Effects of baculovirus infection on intestinal microflora of BmNPV resistant and susceptible strain silkworm

Bombyx mori L. (Lepidoptera: Bombycidae) nucleopolyhedrovirus (BmNPV) is a serious pathogen causing huge economic losses to sericulture. There is growing evidence that the gut microbiota of silkworms plays a critical role in shaping host responses and interactions with viral infection. However, little is known about the differences in the composition and diversity of intestinal microflora, especially with respect to silkworm strain differences and BmNPV infection-induced changes. Here, we aim to explore the differences between BmNPV-resistant strain A35 and susceptible strain P50 silkworm and the impact of BmNPV infection on intestinal microflora in different strains. The 16S rDNA sequencing analysis revealed that the fecal microbial populations were distinct between A35 and P50 and were significantly changed post BmNPV infection in both strains. Further analysis showed that the BmNPV-resistant strain silkworm possessed higher bacterial diversity than the susceptible strain, and BmNPV infection reduced the diversity of intestinal flora assessed by feces in both silkworm strains. In response to BmNPV infection, the abundance of Muribaculaceae increased in P50 and decreased in A35, while the abundance of Enterobacteriaceae decreased in P50 and increased in A35. These results indicated that BmNPV infection had various effects on the abundance of fecal microflora in different silkworm strains. Our findings not only broadened the understanding of host-pathogen interactions but also provided theoretical help for the breeding of resistant strains and healthy rearing of silkworms based on symbiotic bacteria.

Identification of long non-coding RNAs in response to microsporidia infection in Silkworm, Bombyx mori

Microsporidia Nosema bombycis (Nb) is a cellular parasite responsible for pébrine disease in silkworms, significantly impacting the sericulture industry. Long non-coding RNAs (lncRNAs), which are RNA fragments longer than 200 nucleotides, are pivotal in a range of cellular and physiological functions. However, the potential role of silkworm lncRNAs in response to Nb infection remains unknown. This study conducted transcriptome sequencing on both larvae and Nb-infected midguts of silkworms, identifying 1,440 lncRNAs across all examined midgut samples. Within the Nb-infected group, 42 differentially expressed lncRNAs (DElncRNAs) and 305 differentially expressed mRNAs (DEmRNAs) were detected. Functional annotation and pathway analysis showed that these DEmRNAs are mostly involved in metabolism, apoptosis, autophagy, and other key pathways. The co-expression network of DEmRNAs and DElncRNAs illustrates that 1 gene could be regulated by multiple lncRNAs and 1 lncRNA may target multiple genes, indicating that the regulation of lncRNA is intricate and networked. In addition, the DElncRNA-miRNA-mRNA network showed that some DElncRNAs may be involved in the immune response and metabolism through miRNA. Notably, the study observed an increase in lncRNA MSTRG857.1 following Nb infection, which may promote Nb proliferation. These findings offer insights into the complex interplay between insects and microsporidia.

Nonsteaming method improves the nutritional value and utilization efficiency of silkworm artificial diets

Artificial diets for silkworms overcome the seasonal limitations of traditional rearing methods with fresh mulberry leaves. However, the current wet artificial diets, steamed at high temperatures, are not favored by silkworms, and they are cumbersome and challenging to preserve. These conditions adversely affected the development of artificial diet-based sericulture production. In this study, we disinfected dry powder diets with radiation and added distilled water without steaming before use. Then, the nutritional value of finished diets and their impact on silkworm development was assessed. Compared with steamed diets, nonsteamed diets were more attractive to silkworms. Chemical assays showed significantly more essential nutrients for silkworms, including l-ascorbic acid, vitamin B1, vitamin B2, and urease in nonsteamed diets than in steamed diets. Feeding fifth-instar silkworm larvae with nonsteamed diets significantly improved the ammonia utilization efficiency of the diet and increased the cocoon shell rate and diet/silk protein conversion efficiency by 5.9% and 13.3%, respectively. When fed with nonsteamed diets, the abundance of aerobic microorganisms in silkworm intestines increased and the abundance of pathogenic bacteria decreased. Furthermore, the vitality of the silkworm, measured by the dead worm cocoon rate, significantly improved by 16.90%. In summary, preparing sterile wet diets without high-temperature steaming effectively improved the nutritional value of the diet and enhanced silkworm growth.

