Background: Horizontal gene transfer (HGT), a source of genetic variation, is generally considered to facilitate hosts' adaptability to environments. However, convincing evidence supporting the significant contribution of the transferred genes to the evolution of metazoan recipients is rare. Results: In this study, based on sequence data accumulated to date, we used a unified method consisting of similarity search and phylogenetic analysis to detect horizontally transferred genes (HTGs) between prokaryotes and five insect species including Drosophila melanogaster, Anopheles gambiae, Bombyx mori, Tribolium castaneum and Apis mellifera. Unexpectedly, the candidate HTGs were not detected in D. melanogaster, An. gambiae and T. castaneum, and 79 genes in Ap. mellifera sieved by the same method were considered as contamination based on other information. Consequently, 14 types of 22 HTGs were detected only in the silkworm. Additionally, 13 types of the detected silkworm HTGs share homologous sequences in species of other Lepidopteran superfamilies, suggesting that the majority of these HTGs were derived from ancient transfer events before the radiation of Ditrysia clade. On the basis of phylogenetic topologies and BLAST search results, donor bacteria of these genes were inferred, respectively. At least half of the predicted donor organisms may be entomopathogenic bacteria. The predicted biochemical functions of these genes include four categories: glycosyl hydrolase family, oxidoreductase family, amino acid metabolism, and others. Conclusions: The products of HTGs detected in this study may take part in comprehensive physiological metabolism. These genes potentially contributed to functional innovation and adaptability of Lepidopteran hosts in their ancient lineages associated with the diversification of angiosperms. Importantly, our results imply that pathogens may be advantageous to the subsistence and prosperity of hosts through effective HGT events at a large evolutionary scale.

The peritrophic matrix (PM) secreted by the midgut cells of insects is formed by the binding of PM proteins to chitin fibrils. The PM envelops the food bolus, serving as a barrier between the content of the midgut lumen and its epithelium, and plays a protective role for epithelial cells against mechanical damage, pathogens, toxins, and other harmful substances. However, few studies have investigated the characteristics and synthesis factors of the PM in the silkworm, Bombyx mori. Here, we examined the characteristics of the PM in the silkworms. The PM thickness of the silkworms increased gradually during growth, while there was no significant difference in thickness along the entire PM region. Permeability of the PM decreased gradually from the anterior to posterior PM. We also found that PM synthesis was affected by food ingestion and the gut microbiota. Our results are beneficial for future studies regarding the function of the PM in silkworms.

The insect midgut secretes a semi-permeable, acellular peritrophic membrane (PM) that maintains intestinal structure, promotes digestion, and protects the midgut from food particles and pathogenic microorganisms. Peritrophin is an important PM protein (PMP) in the PM. Here, we identified 11 peritrophins with 1–16 chitin binding domains (CBDs) comprising 50–56 amino acid residues. Multiple CBDs in the same peritrophin clustered together, rather than by species. The CBD contained six highly conserved cysteine residues, with the key feature of amino acids between them being CX_11-15CX₅CX_9-14CX_11-12CX_6-7C. Peritrophins with 2 and 4 CBDs (Bm09641 and Bm01504, respectively), and with 1, 8, and 16 CBDs (Bm11851, Bm00185, and Bm01491, respectively) were mainly expressed in the anterior midgut, and throughout the midgut, respectively. Survival rates of transgenic silkworms with Bm01504 overexpression (Bm01504-OE) and knockout (Bm01504-KO) infected with B. morinucleopolyhedrovirus (BmNPV) were significantly higher and lower, whereas expression of the key viral gene, p10, were lower and higher, respectively, compared with wild type (WT). Therefore, Bm01504-OE and Bm01504-KO transgenic silkworms were more and less resistant, respectively, to BmNPV. Bm01504 plays important roles in resisting BmNPV invasion. We provide a new perspective for studying PM function, and reveal how the silkworm midgut resists invasive exogenous pathogenic microorganisms.