The beta-1, 4-N-acetylglucosaminidase 1 gene, selected by domestication and breeding, is involved in cocoon construction of Bombyx mori.

Author summary The cocoon provides a protected space for the metamorphosis of many insect species. Silk protein is a precursor of the fiber used for cocoon construction. Deciphering the genetic basis underlying silk protein synthesis will improve our understanding of cocoon construction and the adaptations of species that construct cocoons. We used the silkworm (Bombyx mori) as a model to identify genes affecting silk protein synthesis and cocoon construction. Quantitative genetic analysis was used to show that beta-1,4-N-acetylglucosaminidase 1(BmGlcNase1), a gene selected during silkworm domestication and breeding, is associated with sericin synthesis. Transgenic-based functional validation confirmed thatBmGlcNase1positively regulates sericin content in the silkworm cocoon. The selective pressure ofGlcNase1in the evolution of insects with cocoons is higher than those without cocoons. This indicates that it has a conserved function in the cocooning process. These results reveal aspects of the genetic basis of silk protein synthesis and the cocoon construction of insects. Holometabolous insects have distinct larval, pupal, and adult stages. The pupal stage is typically immobile and can be subject to predation, but cocoon offers pupal protection for many insect species. The cocoon provides a space in which the pupa to adult metamorphosis occurs. It also protects the pupa from weather, predators and parasitoids. Silk protein is a precursor of the silk used in cocoon construction. We used the silkworm as a model species to identify genes affecting silk protein synthesis and cocoon construction. We used quantitative genetic analysis to demonstrate that beta-1,4-N-acetylglucosaminidase 1(BmGlcNase1) is associated with synthesis of sericin, the main composite of cocoon.BmGlcNase1has an expression pattern coupled with silk gland development and cocoon shell weight (CSW) variation, and CSW is an index of the ability to synthesize silk protein. Up-regulated expression ofBmGlcNase1increased sericin content by 13.9% and 22.5% while down-regulation reduced sericin content by 41.2% and 27.3% in the cocoons of females and males, respectively. Genomic sequencing revealed that sequence variation upstream of theBmGlcNase1transcriptional start site (TSS) is associated with the expression ofBmGlcNase1and CSW. Selective pressure analysis showed thatGlcNase1was differentially selected in insects with and without cocoons (omega(1)= 0.044 vs.omega(2)= 0.154). This indicates that this gene has a conserved function in the cocooning process of insects.BmGlcNase1appears to be involved in sericin synthesis and silkworm cocooning.