Silk is an important natural fiber of high economic value, and thus genetic study of the silkworm is a major area of research. Transcriptome analysis can provide guidance for genetic studies of silk yield traits. In this study, we performed a transcriptome comparison using multiple silkworms with different silk yields. A total of 22 common differentially expressed genes (DEGs) were identified in multiple strains and were mainly involved in metabolic pathways. Among these, seven significant common DEGs were verified by quantitative reverse transcription polymerase chain reaction, and the results coincided with the findings generated by RNA sequencing. Association analysis showed that BGIBMGA003330 and BGIBMGA005780 are significantly associated with cocoon shell weight and encode uridine nucleosidase and small heat shock protein, respectively. Functional annotation of these genes suggest that these play a role in silkworm silk gland development or silk protein synthesis. In addition, we performed principal component analysis (PCA) in combination with wild silkworm analysis, which indicates that modern breeding has a stronger selection effect on silk yield traits than domestication, and imply that silkworm breeding induces aggregation of genes related to silk yield.

Flight loss has occurred in many winged insect taxa. The flightless silkmoth Bombyx mori, is domesticated from the wild silkmoth, Bombyx mandarina, which can fly. In this paper, we studied morphological characteristics attributed to flightlessness in silkmoths. Three domestic flightless B. mori strains and one B. mandarina population were used to compare morphological components of the flight apparatus, including wing characteristics (shape, forewing area, loading, and stiffness), flight muscle (weight, ratio, and microscopic detail) and body mass. Compared with B. mandarina, B. mori strains have a larger body, greater wing loading, more flexible wings and a lower flight muscle ratio. The arrangement in microscopy of dorsal longitudinal flight muscles (DLFMs) of B. mori was irregular. Comparative analysis of the sexes suggests that degeneration of flight muscles and reduction of wing mechanical properties (stiffness) are associated with silkmoth flightlessness. The findings provide important clues for further research of the molecular mechanisms of B. mori flight loss.

The insect glycoside hydrolase family 20 β-N-acetylhexosaminidases (HEXs) are key enzymes involved in chitin degradation. In this study, nine HEX genes in Bombyx mori were identified by genome-wide analysis. Bioinformatic analysis based on the transcriptome database indicated that each gene had a distinct expression pattern. qRT-PCR was performed to detect the expression pattern of the chitooligosaccharidolytic β-N-acetylglucosaminidase (BmChiNAG). BmChiNAG was highly expressed in chitin-rich tissues, such as the epidermis. In the wing disc and epidermis, BmChiNAG has the highest expression level during the wandering stage. CRISPR/Cas9-mediated BmChiNAG deletion was used to study the function. In the BmChiNAG-knockout line, 39.2% of female heterozygotes had small and curly wings. The ultrastructure of a cross-section showed that the lack of BmChiNAG affected the stratification of the wing membrane and the formation of the correct wing vein structure. The molting process of the homozygotes was severely hindered during the larva to pupa transition. Epidermal sections showed that the endocuticle of the pupa was not degraded in the mutant. These results indicate that BmChiNAG is involved in chitin catabolism and plays an important role in the molting and wing development of the silkworm, which highlights the potential of BmChiNAG as a pest control target.