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.

The Exact Timing of Microinjection of Parthenogenetic Silkworm Embryos Is Crucial for Their Successful Transgenesis.

The use of parthenogenetic silkworm (Bombyx mori) strains, which eliminate the problem of recombination, is a useful tool for maintaining transgenic clonal lines. The generation of genetically identical individuals is becoming an important tool in genetic engineering, allowing replication of an existing advantageous trait combination without the mixing that occurs during sexual reproduction. Thus, an animal with a particular genetic modification, such as the ability to produce transgenic proteins, can reproduce more rapidly than by natural mating. One obstacle to the widespread use of parthenogenesis in silkworm genetic engineering is the relatively low efficiency of downstream transgenesis techniques. In this work, we seek to optimize the use of transgenesis in conjunction with the production of parthenogenetic individuals. We found that a very important parameter for the introduction of foreign genes into a parthenogenetic strain is the precise timing of embryo microinjection. Our modification of the original method increased the efficiency of transgene injection as well as the survival rate of injected embryos. We also provide a detailed description of the methodological procedure including a graphical overview of the entire protocol.

Production of cloned transgenic silkworms by breeding non-diapausing parthenogenetic strains.

Clonal transgenic silkworms are useful for the functional analysis of insect genes and for the production of recombinant proteins. Such silkworms have previously been created using an existing ameiotic parthenogenetic strain. However, the process was labor intensive, and the efficiency of producing transgenic silkworms was very low. To overcome this issue, we developed a more convenient and efficient method by breeding non-diapausing parthenogenetic strains. The strains produced non-diapausing eggs only when the embryogenesis of the parent eggs was performed at low temperatures, which could then be used for injecting vector plasmids. This demonstrated that transgenic silkworms could be produced with greater ease and efficiency. To breed the strains, we crossed the existing parthenogenetic strains with bivoltine strains and made F1 and F2 from each cross. Then we selected the silkworms whose eggs have a high ability of parthenogenesis and became non-diapausing. We also demonstrated that the germplasm could be cryopreserved in liquid nitrogen. Thus, this method increases the efficiency and ease of using genetically engineered silkworms to analyze gene function and produce recombinant proteins, potentially impacting various industries.

Mutation in Bombyx mori fibrohexamerin (P25) gene causes reorganization of rough endoplasmic reticulum in posterior silk gland cells and alters morphology of fibroin secretory globules in the silk gland lumen.

Larvae of many lepidopteran species produce a mixture of secretory proteins, known as silk, for building protective shelters and cocoons. Silk consists of a water-insoluble silk filament core produced in the posterior silk gland (PSG) and a sticky hydrophilic coating produced by the middle silk gland (MSG). In Bombyx mori, the fiber core comprises three proteins: heavy chain fibroin (Fib-H), light chain fibroin (Fib-L) and fibrohexamerin (Fhx, previously referred to as P25). To learn more about the role of Fhx, we used transcription activator-like effector nuclease (TALEN) mutagenesis and prepared a homozygous line with a null mutation in the Fhx gene. Our characterization of cocoon morphology and silk quality showed that the mutation had very little effect. However, a detailed inspection of the secretory cells in the posterior silk gland (PSG) of mid-last-instar mutant larvae revealed temporary changes in the morphology of the endoplasmic reticulum. We also observed a morphological difference in fibroin secretory globules stored in the PSG lumen of Fhx mutants, which suggests that their fibroin complexes have a slightly lower solubility. Finally, we performed an LC-MS-based quantitative proteomic analysis comparing mutant and wild-type (wt) cocoon proteins and found a high abundance of a 16 kDa secretory protein likely involved in fibroin solubility. Overall, our study shows that whilst Fhx is dispensable for silk formation, it contributes to the stability of fibroin complexes during intracellular transport and affects the morphology of fibroin secretory globules in the PSG lumen.

Genome engineering and parthenocloning in the silkworm, Bombyx mori.

Genetic engineering of the silkworm, Bombyx mori, opens door to the production of new kinds of silk and to the use of silkworms as proteosynthetic bioreactors. The insertion of foreign genes into silkworm genome and the control of their expression by diverse promoters have become possible but are not yet efficient enough for commercial use. Several methods of gene targeting are being developed to minimize position effect on transgene expression and facilitate cloning. Parthenocloning can be exploited to conserve genetic traits and improve selection and amplification of clones containing genes of interest. Some silkworm clones have been bred for decades as genetically stable female stocks whose unfertilized eggs are induced to develop by heat-shock treatment. Any exclusively female generation contains exact copies of the maternal clone-founder genome. Ovaries transplanted in either direction between the standard and the parthenogenetic genotypes yield eggs capable of parthenocloning. In addition, use ofmale larvae as ovary recipients eliminates diapause in eggs produced in the implants. Unfertilized eggs of some silkworm clones respond also to the cold-shock treatment by producing homozygous fertile sons; cloned females can be crossed with their parthenogenetic sons to obtain progeny homozygous for the transgene in both sexes. Rational exploitation of available parthenozygous pools and the use of parthenocloning methods enable rapid fixation and maintenance of the desired genotypes.

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.