Ectopic expression of BmeryCA in Bombyx mori increases silk yield and mechanical properties by altering the pH of posterior silk gland.

The silk glands are the specialized tissue where silk protein synthesis, secretion, and conformational transitions take place, with pH playing a critical role in both silk protein synthesis and fiber formation. In the present study, we have identified erythrocyte carbonic anhydrase (BmeryCA) belonging to the α-CA class in the silk gland, which is a Zn2+ dependent metalloenzyme capable of efficiently and reversibly catalyzing the hydrated reaction of CO2 to HCO3-, thus participating in the regulation of acid-base balance. Multiple sequence alignments revealed that the active site of BmeryCA was highly conserved. Tissue expression profiling showed that BmeryCA had relatively high expression levels in hemolymph and epidermis but is barely expressed in the posterior silk gland (PSG). By specifically overexpressing BmeryCA in the PSG, we generated transgenic silkworms. Ion-selective microelectrode (ISM) measurements demonstrated that specifically overexpression of BmeryCA in the PSG led to a shift in pH from weakly alkaline to slightly neutral conditions. Moreover, the resultant PSG-specific BmeryCA overexpression mutant strain displayed a significant increase in both silk yield and silk fiber mechanical properties. Our research provided new insights into enhancing silk yield and improving the mechanical properties of silk fibers.

High-throughput and genome-scale targeted mutagenesis using CRISPR in a nonmodel multicellular organism, Bombyx mori.

Large-scale genetic mutant libraries are powerful approaches to interrogating genotype-phenotype correlations and identifying genes responsible for certain environmental stimuli, both of which are the central goal of life science study. We produced the first large-scale CRISPR-Cas9-induced library in a nonmodel multicellular organism, Bombyx mori We developed a piggyBac-delivered binary genome editing strategy, which can simultaneously meet the requirements of mixed microinjection, efficient multipurpose genetic operation, and preservation of growth-defect lines. We constructed a single-guide RNA (sgRNA) plasmid library containing 92,917 sgRNAs targeting promoters and exons of 14,645 protein-coding genes, established 1726 transgenic sgRNA lines following microinjection of 66,650 embryos, and generated 300 mutant lines with diverse phenotypic changes. Phenomic characterization of mutant lines identified a large set of genes responsible for visual phenotypic or economically valuable trait changes. Next, we performed pooled context-specific positive screens for tolerance to environmental pollutant cadmium exposure, and identified KWMTBOMO12902 as a strong candidate gene for breeding applications in sericulture industry. Collectively, our results provide a novel and versatile approach for functional B. mori genomics, as well as a powerful resource for identifying the potential of key candidate genes for improving various economic traits. This study also shows the effectiveness, practicality, and convenience of large-scale mutant libraries in other nonmodel organisms.

Effect of eIF6 on the development of silk glands and silk protein synthesis of the silkworm, Bombyx mori.

The silkworm is a lepidopteran domesticated from the wild silkworm, mostly valued for its efficient synthesis of silk protein. This species' ability to spin silk has supported the 5500-year-old silk industry and the globally known "Silk Road", making the transformation of mulberry leaves into silk of great concern. Therefore, research on the silk-related genes of silkworms and their regulatory mechanisms has attracted increasing attention. Previous studies have revealed that domestic silk gland cells are endoreduplication cells, and their high-copy genome and special chromatin conformation provide conditions for the high expression of silk proteins. In this study, we systematically investigate the expression pattern of eukaryotic initiation factors (eIFs) and identified the eIF6 as a eukaryotic translation initiation factor involved in the synthesis of silk proteins. We generated an eIF6 gene deletion mutant strain of silkworm using the CRISPR/Cas9 system and investigated the function of eIF6 in silk gland development and silk protein synthesis. The results showed that deletion of eIF6 inhibited the individual development of silkworm larvae, inhibited the development of silk glands, and significantly reduced the cocoon layer ratio. Therefore, we elucidated the function of eIF6 in the development of silk glands and the synthesis of silk proteins, which is important for further elucidation of the developmental process of silk glands and the mechanism underlying the ultra-high expression of silk proteins.

