Germ line transformation of the silkworm, Bombyx mori, using the transposable element Minos

Abolition of egg diapause by ablation of suboesophageal ganglion in parental females is compatible with genetic engineering methods

Microinjection of genetic material into non-diapause eggs is required for genetic engineering of silkworms. Besides diapause could be useful for maintaining transgenic lines, a drawback of this technology is that most standard silkworm strains and experimental lines of interest produce diapausing eggs. Several approaches have been developed to abolish diapause but none are very efficient. Here, we investigated the ablation of the suboesophageal ganglion (SG) in female pupae, which is a source of the hormone required to trigger egg diapause, as a mean to abolish diapause. We showed that SG-ablation is a reliable method to produce nondiapause eggs. Additionally, the challenge associated with lower fecundity of females with SG ablation was resolved by injecting pilocarpine into the mated female. We also investigated the suitability of nondiapause eggs laid by SG-ablated females for transgenesis, targeted mutagenesis, and induction of parthenogenetic development. Our results demonstrated SG-ablation to be a useful and simple method for expanding the possibilities associated with genetic engineering in silkworms.

An inducible constitutive expression system in Bombyx mori mediated by phiC31 integrase

Inducible gene-expression systems play important roles in gene functional assays in the post-genome era. Streptomyces phage-derived phiC31 integrase, which mediates an irreversible site-specific cassette exchange between the phage attachment site (attP) and the bacterial attachment site (attB), provides a promising option for the construction of a controllable gene-expression system. Here, we report a phiC31 integrase-mediated promoter flip system (FLIP) for the inducible expression of target genes in silkworm (Bombyx mori). First, we constructed a FLIP reporter system, in which a BmAct4 promoter with enhanced translational efficiency was flanked by the attB and attP sites in a head-to-head orientation and further linked in a reverse orientation to a DsRed reporter gene. The coexpression of a C-terminal modified phiC31-NLS integrase carrying a simian virus 40 (SV40) nuclear localization signal (NLS) effectively flipped the BmAct4 promoter through an attB/attP exchange, thereby activating the downstream expression of DsRed in a silkworm embryo-derived cell line, BmE. Subsequently, the FLIP system, together with a system continuously expressing the phiC31-NLS integrase, was used to construct binary transgenic silkworm lines. Hybridization between FLIP and phiC31-NLS transgenic silkworm lines resulted in the successful flipping of the BmAct4 promoter, with an approximately 39% heritable transformation efficiency in silkworm offspring, leading to the constitutive and high-level expression of DsRed in silkworms, which accounted for approximately 0.81% of the silkworm pupal weight. Our successful development of the FLIP system offers an effective alternative for manipulating gene expression in silkworms and other lepidopteran species.

The neurobiological basis of sleep: Insights from Drosophila

Sleep is a biological enigma that has raised numerous questions about the inner workings of the brain. The fundamental question of why our nervous systems have evolved to require sleep remains a topic of ongoing scientific deliberation. This question is largely being addressed by research using animal models of sleep. Drosophila melanogaster, also known as the common fruit fly, exhibits a sleep state that shares common features with many other species. Drosophila sleep studies have unearthed an immense wealth of knowledge about the neuroscience of sleep. Given the breadth of findings published on Drosophila sleep, it is important to consider how all of this information might come together to generate a more holistic understanding of sleep. This review provides a comprehensive summary of the neurobiology of Drosophila sleep and explores the broader insights and implications of how sleep is regulated across species and why it is necessary for the brain.

Targeted Mutagenesis in Bombyx mori Using TALENs

Bombyx mori is a valuable model organism of high economic importance. Its genome sequence is available, as well as basic genetic and molecular genetic tools and markers. The introduction of genome editing methods based on engineered nucleases enables precise manipulations with genomic DNA, including targeted DNA deletions, insertions, or replacements in the genome allowing gene analysis and various applications. We describe here the use of TALENs which have a simple modular design of their DNA-binding domains, are easy to prepare and proved to be efficient in targeting of a wide range of cleavage sites. Our procedure often allows the production of individuals carrying homozygous mutations as early as in the G1 generation.

Construction and long term preservation of clonal transgenic silkworms using a parthenogenetic strain