Bivoltine cocoon color sex-limited breeds of Bombyx mori (Lepidoptera: Bombycidae) show enhanced economic performance and fecundity following spermidine supplementation

Sericulture has immense economic significance. Separating male and female silkworm pupae for egg production in grainage is a laborious task; hence, sex-limited breeds for cocoon color are advantageous for this process. The major constraint in sex-limited breeds is their low fecundity. Sex-limited female moths lay a lower number of eggs than nonsex-limited breeds. Polyamine, spermidine was shown to improve fecundity in several organisms, including the silkworm, Bombyx mori L. In the present study, cocoon color sex-limited breeds, HTO2SL, APS27SL, and SLFC27, were selected and fed with spermidine to improve fecundity and nutritional efficiency. The fifth-instar silkworm larvae of the selected breeds were subjected to standard rearing conditions with and without spermidine supplementation up to spinning. The spermidine-supplemented sex-limited breeds exhibited a significant increase in fecundity, nutritional indices, and economic parameters compared with the control sex-limited breeds. Among the 3 sex-limited breeds tested, the performance of APS27SL improved significantly.

Functional characterization of Bombyx mori (Lepidoptera: Bombycidae) C-type lectin 5

C-type lectins (CTLs) are an important family of pattern recognition receptors (PRRs) that regulate immune responses. The CTL5 gene of the silkworm Bombyx mori L. (Lepidoptera: Bombycidae) encodes a protein comprised of 223 amino acids, containing a signal peptide and a carbohydrate recognition domain (CRD). Our previous study showed that CTL5 can facilitate the clearance of bacteria from larval hemocoel but the underlying mechanisms are unclear. In this study, we found that CTL5 was mainly expressed in fourth-instar larvae, adult moths, and the larval epidermis. CTL5 expression showed differential responses to both pathogenic stimuli and the molting hormone 20-hydroxyecdysone. The full-length (FL) and truncated (ΔN/ΔC/ΔNC) CTL5 recombinant proteins can bind to hemocytes, polysaccharides, bacteria, and spores of the entomopathogenic fungus Beauveria bassiana. Yeast 2-hybrid assays showed that the recombinant proteins can interact with integrin β2-β5 subunits. Recombinant proteins increased the phagocytic rate of hemocytes. Injection of recombinant CTL5 stimulated the expression of many immune genes in hemocytes, mainly antimicrobial peptides and immune signaling molecules. Additionally, transcriptomic sequencing of CTL5-stimulated hemocytes revealed 265 upregulated and 580 downregulated genes. Functional enrichment and the gene set enrichment analyses showed that differentially expressed genes were mainly enriched in innate immune responses and signaling. Our study suggests that CTL5 may act as an opsonin to enhance the clearance of pathogens by regulating both humoral and cellular responses.

Highly accurate genome assembly of an improved high-yielding silkworm strain, Nichi01

The silkworm (Bombyx mori) is an important lepidopteran model insect and an industrial domestic animal traditionally used for silk production. Here, we report the genome assembly of an improved Japanese strain Nichi01, in which the cocoon yield is comparable to that of commercial silkworm strains. The integration of PacBio Sequel II long-read and ddRAD-seq-based high-density genetic linkage map achieved the highest quality genome assembly of silkworms to date; 22 of the 28 pseudomolecules contained telomeric repeats at both ends, and only four gaps were present in the assembly. A total of 452 Mbp of the assembly with an N50 of 16.614 Mbp covered 99.3% of the complete orthologs of the lepidopteran core genes. Although the genome sequence of Nichi01 and that of the previously reported low-yielding tropical strain p50T assured their accuracy in most regions, we corrected several regions, misassembled in p50T, in our assembly. A total of 18,397 proteins were predicted using over 95 Gb of mRNA-seq derived from 10 different organs, covering 96.9% of the complete orthologs of the lepidopteran core genes. The final assembly and annotation files are available in KAIKObase (https://kaikobase.dna.affrc.go.jp/index.html) along with a genome browser and BLAST searching service, which would facilitate further studies and the breeding of silkworms and other insects.