Genetically engineered Blue silkworm capable of synthesizing natural blue pigment.

Synthetic biology is an eco-friendly and sustainable approach for the production of compounds, particularly used when the production processes involve toxic reagents. In this study, we used the silk gland of silkworm to produce indigoidine, a valuable natural blue pigment that cannot be synthesized naturally in animals. We genetically engineered these silkworms by integrating the indigoidine synthetase (idgS) gene from S. lavendulae and the PPTase (Sfp) gene from B. subtilis into the silkworm genome. In the resulting Blue silkworm, indigoidine was detected at a high level in the posterior silk gland (PSG), spanning all developmental stages from larvae to adults, without affecting silkworm growth or development. This synthesized indigoidine was secreted from the silk gland and subsequently stored in the fat body, with only a small fraction being excreted by the Malpighian tubule. Metabolomic analysis revealed that Blue silkworm efficiently synthesized indigoidine by upregulating l-glutamine, the precursor of indigoidine, and succinate, which is related to energy metabolism in the PSG. This study represents the first synthesis of indigoidine in an animal and therefore opens a new avenue for the biosynthesis of natural blue pigments and other valuable small molecules.

Combined CRISPR toolkits reveal the domestication landscape and function of the ultra-long and highly repetitive silk genes.

Highly repetitive sequences play a major structural and function role in the genome. In the present study, we developed Cas9-assisted cloning and SMRT sequencing of long repetitive sequences (CACS) to sequence and manipulate highly repetitive genes from eukaryotic genomes. CACS combined Cas9-mediated cleavage of a target segment from an intact genome, Gibson assembly cloning, and PacBio SMRT sequencing. Applying CACS, we directly cloned and sequenced the complete sequences of fibroin heavy chain (FibH) genes from 17 domesticated (Bombyx mori) and 7 wild (Bombyx mandarina) silkworms. Our analysis revealed the unique fine structure organization, genetic variations, and domestication dynamics of FibH. We also demonstrated that the length of the repetitive regions determined the mechanical properties of silk fiber, which was further confirmed by Cas9 editing of FibH. CACS is a simple, robust, and efficient approach, providing affordable accessibility to highly repetitive regions of a genome. STATEMENT OF SIGNIFICANCE: Silkworm silk is the earliest and most widely used animal fiber, and its excellent performance mainly depends on the fibroin heavy chain (FibH) protein. The FibH gene is the main breakthrough in understanding the formation mechanism and improvement of silk fiber. In the study, we developed a CACS method for characterizing the fine structure and domestication landscape of 24 silkworm FibH genes. We used CRISPR/Cas9 to edit the repetitive sequence of FibH genes, revealing the relationship between FibH genes and mechanical properties of silkworm silk. Our study is helpful in modifying silk genes to manipulate other valuable highly repetitive sequences, and provides insight for silkworm breeding.

Comparison of Long-Read Methods for Sequencing and Assembly of Lepidopteran Pest Genomes.

Lepidopteran species are mostly pests, causing serious annual economic losses. High-quality genome sequencing and assembly uncover the genetic foundation of pest occurrence and provide guidance for pest control measures. Long-read sequencing technology and assembly algorithm advances have improved the ability to timeously produce high-quality genomes. Lepidoptera includes a wide variety of insects with high genetic diversity and heterozygosity. Therefore, the selection of an appropriate sequencing and assembly strategy to obtain high-quality genomic information is urgently needed. This research used silkworm as a model to test genome sequencing and assembly through high-coverage datasets by de novo assemblies. We report the first nearly complete telomere-to-telomere reference genome of silkworm Bombyx mori (P50T strain) produced by Pacific Biosciences (PacBio) HiFi sequencing, and highly contiguous and complete genome assemblies of two other silkworm strains by Oxford Nanopore Technologies (ONT) or PacBio continuous long-reads (CLR) that were unrepresented in the public database. Assembly quality was evaluated by use of BUSCO, Inspector, and EagleC. It is necessary to choose an appropriate assembler for draft genome construction, especially for low-depth datasets. For PacBio CLR and ONT sequencing, NextDenovo is superior. For PacBio HiFi sequencing, hifiasm is better. Quality assessment is essential for genome assembly and can provide better and more accurate results. For chromosome-level high-quality genome construction, we recommend using 3D-DNA with EagleC evaluation. Our study references how to obtain and evaluate high-quality genome assemblies, and is a resource for biological control, comparative genomics, and evolutionary studies of Lepidopteran pests and related species.