For the functional analysis of insect genes as well as for the production of recombinant proteins for biomedical use, clonal transgenic silkworms are very useful. We examined if they could be produced in the parthenogenetic strain that had been maintained for more than 40years as a female line in which embryogenesis is induced with nearly 100% efficiency by a heat shock treatment of unfertilized eggs. All individuals have identical female genotype. Silkworm transgenesis requires injection of the DNA constructs into the non-diapausing eggs at the preblastodermal stage of embryogenesis. Since our parthenogenetic silkworms produce diapausing eggs, diapause programing was eliminated by incubating ovaries of the parthenogenetic strain in standard male larvae. Chorionated eggs were dissected from the implants, activated by the heat shock treatment and injected with the transgene construct. Several transgenic individuals occurred in the daughter generation. Southern blotting analysis of two randomly chosen transgenic lines VTG1 and VTG14 revealed multiple transgene insertions. Insertions found in the parental females were transferred to the next generation without any changes in their sites and copy numbers, suggesting that transgenic silkworms can be maintained as clonal strains with homozygous transgenes. Cryopreservation was developed for the storage of precious genotypes. As shown for the VTG1 and VTG14 lines, larval ovaries can be stored in DMSO at the temperature of liquid nitrogen, transferred to Grace's medium during defrosting, and then implanted into larvae of either sex of the standard silkworm strains C146 and w1-pnd. Chorionated eggs, which developed in the implants, were dissected and activated by the heat shock to obtain females (nearly 100% efficiency) or by a cold shock to induce development to both sexes in 4% of the eggs. It was then possible to establish bisexual lines homozygous for the transgene.

Highly efficient multiplex targeted mutagenesis and genomic structure variation in Bombyx mori cells using CRISPR/Cas9

Bombyx mori is an economically important insect and a model organism for studying lepidopteran and arthropod biology. Using a highly efficient CRISPR/Cas9 system, we showed that this system could mediate highly efficient targeted genome editing of a single gene locus, large chromosomal deletion or inversion, and also multiplex genome editing of 6 genes simultaneously in BmNs cell line derived from B. mori. The simplicity and high efficiency of our system provide unprecedented possibilities for researchers to implement precise and sophisticated manipulation of a chosen B. mori gene in BmNs cells easily in a limited time course, and perhaps new opportunities for functional genomics of B. mori and other lepidopteran insects.

In vivo site-specific integration of transgene in silkworm via PhiC31 integrase-mediated cassette exchange

Current techniques for genetic engineering of the silkworm Bombyx mori genome utilize transposable elements, which result in positional effects and insertional mutagenesis through random insertion of exogenous DNA. New methods for introducing transgenes at specific positions are therefore needed to overcome the limitations of transposon-based strategies. Although site-specific recombination systems have proven powerful tools for genome manipulation in many organisms, their use has not yet been well established for the integration of transgenes in the silkworm. We describe a method for integrating target genes at pre-defined chromosomal sites in the silkworm via phiC31/att site-specific recombination system-mediated cassette exchange. Successful recombinase-mediated cassette exchange (RMCE) was observed in the two transgenic target strains with an estimated transformation efficiency of 3.84-7.01%. Our results suggest that RMCE events between chromosomal attP/attP target sites and incoming attB/attB sites were more frequent than those in the reciprocal direction. This is the first report of in vivo RMCE via phiC31 integrase in the silkworm, and thus represents a key step toward establishing genome manipulation technologies in silkworms and other lepidopteran species.

Advances in silkworm studies accelerated by the genome sequencing of Bombyx mori

Significant progress has been achieved in silkworm (Bombyx mori) research since the last review on this insect was published in this journal in 2005. In this article, we review the new and exciting progress and discoveries that have been made in B. mori during the past 10 years, which include the construction of a fine genome sequence and a genetic variation map, the evolution of genomes, the advent of functional genomics, the genetic basis of silk production, metamorphic development, immune response, and the advances in genetic manipulation. These advances, which were accelerated by the genome sequencing project, have promoted B. mori as a model organism not only for lepidopterans but also for general biology.

piggyBac transposon remobilization and enhancer detection in Anopheles mosquitoes

Technical advances in mosquito biology are enabling the development of new approaches to vector control. Absent are powerful forward-genetics technologies, such as enhancer and gene traps, that permit determination of gene functions from the phenotypes arising from transposon insertion mutations. We show that the piggyBac transposon is highly active in the germline of the human malaria vector Anopheles stephensi. Up to 6% of the progeny from transgenic A. stephensi containing a single 6-kb piggyBac element with a marker gene expressing EGFP had the vector in new genomic locations when piggyBac transposase was provided in trans from a second integrated transgene. The active transposition of piggyBac resulted in the efficient detection of enhancers, with ~10% of the progeny with piggyBac in new locations with novel patterns of EGFP expression in third and fourth instar larvae and in adults. The availability of advanced transgenic capabilities such as efficient transposon-based forward-genetics technologies for Anopheles mosquitoes not only will accelerate our understanding of mosquito functional genomics and the development of novel vector and disease transmission control strategies, but also will enable studies by evolutionary developmental biologists, virologists, and parasitologists.

