Abstract: 【Aim】 To analyze the differences in the midgut bacterial composition among different varieties of Bombyx mori with different vitalities and cocoon shell ratios, and their effects on the related traits such as vitality and cocoon shell ratio. 【Methods】 Based on the results of the long-term feeding survey on B. mori resource, the variety 932G with high vitality and the variety 2041J with high silk yield were selected as experimental materials, the midguts of the 5th instar larva and pupa were collected, and the 16S rDNA sequences of the midgut bacteria were sequenced and analyzed by the high-throughput sequencing platform to compare the differences in the composition and function of the midgut bacteria between different varieties and different developmental stages of B. mori. 【Results】 A total of 399 and 453 operational taxonomic units (OTUs) of the midgut bacteria were obtained from 932G and 2041J at the 5th instar larval stage, and 138 and 162 OTUs of the midgut bacteria were obtained from 932G and 2041J at the pupal stage, respectively. The dominant phyla of the midgut bacteria of B. mori at the 5th instar larval stage were Proteobacteria, Firmicutes and Actinobacteriota, and the dominant genus was Methylobacterium with the highest abundance, followed by Staphylococcus. However, there were significant differences in the relative abundance of some genera between the two varieties. The relative abundance of some genera including Devosia, Ralstonia, Nitrospira, Brachybacterium, Rothia, Lawsonella etc. in the midgut of the 5th instar larva of 932G was significantly higher than that in the midgut of the 5th instar larva of 2041J. The relative abundance of Pseudomonas, Methylobacterium, Acinetobscter, Cloacibacterium, Leuconostoc, Propionibacteriaceae, Psychrobacter, Sphingobium, Bacteroides etc. in the midgut of the 5th instar larva of 2041J was significantly higher than that in the midgut of the 5th instar larva of 932G. There were 77% and 78% functional genes in the midgut bacteria of the 5th instar larvae of 932G and 2041J enriched on the metabolic pathways of KEGG, respectively, followed by those of the function genes enriched on the environmental information processing and genetic information processing. The proportions of the midgut bacterial communities involved in nitrate reduction, nitrogen respiration, nitrate respiration, nitrite respiration and nitrogen fixation in the 5th instar larva of 932G were higher than those in 2041J. The proportions of the midgut bacterial communities involved in chemical heterotrophic, urea decomposition and methanol oxidation in the 5th instar larva of 2041J were higher than those in the 5th instar larva of 932G. The difference in the midgut bacterial composition between the 5th instar larva and pupa was significant. Erwinia of Proteobacteria in the midgut of the pupa was the dominant genus, and there was no significant difference in the relative abundance of Erwinia between the two varieties at the pupal stage. The relative abundance of functional genes involved in amino acid transport and metabolism, extracellular secretion and transport, carbohydrate transport and metabolism, and inorganic salt transport and metabolism of the midgut bacteria in the pupa was significantly higher than that in the 5th instar larva of 932G.【Conclusion】 There were significant differences in the midgut bacterial composition and predicted function between the varieties 932G with high vitality and 2041J with high silk yield of B. mori at the 5th instar larval stage. The composition and function of the midgut bacteria of B. mori at the pupal stage were significantly different from those of B. mori at the 5th instar larval stage, and there was no significant difference in the relative abundance of midgut bacteria between the two varieties at the pupal stage. These research results can provide a reference for further exploring the role of gut microorganisms in stress resistance, drug resistance, disease resistance, protein synthesis and transformation of B. mori and variety breeding.

Key words: Bombyx mori, variety, high-throughput sequencing, gut microbiota, 5th instar larval stage, pupal stage