgene with strong eye-specific expression of the enhanced green fluorescent protein (EGFP) marker gene regulated by the artificial 3xP3 promoter. Southern blot hybridization and inverse PCR analyses of genomic DNA demonstrated a precise piggyBac-mediated, single copy insertion of the pBac[3xP3-EGFP afm,Vg-DefA] transposon in each transgenic line. For each line, genetic analysis confirmed stability and integrity of the entire transposon construct in the mosquito genome through the G2-G6 generations. Successful establishment of homozygous transgenic lines indicated that in both cases a non-lethal integration of the transposon into the mosquito genome had occurred. The 3xP3-EGFP marker was tested in mosquitoes with different genetic backgrounds. In white-eyed transgenic mosquitoes, the strong eye-specific expression of GFP was observed throughout all stages of development, starting from newly hatched first instar larvae to adults. A similar level and pattern of fluorescence was observed in red-eyed mosquitoes that were generated by crossing the 3xP3-EGFP transformants with the kh(w) white-eye mosquitoes transformed with the Drosophila cinnabar gene. Importantly, the utility of the 3xP3-EGFP, as marker gene for transformation of wild type mosquitoes, was demonstrated by strong eye-specific GFP expression in larval and pupal stages of black-eyed hybrids of the 3xP3-EGFP white-eye transformants and the wild type Rockefeller/UGAL strain. Finally, analysis of the Vg-DefA transgene expression in transformants from two established lines demonstrated strong blood-meal activation and fat-body-specific expression regulated by the Vg 1.8-kb 5' upstream region.

Stable, germ-line transformation of Culex quinquefasciatus (Diptera: Culicidae)

A Hermes-based transposable element transformation system incorporating an enhanced green fluorescent protein (EGFP) marker was used to produce two transgenic lines of Culex quinquefasciatus (Say). The transformation frequency was approximately 12% and transformation of Culex was shown to be dependent on the presence of Hermes transposase. Injected Culex embryos were treated with four different heat shock regimes, two of which produced transformed individuals. These individuals were mated with wild-type mosquitoes and produced offspring which expressed the dominant EGFP gene in Mendelian ratios predicted for the stable integration of a gene at a single locus. The two transformed lines displayed distinct patterns of phenotypic expression, the expression of which has remained stable after fifteen generations. In these transgenic lines both the Hermes element and flanking plasmid DNA integrated into the Culex genome, as has been previously seen in Hermes-mediated transgenic strains of Aedes aegypti (L.). The high frequency of Culex transformation together with the dependence on the presence of Hermes transposase suggests that, as for Ae. aegypti, this mode of transposition into the germ-line genome occurs by an alternate mechanisms to the cut and paste type of transposition seen for this element in other insect species and in the somatic nuclei of mosquitoes. This is the first report of the genetic transformation of a species in the genus Culex and demonstrates that this medically important mosquito species can now, along with several other Culicine and Anopheline mosquito species, be genetically manipulated.

Hermes-mediated germ-line transformation of the Mediterranean fruit fly Ceratitis capitata

We report the use of the Hermes transposable element for germ-line transformation of the Mediterranean fruit fly, Ceratitis capitata. Hermes was able to genetically transform this insect at an estimated frequency between 0.6 and 1.1%, which is comparable to the transformation frequencies obtained for this species when using other transposable elements. Hermes integrates into the medfly genome by a cut-and-paste mechanism and the sequences integrated into the genome are delimited by the terminal nucleotides of the Hermes inverted terminal repeats. Integration resulted in the generation of 8 bp target site duplications, the sequences of which conformed to the target site duplications generated by hAT element transposition in insects. The Hermes element is one additional genetic tool that can be deployed in manipulating and characterizing the medfly genome.

Eight novel families of miniature inverted repeat transposable elements in the African malaria mosquito, Anopheles gambiae

Eight novel families of miniature inverted repeat transposable elements (MITEs) were discovered in the African malaria mosquito, Anopheles gambiae, by using new software designed to rapidly identify MITE-like sequences based on their structural characteristics. Divergent subfamilies have been found in two families. Past mobility was demonstrated by evidence of MITE insertions that resulted in the duplication of specific TA, TAA, or 8-bp targets. Some of these MITEs share the same target duplications and similar terminal sequences with MITEs and other DNA transposons in human and other organisms. MITEs in A. gambiae range from 40 to 1340 copies per genome, much less abundant than MITEs in the yellow fever mosquito, Aedes aegypti. Statistical analyses suggest that most A. gambiae MITEs are in highly AT-rich regions, many of which are closely associated with each other. The analyses of these novel MITEs underscored interesting questions regarding their diversity, origin, evolution, and relationships to the host genomes. The discovery of diverse families of MITEs in A. gambiae has important practical implications in light of current efforts to control malaria by replacing vector mosquitoes with genetically modified refractory mosquitoes. Finally, the systematic approach to rapidly identify novel MITEs should have broad applications for the analysis of the ever-growing sequence databases of a wide range of organisms.