A molecular cell biology toolkit for the study of meiosis in the silkworm Bombyx mori

Meiosis is usually described as four essential and sequential processes: (1) homolog pairing; (2) synapsis, mediated by the synaptonemal complex (SC); (3) crossing over; and (4) segregation. In this canonical model, the maturation of crossovers into chiasmata plays a vital role in holding homologs together and ensuring their segregation at the first meiotic division. However, Lepidoptera (moths and butterflies) undergo three distinct meiotic processes, only one of which is canonical. Lepidoptera males utilize two meiotic processes: canonical meiosis that produces nucleated fertile sperm, and a noncanonical meiosis that produces anucleated nonfertile sperm which are nonetheless essential for reproduction. Lepidoptera females, which carry heteromorphic sex chromosomes, undergo a completely achiasmate (lacking crossovers) meiosis, thereby requiring an alternative mechanism to ensure proper homolog segregation. Here we report the development of a molecular cell biology toolkit designed to properly analyze features of meiosis, including the SC structure and function, in the silkworm Bombyx mori. In addition to standard homology searches to identify Bombyx orthologs of known SC encoding genes, we developed an ortholog discovery app (Shinyapp) to identify Bombyx orthologs of proteins involved in several meiotic processes. We used this information to clone genes expressed in the testes and then created antibodies against their protein products. We used the antibodies to confirm the localization of these proteins in normal male spermatocytes, as well as using in vitro assays to confirm orthologous interactions. The development of this toolkit will facilitate further study of the unique meiotic processes that characterize meiosis in Lepidoptera.

Identification and characterization of a serine racemase in the silkworm Bombyx mori

The pupae of lepidopterans contain high concentrations of endogenous d-serine. In the silkworm Bombyx mori, d-serine is negligible during the larval stage but increases markedly during the pupal stage, reaching 50% of the total free serine. However, the physiological function of d-serine and the enzyme responsible for its production are unknown. Herein, we identified a new type of pyridoxal 5'-phosphate (PLP)-dependent serine racemase (SR) that catalyzes the racemization of l-serine to d-serine in B. mori. This silkworm SR (BmSR) has an N-terminal PLP-binding domain that is homologous to mammalian SR and a C-terminal putative ligand-binding regulatory-like domain (ACT-like domain) that is absent in mammalian SR. Similar to mammalian SRs, BmSR catalyzes the racemization and dehydration of both serine isomers. However, BmSR is different from mammalian SRs as evidenced by its insensitivity to Mg2+/Ca2+ and Mg-ATP-which are required for activation of mammalian SRs-and high d-serine dehydration activity. At the pupal stage, the SR activity was predominantly detected in the fat body, which was consistent with the timing and localization of BmSR expression. The results are an important first step in elucidating the physiological significance of d-serine in lepidopterans.

Mitochondrial Genome Architecture and Evolutionary Origin of the Yao Silkworm, a Living Fossil of the Domestic Silkworm Bombyx mori (Lepidoptera: Bombycidae)

The Yao silkworm is a unique silkworm resource producing yellow flat plate silk that has only been reared by the Baiku Yao ethnic group in Nandan County, Guangxi Province, China for a thousand years. Here, we report the mitochondrial genomes (mitogenomes) of five Yao silkworm strains and 10 local Guangxi strains of the domestic silkworm (Bombyx mori) L. (Lepidoptera: Bombycidae), and use the resulting mitogenomes and the available Bombyx mitogenomes to characterize their genome architecture and trace the evolutionary origin of the Yao silkworm. The five Yao silkworm mitogenomes exhibited genome architectures identical to typical set of 37 mitochondrial genes (13 protein-coding genes, 22 transfer RNAs, and two ribosomal RNAs) and a high level of genome sequence similarity with the domestic silkworm. Mitogenome-based phylogenetic reconstruction provided solid evidence that the Yao silkworm shares a common ancestor with the domestic silkworm. Sliding window analysis uncovered a distinct variation pattern in the mitogenome between the Yao silkworm and the other domestic silkworm strains. The phylogenetic analyses revealed a basal placement of the Yao silkworm among all available domestic silkworm strains, indicating that the Yao silkworm is an ancient population of the domestic silkworm. Our data indicated that the Yao silkworm (B. mori) is a lineage of the domestic silkworm, which for the first time provides insights into the origin of the Yao silkworm.

