Abstract:  【Aim】 The small cocoon mutant sc was discovered among the offspring of space silkworm (Bombyx mori), exhibiting slow larval development and reduced consumption of mulberry leaves. We speculate that genes related to growth and development pathways of the sc mutant may be affected by the gene mutation. Our previous research indicated that the sc mutant is controlled by a pair of recessive genes located on the 3rd linkage group of B. mori, but the responsible gene has not yet been identified. This study aims to provide insights into the identification of the responsible gene for the sc mutant and the analysis of its molecular mechanisms through comparative transcriptomic analysis between the sc mutant and the normal cocoon strain (TG) derived from space B. mori.【Methods】 The head and midgut tissues from the day-4 5th instar larvae of sc and TG were collected for transcriptome sequencing (RNA-seq), respectively. The differentially expressed genes (DEGs) were obtained by comparative transcriptome analysis. Then GO annotation and KEGG enrichment analysis of DEGs were performed. qRT-PCR was employed to validate the expression levels of randomly selected DEGs in sc and TG and investigate the expression levels of the genes of interest in sc. 【Results】 A total of 1 528 DEGs were detected in heads in the comparison group TG vs sc, with 820 DEGs showing up-regulated expression and 708 DEGs showing down-regulated expression. Similarly, 1 401 DEGs were identified in the midguts of the comparison group TG vs sc, with 683 DEGs showing up-regulated expression and 718 DEGs showing down-regulated expression. The GO analysis indicated that in biological processes, the majority of DEGs in the head and midgut were implicated in cellular process, metabolic process, biological regulation, response to stimulus, etc. In terms of molecular functions, most DEGs were associated with binding, catalytic activity, structural molecule activity, transporter activity and ATP-dependent activity. DEGs in the head and midgut were implicated in signaling pathways associated with the growth and development of B. mori, including the Hippo, Insulin and mTOR pathways. The qRT-PCR analysis revealed that the gene expression trend was consistent with the transcriptome sequencing result, and compared to TG, the sc mutant had the key genes BMSK0008105, BMSK0009907, BMSK0002689, BMSK0000286, BMSK0012340 and BMSK00083629 involved in growth and development signaling pathways with differential expression. 【Conclusion】 The differential expression of the critical genes in growth and development signaling pathways of sc and TG disturbs the key physiological processes like energy metabolism, organogenesis and cell growth, proliferation and apoptosis, thereby affecting the body development of the small cocoon mutant sc. These findings contribute to understanding the molecular mechanisms underlying the formation of the sc mutant and offer valuable experimental data for further exploration into the regulation of B. mori body size.

Key words: Bombyx mori, space silkworm, small cocoon mutant, transcriptome sequencing, differentially expressed genes, functional annotation