Bmovo-1 Regulates Ovary Size in the Silkworm,Bombyx mori

The regulation of antagonistic OVO isoforms is critical for germline formation and differentiation in Drosophila. However, little is known about genes related to ovary development. In this study, we cloned the Bombyx mori ovo gene and investigated its four alternatively spliced isoforms. BmOVO-1, BmOVO-2 and BmOVO-3 all had four C2H2 type zinc fingers, but differed at the N-terminal ends, while BmOVO-4 had a single zinc finger. Bmovo-1, Bmovo-2 and Bmovo-4 showed the highest levels of mRNA in ovaries, while Bmovo-3 was primarily expressed in testes. The mRNA expression pattern suggested that Bmovo expression was related to ovary development. RNAi and transgenic techniques were used to analyze the biological function of Bmovo. The results showed that when the Bmovo gene was downregulated, oviposition number decreased. Upregulation of Bmovo-1 in the gonads of transgenic silkworms increased oviposition number and elevated the trehalose contents of hemolymph and ovaries. We concluded that Bmovo-1 was involved in protein synthesis, contributing to the development of ovaries and oviposition number in silkworms.     

Identification and Expression Analysis of Ras Gene in Silkworm,Bombyx mori

Ras proteins play important roles in development especially for cell proliferation and differentiation in various organisms. However, their functions in the most insect species are still not clear. We identified three ras cDNAs from the silk worm, Bombyx mori. These sequences corresponded to three Ras of Drosophila melanogaster, but not to three mammalian Ras (H-Ras, K-Ras, N-Ras). Subsequently, the expression profiles of ras were investigated by quantitative real-time PCR using whole body of individuals from the embryonic to adult stages, and various tissues of 4th and 5th instar larvae. Each of three Bombyx ras showed different expression patterns. We also showed membrane localization of their products. These results indicate that the three Bombyx Ras are functional and have different roles.     

Comparative Proteome Analysis of Multi-Layer Cocoon of the Silkworm,Bombyx mori

Bombyx mori cocoon has a multi-layer structure that provides optimal protection for silkworm pupa. Research on the mechanical properties of the multi-layer structure revealed structure-property relationships of the cocoon. Here, we investigated the protein components of the B. mori cocoon in terms of its multi-layer structure. Liquid chromatography-tandem mass spectrometry identified 286 proteins from the multiple cocoon layers. In addition to fibroins and sericins, we identified abundant protease inhibitors, seroins and proteins of unknown function. By comparing protein abundance across layers, we found that the outermost layer contained more sericin1 and protease inhibitors and the innermost layer had more seroin1. As many as 36 protease inhibitors were identified in cocoons, showing efficient inhibitory activities against a fungal protease. Thus, we propose that more abundant protease inhibitors in the outer cocoon layers may provide better protection for the cocoon. This study increases our understanding of the multi-layer mechanism of cocoons, and helps clarify the biological characteristics of cocoons. The data have been deposited to the ProteomeXchange with identifier PXD001469.     

Evolutionarily Conserved Repulsive Guidance Role of Slit in the Silkworm Bombyx mori

Axon guidance molecule Slit is critical for the axon repulsion in neural tissues, which is evolutionarily conserved from planarians to humans. However, the function of Slit in the silkworm Bombyx mori was unknown. Here we showed that the structure of Bombyx mori Slit (BmSlit) was different from that in most other species in its C-terminal sequence. BmSlit was localized in the midline glial cell, the neuropil, the tendon cell, the muscle and the silk gland and colocalized with BmRobo1 in the neuropil, the muscle and the silk gland. Knock-down of Bmslit by RNA interference (RNAi) resulted in abnormal development of axons and muscles. Our results suggest that BmSlit has a repulsive role in axon guidance and muscle migration. Moreover, the localization of BmSlit in the silk gland argues for its important function in the development of the silk gland.     

Heritable Genome Editing with CRISPR/Cas9 in the Silkworm,Bombyx mori

We report the establishment of an efficient and heritable gene mutagenesis method in the silkworm Bombyx mori using modified type II clustered regularly interspaced short palindromic repeats (CRISPR) with an associated protein (Cas9) system. Using four loci Bm-ok, BmKMO, BmTH, and Bmtan as candidates, we proved that genome alterations at specific sites could be induced by direct microinjection of specific guide RNA and Cas9-mRNA into silkworm embryos. Mutation frequencies of 16.7–35.0% were observed in the injected generation, and DNA fragments deletions were also noted. Bm-ok mosaic mutants were used to test for mutant heritability due to the easily determined translucent epidermal phenotype of Bm-ok-disrupted cells. Two crossing strategies were used. In the first, injected Bm-ok moths were crossed with wild-type moths, and a 28.6% frequency of germline mutation transmission was observed. In the second strategy, two Bm-ok mosaic mutant moths were crossed with each other, and 93.6% of the offsprings appeared mutations in both alleles of Bm-ok gene (compound heterozygous). In summary, the CRISPR/Cas9 system can act as a highly specific and heritable gene-editing tool in Bombyx mori.     