Effect of Lactic Acid Supplementation on the Growth and Reproduction of Bombyx mori (Lepidopteria: Bombycidae)

Lactic acid is widely used in the food, drugs, cosmetics, and other industries to maintain the microbial stability of low-pH products. However, it is unclear whether lactic acid can affect silkworm (Bombyx mori) growth and reproduction. This study investigated the effects of lactic acid on the growth and reproduction of the silkworm. We analyzed the growth, cocoon quality, and reproductive performance of fifth instar larvae fed on mulberry leaves saturated with different concentrations (0.01, 0.1, 1, and 10%) of lactic acid and the control. Results showed that 0.01, 0.1, and 1% lactic acid supplementation positively affects growth and female cocoon quality, with increased larval weight and female cocoon shell weight compared to the control group. In contrast, 10% lactic acid was toxic to the larvae and significantly decreased growth, leading to larval death. Our study provides a basic reference for the optimal amount of preservatives. In addition, this study can be a desirable intervention for sericulturists and can play an important role in getting high return from silkworm-rearing activities.

A Pattern Recognition Receptor C-type Lectin-S6 (CTL-S6) is Involved in the Immune Response in the Silkworm (Lepidoptera: Bombycidae)

Insect innate immunity is initiated by the special recognition and binding of the foreign pathogens, which is accomplished by the pattern recognition receptors (PRRs). As an important type of PRRs, C-type lectins (CTLs) play various roles in insect innate immunity, including pathogen recognition, stimulation of prophenoloxidase, regulation of cellular immunity and so on. In this study, we have cloned the full-length cDNA of a CTL gene named CTL-S6 from the silkworm, Bombyx mori. The open reading frame (ORF) of B. mori CTL-S6 encodes 378 amino acids, which contain a secretion signal peptide. The mRNA of CTL-S6 exhibited the highest transcriptional level in the midgut. Its transcriptional level increased dramatically in fat body and hemocytes upon Escherichia coli or Micrococcus luteus challenge. Purified recombinant CTL-S6 could bind to bacterial cell wall components, including peptidoglycan (PGN, from Bacillus subtilis) and lipopolysaccharide (LPS, from E. coli 0111:B4), and recombinant CTL-S6 was involved in the encapsulation and melanization of hemocytes. Furthermore, the addition of recombinant CTL-S6 to the hemolymph of silkworm resulted in a significant increase in phenoloxidase activity. Overall, our results indicated that B. mori CTL-S6 may serve as a PRR for the recognition of foreign pathogens, prophenoloxidase pathway stimulation and involvement in the innate immunity.

Identification and Characterization of Aldehyde Oxidase 5 in the Pheromone Gland of the Silkworm (Lepidoptera: Bombycidae)

Aldehyde oxidases (AOXs) are a subfamily of cytosolic molybdo-flavoenzymes that play critical roles in the detoxification and degradation of chemicals. Active AOXs, such as AOX1 and AOX2, have been identified and functionally analyzed in insect antennae but are rarely reported in other tissues. This is the first study to isolate and characterize the cDNA that encodes aldehyde oxidase 5 (BmAOX5) in the pheromone gland (PG) of the silkworm, Bombyx mori. The size of BmAOX5 cDNA is 3,741 nucleotides and includes an open reading frame, which encodes a protein of 1,246 amino acid residues. The theoretical molecular weight and isoelectric point of BmAOX5 are approximately 138 kDa and 5.58, respectively. BmAOX5 shares a similar primary structure with BmAOX1 and BmAOX2, containing two [2Fe-2S] redox centers, a FAD-binding domain, and a molybdenum cofactor (MoCo)-binding domain. RT-PCR revealed BmAOX5 to be particularly highly expressed in the PG (including ovipositor) of the female silkworm moth, and the expression was further confirmed by in situ hybridization, AOX activity staining, and anti-BmAOX5 western blotting. Further, BmAOX5 was shown to metabolize aromatic aldehydes, such as benzaldehyde, salicylaldehyde, and vanillic aldehyde, and fatty aldehydes, such as heptaldehyde and propionaldehyde. The maximum reaction rate (Vmax) of benzaldehyde as substrate was 21 mU and Km was 1.745 mmol/liter. These results suggested that BmAOX5 in the PG could metabolize aldehydes in the cytoplasm for detoxification or participate in the degradation of aldehyde pheromone substances and odorant compounds to identify mating partners and locate suitable spawning sites.