Homology of dipteran bristles and lepidopteran scales: requirement for the Bombyx mori achaete-scute homologue ASH2

Lepidopteran wing scales and Drosophila bristles are considered homologous structures on the basis of the similarities in their cell lineages. However, the molecular mechanisms underlying scale development are essentially unknown as analysis of gene function in Lepidoptera is sorely limited. In this study, we used the Bombyx mori mutant scaleless (sl), which displays a nearly complete loss of wing scales, to explore the mechanism of lepidopteran wing-scale formation. We found that Bm-ASH2, one of four Bombyx achaete-scute homologs, is highly expressed in early pupal wings of wild-type silkworms, but its expression is severely reduced in sl pupal wings. Through molecular characterization of the mutant locus using luciferase and gel shift assays, genetic analysis of recombining populations, and in vivo rescue experiments, we provide evidence that a 26-bp deletion within the Bm-ASH2 promoter is closely linked to the sl locus and leads to loss of Bm-ASH2 expression and the scaleless-wings phenotype. Thus, the Bm-ASH2 appears to play a critical role in scale formation in B. mori. This finding supports the proposed homology of lepidopteran scales and dipteran bristles and provides evidence for conservation of the genetic pathway in scale/bristle development at the level of gene function.

The NemaGENETAG initiative: large scale transposon insertion gene-tagging in Caenorhabditis elegans

The nematode Caenorhabditis elegans is a widely appreciated, powerful platform in which to study important biological mechanisms related to human health. More than 65% of human disease genes have homologues in the C. elegans genome, and essential aspects of mammalian cell biology, neurobiology and development are faithfully recapitulated in this organism. The EU-funded NemaGENETAG project was initiated with the aim to develop cutting-edge tools and resources that will facilitate modelling of human pathologies in C. elegans, and advance our understanding of animal development and physiology. The main objective of the project involves the generation and evaluation of a large collection of transposon-tagged mutants. In the process of achieving this objective the NemaGENETAG consortium also endeavours to optimize and automate existing transposon-mediated mutagenesis methodologies based on the Mos1 transposable element, in addition to developing alternatives using other transposon systems. The final product of this initiative-a comprehensive collection of transposon-tagged alleles-together with the acquisition of efficient transposon-based tools for mutagenesis and transgenesis in C. elegans, should yield a wealth of information on gene function, immediately relevant to key biological processes and to pharmaceutical research and development.

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.

From genome to proteome: great progress in the domesticated silkworm (Bombyx mori L.)

As the only truly domesticated insect, the silkworm not only has great economic value, but it also has value as a model for genetics and molecular biology research. Genomics and proteomics have recently shown vast potential to be essential tools in domesticated silkworm research, especially after the completion of the Bombyx mori genome sequence. This paper reviews the progress of the domesticated silkworm genome, particularly focusing on its genetic map, physical map and functional genome. This review also presents proteomics, the proteomic technique and its application in silkworm research.

How functional genomics and genetics complements insect endocrinology

Insects are the most abundant animal group on Earth and have been the subject of genetic and physiological studies since the beginning of the 19th century. The public interest in understanding their biology increased as many insects have proven to exert a severe impact on human welfare and the environment. To trigger insect physiological and endocrinological research, the genome of several economical and ecological important insect species was recently sequenced. Following the availability of these genomic data many so called 'post-genomic' technologies have been developed to characterise gene function and to unravel signalling pathways underlying biological processes. For some species genomic research is further complemented with mutagenesis and reverse genetic studies. In the following, we present an overview of genomic and functional genetic methodologies that boosted endocrine research in insects.

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.

A new method for the modification of fibroin heavy chain protein in the transgenic silkworm

We constructed a new plasmid vector for the production of a modified silk fibroin heavy chain protein (H-chain) in the transgenic silkworm. The plasmid (pHC-null) contained the promoter and the 3' region of a gene encoding the H-chain and the coding regions for the N-terminal domain and the C-terminal domain of the H-chain. For the model protein, we cloned a foreign gene that encoded EGFP between the N-terminal domain and the C-terminal domain in pHC-null and generated transgenic silkworms that produced a modified H-chain, HC-EGFP. Transgenic silkworms produced HC-EGFP in the posterior part of silk gland cells, secreted it into the lumen of the gland, and produced a cocoon with HC-EGFP as part of the fibroin proteins. N-terminal sequencing of HC-EGFP localized the signal sequence cleavage site to between positions A((21)) and N((22)). These results indicate that our new plasmid successfully produced the modified H-chain in a transgenic silkworm.

Patterns on the insect wing

The evolution of wings and the adaptive advantage they provide have allowed insects to become one of the most evolutionarily successful groups on earth. The incredible diversity of their shape, size, and color patterns is a direct reflection of the important role wings have played in the radiation of insects. In this review, we highlight recent studies on both butterflies and Drosophila that have begun to uncover the types of genetic variations and developmental mechanisms that control diversity in wing color patterns. In butterflies, these analyses are now possible because of the recent development of a suite of genomic and functional tools, such as detailed linkage maps and transgenesis. In one such study, extensive linkage mapping in Heliconius butterflies has shown that surprisingly few, and potentially homologous, loci are responsible for several major pattern variations on the wings of these butterflies. Parallel work on a clade of Drosophila has uncovered how cis-regulatory changes of the same gene correlate with the repeated gain and loss of pigmented wing spots. Collectively, our understanding of formation and evolution of color pattern in insect wings is rapidly advancing because of these recent breakthroughs in several different fields.