A comprehensive review of silkworm genetic improvement strategies for enhancing silk production

Introduction

Sericulture is an important agro-based industry that provides employment and income for millions of rural households worldwide [1] . The domesticated silkworm Bombyx mori is the primary species used for commercial silk production due to its ability to produce high-quality silk fibers. Silk is valued for its unique properties such as high tensile strength, elasticity, and smooth texture, making it useful in textile, biomedical, and industrial applications. The productivity of sericulture largely depends on the genetic potential of silkworm strains and the quality of mulberry leaves used as feed. Over the past century, significant progress has been made in silkworm breeding to develop strains with improved silk yield, disease resistance, and environmental adaptability [2][3][4] . Traditional breeding methods such as selection, hybridization, and crossbreeding have played a key role in enhancing silkworm productivity. However, conventional breeding techniques often require long breeding cycles and may not precisely target specific genetic traits. With the advancement of molecular genetics and biotechnology, new approaches such as marker-assisted selection, genomic analysis, and gene editing have emerged as powerful tools for improving silkworm strains. These technologies allow scientists to identify genes associated with desirable traits and manipulate them to enhance silk production and cocoon quality [5] . Recent advances in genome sequencing and gene-editing technologies have significantly improved the understanding of silkworm genetics. Modern techniques such as CRISPR-Cas9 enable precise modification of genes related to silk protein synthesis, disease resistance, and stress tolerance, offering new possibilities for genetic improvement in sericulture. Therefore, integrating traditional breeding methods with modern molecular approaches has become essential for developing improved silkworm strains capable of meeting the increasing global demand for silk.

Conventional Breeding Approaches in Silkworm Genetic Improvement

Traditional breeding methods have been widely used in sericulture to improve the genetic performance of silkworm strains. These methods primarily involve selection, hybridization, and crossbreeding to combine desirable traits from different strains.

Selection Breeding

Selection breeding involves choosing superior individuals based on desirable traits such as cocoon weight, shell ratio, larval survival, and silk yield. Through repeated selection over several generations, breeders can gradually improve the genetic potential of silkworm populations. Selection breeding has successfully produced several improved silkworm varieties with higher silk productivity and better adaptability to different environmental conditions [6] . However, this approach is time-consuming and may require many generations to achieve significant improvements.

Hybridization

Hybridization is another important breeding technique used to enhance silk production. It involves crossing two genetically different silkworm strains to produce hybrid offspring that exhibit hybrid vigor (heterosis). Hybrid silkworms often show improved growth rate, disease resistance, and cocoon yield compared to their parental lines. Hybrid silkworm varieties are widely used in commercial sericulture because they provide higher productivity and better economic returns for farmers [7][8] . Many countries have developed high-yielding bivoltine hybrids through systematic breeding programs.

Crossbreeding and Line Breeding

Crossbreeding involves mating individuals from different strains or breeds to combine beneficial traits such as high silk yield and environmental adaptability. Line breeding, on the other hand, focuses on maintaining specific desirable genetic traits within a population by controlled breeding within closely related individuals [9] . These conventional breeding approaches have contributed significantly to the development of improved silkworm strains used in modern sericulture.

Molecular Breeding and Marker-Assisted Selection

Recent advancements in molecular genetics have enabled the use of molecular markers for silkworm breeding programs. Marker-assisted selection (MAS) allows breeders to identify specific genes associated with desirable traits such as silk yield, cocoon weight, and disease resistance [10] . Quantitative trait loci (QTL) analysis has been widely used to identify genomic regions controlling economically important traits in silkworms. Researchers have discovered genes associated with silk protein synthesis and larval development that can be used as markers for genetic improvement. Molecular markers such as simple sequence repeats (SSR), single nucleotide polymorphisms (SNPs), and amplified fragment length polymorphisms (AFLP) have been used to analyze genetic diversity and assist breeding programs. These markers enable faster identification of superior silkworm strains and improve the efficiency of breeding strategies [11] . Marker-assisted breeding helps reduce the time required for developing improved silkworm varieties compared to conventional breeding methods. By selecting individuals with specific genetic markers, breeders can ensure that desirable traits are passed on to future generations.

Genetic Engineering and Transgenic Silkworms

Genetic engineering has opened new opportunities for improving silkworm productivity and silk quality. Transgenic technology allows scientists to introduce foreign genes into the silkworm genome to enhance specific traits. For example, researchers have introduced spider silk genes into silkworms to produce silk fibers with improved mechanical strength. Transgenic silkworms have also been developed to produce recombinant proteins for biomedical and industrial applications. [12] Genetic engineering techniques have also been used to enhance disease resistance in silkworms by modifying genes involved in immune responses. This approach helps reduce the impact of viral and bacterial infections that commonly affect silkworm populations. Although transgenic technology offers many advantages, its application in sericulture is still limited due to regulatory concerns and biosafety considerations.

Genome Editing Technologies in Silkworm Improvement

Genome editing technologies such as CRISPR-Cas9, TALENs, and zinc-finger nucleases have revolutionized genetic research in silkworms. Among these tools, CRISPR-Cas9 is the most widely used due to its efficiency, precision, and simplicity. CRISPR technology enables scientists to modify specific genes associated with silk production, growth regulation, and disease resistance [13][14][15][16][17] . Researchers have successfully used CRISPR to increase silk yield by manipulating genes involved in silk gland development and protein synthesis. Genome editing also allows the creation of silkworm strains resistant to viral pathogens such as Bombyx mori nucleopolyhedrovirus (BmNPV), which is a major cause of crop loss in sericulture [18][19][20][21] . Furthermore, CRISPR technology enables the insertion of genes that improve silk fiber properties or allow the production of novel biomaterials [22] . These advances demonstrate the potential of genome editing to transform silkworm breeding and improve the sustainability of sericulture.

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