Deep Sequencing Reveals the Comprehensive CRISPR-Cas9 Editing Spectrum in Bombyx mori.

Application of the clustered regularly interspaced short palindromic repeats associated 9 (CRISPR-Cas9) technology has revolutionized biology by greatly enhancing the ability to introduce mutations into DNA for research and prospective therapeutic purposes. However, the understanding of Cas9 editing outcomes is still limited. Previously, it was considered that Cas9 introduces stochastic insertions or deletions (indels) at the target site. In the current study, we performed in vivo multiplex editing, deep sequencing, and comprehensive analysis of its editing outcomes in Bombyx mori (B. mori). A total of 31161 editing events from 9 single-guide RNA (sgRNA) sites in 16 individuals were generated and analyzed, and we found that Cas9 introduces mutations with some regularity rather than via stochastic indels. The editing efficiency varies with sgRNA sequences, individuals, and orientation. Small deletions account for the vast majority of mutated sequences, followed by a small fraction of substitutions and insertions. The most likely mutations are deletions between two microhomologous sequences or single-base deletions at the cleavage site in the absence of microhomologous pairs. Insertions are formed by diverse mechanisms, including direct acquisition of free genomic fragments, duplication of broken ends, replication of adjacent sequences, or random addition of free nucleotides. The above results indicate that the Cas9 editing spectrum is reproducible and predictable. Thus, our findings enable a deeper understanding of Cas9-mediated mutagenesis and better design of genome editing experiments, as well as elucidate the DNA double-strand break repair processes in B. mori.

In-depth transcriptome unveils the cadmium toxicology and a novel metallothionein in silkworm.

Heavy metal pollution has gradually become a major global issue. It is so far reaching in part because heavy metals are absorbed by soil and affect almost all species via ecological cycles. Silkworms (Bombyx mori) are poisoned by heavy metals through a soil-mulberry-silkworm system, which inhibits larval growth and development and leads to a decrease in silk production. In the present study, we performed transcriptome sequencing of larval midgut with cadmium exposure to explore the toxicological mechanism of heavy metal, and found that the following potential pathways may be involved in cadmium infiltration: endocytosis, oxidative phosphorylation, and MAPK signaling. Moreover, we identified a novel metallothionein in silkworm, which is inhibited by cadmium exposure and able to improve heavy metal tolerance in B. mori cell lines and Escherichia coli. We also generated a transgenic silkworm strain overexpressing metallothionein and the result showed that metallothionein observably enhanced larval viability under cadmium exposure. This study used RNA sequencing to reveal a mechanism for cadmium toxicology, and identified and functionally verified BmMT, offering a new potential heavy metal-tolerant silkworm variety.

The novel insight into the outcomes of CRISPR/Cas9 editing intra- and inter-species.

The CRISPR/Cas (clustered regularly interspaced short palindromic repeat technology/CRISPR-associated protein) is a widely used and powerful research tool in biosciences and a promising therapeutic agent for treating genetic diseases. Mutations induced by Cas9 are generally considered stochastic and unpredictable, thus hindering its applications where precise genetic alternations are required. Here, through deep sequencing and analysis of genome editing outcomes of multiple sites in four distinct species, we found that Cas9-induced mutations are coincident in mutation types but are significantly different in indel patterns among species. In human and mouse cells, indels were almost evenly distributed at both ends of the cleavage sites. However, the indels mainly appeared at the upstream of cleavage sites in Bombyx mori, while they predominantly occurred downstream of the cleavage sites in the zebrafish Danio rerio. We also found that within a species, indel patterns are sequence dependent, wherein deletions between two adjacent micro-homology sequences were the most frequently observed mutations in the repair spectrum. These results suggested the species differences in DNA repair processes during Cas9-induced gene editing, and the important role of sequence structure at the target site in predicting the gene editing outcome.