Sensitivity of Polyvoltine Thai Strains of Bombyx mori to a BmNPV Isolate From Mahasarakham

Virus infection by the Bombyx mori nucleopolyhedrovirus (BmNPV) is the most severe disease in Thai sericultural practice of polyvoltine silkworms. Here, we characterized a newly isolated BmNPV isolated from the Mahasarakham province in Thailand (BmNPV-MSU). The purity and morphology of BmNPV-MSU were examined using light microscopy and scanning electron microscopy. The polyhedral inclusion bodies (PIBs) of BmNPV-MSU appeared in tetragonal, hexagonal, octagonal, and globular forms. The virions were both single and multiple embedded as observed by transmission electron microscopy. We also determined the virulence of BmNPV-MSU for six different Thai polyvoltine strains by LC50 and time to death after infection. The LC50 values of Nang Lai, NK04, and Sam Rong strains were 5.05-1.52 × 107 PIBs per ml and mortality peaked 7- to 8-d after inoculation. For Nang Noi, SP2, and RE05 strains the LC50 values were 7.91-1.82 × 106 PIBs/ml and mortality peaked 4-5 d after inoculation, thus having lower chance of survival to infection by BmNPV-MSU.

Can Feeding of Silkworm on Different Mulberry Variety Affect Its Performance?

This study attempted to identify a suitable host plant for rearing hybrid 31*32 silkworms. Four varieties of mulberry leaves including Ichinose, Kenmochi, Kines, and local were supplied to hybrid 31*32 of silkworm and their performance was assessed. Variables measured included nutrition, biochemistry, and economic parameters of silkworm. Number of cocoons, individual and total cocoon weight, cocoon shell weight, and cocoon yield/10,000 per larvae were significantly greater on the Kines variety compared with the others. Similarly, nutritional indices including efficiency of conversion of ingested food (ECI), efficiency of conversion of digested food (ECD), relative growth rate (RGR), and consumption index (CI) were greater for the larvae fed on Kines. Digestive and antioxidant enzyme activity of the larvae differed with variety. Our results suggest that Kines could be considered as an appropriate host plant for rearing of silkworms based on the study parameters.

Proteomic Identification of Immune-Related Silkworm Proteins Involved in the Response to Bacterial Infection

Bombyx mori (Lepidoptera: Bombycidae) is an important economic insect and a classic Lepidopteran model system. Although immune-related genes have been identified at a genome-wide scale in the silkworm, proteins involved in immune defense of the silkworm have not been comprehensively characterized. In this study, two types of bacteria were injected into the silkworm larvae, Gram-negative Escherichia coli (Enterobacteriales: Enterobacteriaceae), or Gram-positive Staphylococcus aureus (Bacillales: Staphylococcaceae). After injection, proteomic analyses of hemolymph were performed by liquid chromatography-tandem mass spectrometry. In total, 514 proteins were identified in the uninduced control group, 540 were identified in the E. coli-induced group, and 537 were identified in the S. aureus-induced group. Based on Uniprot annotations, 32 immunological recognition proteins, 28 immunological signaling proteins, and 21 immunological effector proteins were identified. We found that 127 proteins showed significant upregulation, including 10 immunological recognition proteins, 4 immunological signaling proteins, 11 immunological effector proteins, and 102 other proteins. Using real-time quantitative polymerase chain reaction in the fat body, we verified that immunological recognition proteins, signaling proteins, and effector proteins also showed significant increases at the transcriptional level after infection with E. coli and S. aureus. Five newly identified proteins showed upregulation at both protein and transcription levels after infection, including 30K protein, yellow-d protein, chemosensory protein, and two uncharacterized proteins. This study identified many new immune-related proteins, deepening our understanding of the immune defense system in B. mori. The data have been deposited to the iProX with identifier IPX0001337000.

A Comparative Whole Genome Sequence Analysis Leads to Identification of Repeat-Associated Evolutionarily Conserved miRNAs in Bombyx mori (Lepidoptera: Bombycidae)

MicroRNAs (miRNAs) are evolutionarily conserved small noncoding RNAs, which play important regulatory roles in various biological processes. In this study, we have developed a computational approach for detecting conserved miRNAs based on comparison of whole genome sequences of closely related species by considering various key features of experimentally validated miRNAs. By applying this approach, we have identified 34 new miRNAs from Bombyx mori (L.), which are also conserved in Drosophila melanogaster (Diptera: Drosophilidae) and Anopheles gambiae (Diptera: Culicidae). Most of these miRNAs were associated with repeat region of the genome. We did an expression analysis of the 34 newly predicted miRNAs and found that 30 of these miRNAs are expressing in different tissues of B. mori. Besides, we have also predicted the putative targets of these miRNAs in B. mori based on several known characteristic features of miRNA::mRNA duplexes and found that these targets include diverse range of functions, suggesting multiple layers of gene regulation of various important biological processes.