Chitin in the Silk Gland Ducts of the Spider Nephila edulis and the Silkworm Bombyx mori

Here we report the detection and localisation of chitin in the cuticle of the spinning ducts of both the spider Nephila edulis and the silkworm Bombyx mori. Our observations demonstrate that the duct walls of both animals contain chitin notwithstanding totally independent evolutionary pathways of the systems. We conclude that chitin may well be an essential component for the construction of spinning ducts; we further conclude that in both species chitin may indicate the evolutionary origin of the spinning ducts.     

Translationally Controlled Tumor Protein,a Dual Functional Protein Involved in the Immune Response of the Silkworm,Bombyx mori

Insect gut immunity is the first line of defense against oral infection. Although a few immune-related molecules in insect intestine has been identified by genomics or proteomics approach with comparison to well-studied tissues, such as hemolymph or fat body, our knowledge about the molecular mechanism underlying the gut immunity which would involve a variety of unidentified molecules is still limited. To uncover additional molecules that might take part in pathogen recognition, signal transduction or immune regulation in insect intestine, a T7 phage display cDNA library of the silkworm midgut is constructed. By use of different ligands for biopanning, Translationally Controlled Tumor Protein (TCTP) has been selected. BmTCTP is produced in intestinal epithelial cells and released into the gut lumen. The protein level of BmTCTP increases at the early time points during oral microbial infection and declines afterwards. In vitro binding assay confirms its activity as a multi-ligand binding molecule and it can further function as an opsonin that promotes the phagocytosis of microorganisms. Moreover, it can induce the production of anti-microbial peptide via a signaling pathway in which ERK is required and a dynamic tyrosine phosphorylation of certain cytoplasmic membrane protein. Taken together, our results characterize BmTCTP as a dual-functional protein involved in both the cellular and the humoral immune response of the silkworm, Bombyx mori.     

BmPAH Catalyzes the Initial Melanin Biosynthetic Step in Bombyx mori

Pigmentation during insect development is a primal adaptive requirement. In the silkworm, melanin is the primary component of larval pigments. The rate limiting substrate in melanin synthesis is tyrosine, which is converted from phenylalanine by the rate-limiting enzyme phenylalanine hydroxylase (PAH). While the role of tyrosine, derived from phenylalanine, in the synthesis of fiber proteins has long been known, the role of PAH in melanin synthesis is still unknown in silkworm. To define the importance of PAH, we cloned the cDNA sequence of BmPAH and expressed its complete coding sequence using the Bac-to-Bac baculovirus expression system. Purified recombinant protein had high PAH activity, some tryptophan hydroxylase activity, but no tyrosine hydroxylase activity, which are typical properties of PAH in invertebrates. Because melanin synthesis is most robust during the embryonic stage and larval integument recoloring stage, we injected BmPAH dsRNA into silkworm eggs and observed that decreasing BmPAH mRNA reduced neonatal larval tyrosine and caused insect coloration to fail. In vitro cultures and injection of 4^th instar larval integuments with PAH inhibitor revealed that PAH activity was essential for larval marking coloration. These data show that BmPAH is necessary for melanin synthesis and we propose that conversion of phenylalanine to tyrosine by PAH is the first step in the melanin biosynthetic pathway in the silkworm.     

Establishment of Tools for Neurogenetic Analysis of Sexual Behavior in the Silkmoth,Bombyx mori

Background

Silkmoth, Bombyx mori, is an ideal model insect for investigating the neural mechanisms underlying sex pheromone-induced innate behavior. Although transgenic techniques and the GAL4/UAS system are well established in the silkmoth, genetic tools useful for investigating brain function at the neural circuit level have been lacking.

Results

In the present study, we established silkmoth strains in which we could visualize neural projections (UAS-mCD8GFP) and cell nucleus positions (UAS-GFP.nls), and manipulate neural excitability by thermal stimulation (UAS-dTrpA1). In these strains, neural projections and nucleus position were reliably labeled with green fluorescent protein in a GAL4-dependent manner. Further, the behavior of silkworm larvae and adults could be controlled by GAL4-dependent misexpression of dTrpA1. Ubiquitous dTrpA1 misexpression led both silkmoth larvae and adults to exhibit seizure-like phenotypes in a heat stimulation-dependent manner. Furthermore, dTrpA1 misexpression in the sex pheromone receptor neurons of male silkmoths allowed us to control male sexual behavior by changing the temperature. Thermally stimulated male silkmoths exhibited full sexual behavior, including wing-flapping, orientation, and attempted copulation, and precisely approached a thermal source in a manner similar to male silkmoths stimulated with the sex pheromone.