Eight novel families of miniature inverted repeat transposable elements in the African malaria mosquito, Anopheles gambiae

Eight novel families of miniature inverted repeat transposable elements (MITEs) were discovered in the African malaria mosquito, Anopheles gambiae, by using new software designed to rapidly identify MITE-like sequences based on their structural characteristics. Divergent subfamilies have been found in two families. Past mobility was demonstrated by evidence of MITE insertions that resulted in the duplication of specific TA, TAA, or 8-bp targets. Some of these MITEs share the same target duplications and similar terminal sequences with MITEs and other DNA transposons in human and other organisms. MITEs in A. gambiae range from 40 to 1340 copies per genome, much less abundant than MITEs in the yellow fever mosquito, Aedes aegypti. Statistical analyses suggest that most A. gambiae MITEs are in highly AT-rich regions, many of which are closely associated with each other. The analyses of these novel MITEs underscored interesting questions regarding their diversity, origin, evolution, and relationships to the host genomes. The discovery of diverse families of MITEs in A. gambiae has important practical implications in light of current efforts to control malaria by replacing vector mosquitoes with genetically modified refractory mosquitoes. Finally, the systematic approach to rapidly identify novel MITEs should have broad applications for the analysis of the ever-growing sequence databases of a wide range of organisms.

A current perspective on insect gene transformation

The genetic transformation of non-drosophilid insects is now possible with several systems, with germ-line transformation reported in published and unpublished accounts for about 12 species using four different transposon vectors. For some of these species, transformation can now be considered routine. Other vector systems include viruses and bacterial symbionts that have demonstrated utility in species and applications requiring transient expression, and for some, the potential exists for genomic integration. Many of these findings are quite recent, presenting a dramatic turning point in our ability to study and manipulate agriculturally and medically important insects. This review discusses these findings from the perspective of all the contributions that has made this technology a reality, the research that has yet to be done for its safe and efficient use in a broader range of species, and an overview of the available methodology to effectively utilize these systems.

Genetic transformation systems in insects

The past 5 years have witnessed the emergence of techniques that permit the stable genetic transformation of a number of non-drosophilid insect species. These transposable-element-based strategies, together with virus-based techniques that allow the expression of genes to be quickly examined in insects, provide insect scientists with a first generation of genetic tools that can begin to be harnessed to further increase our understanding of gene function and regulation in insects. We review and compare the characteristics of these gene transfer systems and conclude that, although significant progress has been made, these systems still do not meet the requirements of robust genetic tools. We also review risk assessment issues arising from the generation and probable release of genetically engineered insects.

Purified mariner (Mos1) transposase catalyzes the integration of marked elements into the germ-line of the yellow fever mosquito, Aedes aegypti

Derivatives of the mariner transposable element, Mos1, from Drosophila mauritiana, can integrate into the germ-line of the yellow fever mosquito, Aedes aegypti. Previously, the transposase required to mobilize Mos1 was provided in trans by a helper plasmid expressing the enzyme under the control of the D. psuedoobscura heat-shock protein 82 promoter. Here we tested whether purified recombinant Mos1 transposase could increase the recovery of Ae. aegypti transformants. Mos1 transposase was injected into white-eyed, kh(w)/kh(w), Ae. aegypti embryos with a Mos1 donor plasmid containing a copy of the wild-type allele of the D. melanogaster cinnabar gene. Transformed mosquitoes were recognized by partial restoration of eye color in the G(1) animals and confirmed by Southern analyses of genomic DNA. At Mos1 transposase concentrations approaching 100 nM, the rate of germ-line transformants arising from independent insertions in G(0) animals was elevated 2-fold compared to that seen in experiments with helper plasmids. Furthermore, the recovery of total G(1) transformants was increased 7.5-fold over the frequency seen with co-injected helper plasmid. Southern blot analyses and gene amplification experiments confirmed the integration of the transposons into the mosquito genome, although not all integrations were of the expected cut-and-paste type transposition. The increased frequency of germ-line integrations obtained with purified transposase will facilitate the generation of Mos1 transgenic mosquitoes and the application of transgenic approaches to the biology of this important vector of multiple pathogens.

Transformation of Stomoxys calcitrans with a Hermes gene vector

The ability of the Hermes transposable element to function as a germ line transformation vector was tested in the stable fly, Stomoxys calcitrans. Plasmid-based transposable element mobility assays indicated moderate mobility of Hermes in this species. Germline transformants were created using a Hermes element containing the enhanced green fluorescent protein (EGFP) under the regulatory control of the promoter from Actin5C gene of Drosophila melanogaster. Approximately 4% of the fifty-five adults that developed from the 1903 G(0) embryos injected with the vector produced transgenic progeny. In the four transgenic lines established, the EGFP expression pattern was distinctly nonuniform and levels of expression were low. Promoters other than the one from the Actin5C gene of D. melanogaster should be considered for widespread, constitutive expression. All transgenic lines contained multiple (2-4) integrated Hermes elements. Hermes integration events occurred through a canonical cut-and-paste mechanism.

Green fluorescent protein as a genetic marker in transgenic Aedes aegypti

We report here the use of the enhanced green fluorescent protein (EGFP) from the jellyfish, Aequorea victoria, as a genetic marker for the genetic transformation of mosquitoes. The EGFP gene, under the control of the actin5C promoter of Drosophila melanogaster was inserted into the Hermes transposable element. Preblastoderm embryos of a wild-type strain of the yellow fever mosquito, Aedes aegypti, were microinjected with this plasmid, together with a helper plasmid containing the Hermes transposase placed under the control of the D. melanogaster hsp70 promoter. Somatic EGFP expression was observed during early instars in approximately one-half of all G0 individuals. Two G1 individuals arising from a G0 female displayed high levels of EGFP gene expression during all stages of development. EGFP was transmitted in a Mendelian fashion to the G2 and G3 generations and molecular analysis confirmed the presence of the Hermes[actin5C:EGFP] gene in these insects. These results clearly demonstrate that EGFP can be used as an effective genetic marker in wild-type Ae. aegypti and most likely in other mosquito species as well.