Conclusion

These findings indicate that a thermogenetic approach using dTrpA1 is feasible in Lepidopteran insects and thermogenetic analysis of innate behavior is applicable in the silkmoth. These tools are essential for elucidating the relationships between neural circuits and function using neurogenetic methods.     

The Effect of W Chromosome Origin on Sex-Chromosome Pairing in ZZWW Tetraploid Females of the Domesticated Silkworm, Bombyx Mori, and the Congenic Wild Silkworm, Bombyx Mandarina

To analyze the degree of pairing of the Z and W chromosomes in ZZWW tetraploid female silkworms that have the W chromosomes of the domesticated silkworm, Bombyx mori, and those of the wild silkworm, Bombyx mandarina, we induced two types of ZZWW tetraploid female silkworms (Cr4n, Wr4n) through cold treatment of the eggs. The Wr4n female is congenic to the Cr4n female for W chromosomes; namely, the W chromosomes of the Wr4n female are derived from those of B. mandarina. Each of the sex ratios (/) in filial triploids from the Cr4n females was shown to be in the range of 3.9–5.3 (4.6 as an average of six cases). On the other hand, each of the sex ratios (/) in filial triploids from the Wr4n females was shown to be in the range of 6.2–9.0 (6.9 as an average of nine cases). The results of a t-test indicated that the difference in sex ratios in the two groups is highly significant (at the 0.1% level). These results suggest that, in the meiosis of the ZZWW tetraploid female, the frequency of pairing of the W chromosome of B. mandarina and the Z chromosome of B. mori is lower than that of the pairing of the W and Z chromosomes of B. mori. Furthermore, the t-test results are evidence that the W chromosomes have undergone significant evolutional change.     

A Baculovirus Immediate-Early Gene,ie1, Promoter Drives Efficient Expression of a Transgene in Both Drosophila melanogaster and Bombyx mori

Many promoters have been used to drive expression of heterologous transgenes in insects. One major obstacle in the study of non-model insects is the dearth of useful promoters for analysis of gene function. Here, we investigated whether the promoter of the immediate-early gene, ie1, from the Bombyx mori nucleopolyhedrovirus (BmNPV) could be used to drive efficient transgene expression in a wide variety of insects. We used a piggyBac-based vector with a 3xP3-DsRed transformation marker to generate a reporter construct; this construct was used to determine the expression patterns driven by the BmNPV ie1 promoter; we performed a detailed investigation of the promoter in transgene expression pattern in Drosophila melanogaster and in B. mori. Drosophila and Bombyx belong to different insect orders (Diptera and Lepidoptera, respectively); however, and to our surprise, ie1 promoter-driven expression was evident in several tissues (e.g., prothoracic gland, midgut, and tracheole) in both insects. Furthermore, in both species, the ie1 promoter drove expression of the reporter gene from a relatively early embryonic stage, and strong ubiquitous ie1 promoter-driven expression continued throughout the larval, pupal, and adult stages by surface observation. Therefore, we suggest that the ie1 promoter can be used as an efficient expression driver in a diverse range of insect species.     

Effect of Carotenoid Depletion on the Hatching Rhythm of the Silkworm, Bombyx mori

To investigate the functional involvement of carotenoid in the insect circadian rhythm, we observed the effect of carotenoid depletion on the hatching patterns of the silkworm under several light conditions. When eggs were transferred from continuous light (LL) to continuous darkness (DD), an overt hatching rhythm was initiated. The first hatching peak, which was observed at 13.2 h after the transfer in the carotenoid-depleted silkworm, was reduced remarkably in both control groups (carotenoid-rich and carotenoid-depleted but with lutein supplementation), though subsequent peaks occurred with similar timing. Under LD 4 : 20, LD 12 : 12 and LD 20 :4, hatching patterns depended on the dietary carotenoid and the light intensity of the photophase. At a low light intensity, carotenoid depletion suppressed hatching as a masking effect just after light-on. Under LD 4 : 20 at a low light illumination, hatchings in the carotenoid-depleted silkworm were observed during scotophase, but few larvae hatched during scotophase in the control groups. Our findings suggest that carotenoid is not indispensable for the photoreception, formation and entrainment of the circadian hatching rhythm, but that a carotenoid-dependent process that is induced by light-off and damps out in an hour-glass mode, suppresses the hatching during darkness without affecting the circadian oscillation. The possibility that this carotenoid-dependent process is involved in the photoperiodic induction in the silkworm was discussed.