3.1. Silk and silk gland of Spodoptera frugiperda
The 1st-3rd instar larvae of S.frugiperda can spit silk and float in the air to move to plants on the wind. Third instar larvae were placed in a capillary tube and shaken gently, which caused the larvae to spit and droop (Fig. 1. a). Insect silk is a composite fibre with silk fibroin as the centre and several layers of sericin on the outside. Sericin is divided into inner-layer sericin, middle-layer sericin and outer-layer sericin (Fig. 1. c).The morphology of the silk gland can be divided into three regions: the posterior silk gland (PSG), middle silk gland (MSG) and anterior silk gland (ASG). In addition, there is a spigot at the front of the silk gland. Pairs of glands are located on the ventral side of the intestine on both sides, and the spinner is located in the insect’s mouth (Fig. 1. b). Silk fibroin is synthesized from the posterior silk gland, and sericin is synthesized from the middle silk gland, which is processed by the anterior silk gland and finally emerges from the mouth as solid silk through the spinner. The anterior silk gland of S.frugiperda consists of elongated ducts, and there is no obvious demarcation point between the middle and posterior silk gland, presenting a typical N-shaped structure. The posterior silk gland is short and slender, without folding or bending. By observing the morphology of the silk gland of S.frugiperda, we found that it does not have a developed posterior silk gland like Bombyx mori. The development of the posterior silk gland may have a close relationship with the number of insects spinning, and the underdeveloped posterior silk gland limits the ability to synthesize silk. Therefore, S.frugiperda cannot spin as large an amount of silk as Bombyx mori (Fig. 1. b).
3.2. Effects of trace azadirachtin on Spodoptera frugiperda spinning behaviour
Scanning electron microscopy observation of the 3rd instar S.frugiperda showed that the surface of the control S.frugiperda silk was smooth and free of cracks. After the insect silk is synthesized in the bilateral silk glands, it is processed and compressed by the silk press and spigot to a solid state. In this process, it is possible for the double-sided silk gathered in the spinner not to be well fused, resulting in depressions in the middle of the silk (Fig. 2. a, c). After exposure to traces of 0.3 mg/kg azadirachtin for 48 h, there was obvious unevenness on the surface of the silk. The sericin in the anterior silk gland did not perfectly encapsulate the silk fibroin. Compared with the control group, the sol phenomenon was more prone to occur, and swelling and rupture occurred more easily (Fig. 2. b, d). Analysis of the spinning of the 3rd instar S. frugiperda showed that there was no significant difference in spitting ability after 24 h of feeding on azadirachtin. The spinning rate of S. frugiperda fed 0.3 mg/kg azadirachtin increased at 48 h, while no difference was observed at 3 mg/kg, however, the spinning rate was significantly reduced after 72 h at both doses (Fig. 2. e).
3.3. Comparison of the silk gland in terms of morphology, pathology and ultrastructure
Under normal conditions, S.frugiperda grows gradually, but larval growth and development are inhibited by feeding on azadirachtin (Fig. a, d, g, S1). To further understand the effect of azadirachtin on the spinning behaviour of the S.frugiperda silk gland, pathological sections of the silk gland of S.frugiperda were obtained. The silk gland is the site of silk formation and storage, which is closely related to the secretion of silk. The pathological sections showed that the control had no abnormal pathological changes, the gland cavity was full, the entire silk gland was not damaged, and the silk glands gradually became larger as the larvae grew (Fig. 3. b, e, h). After 24 h of exposure to 0.3 mg/kg azadirachtin, the silk gland was still normal, and the protein still aggregated in large amount (Fig. 3. c). However, after 48 h, it could be clearly observed that the protein aggregation in the silk gland was significantly weakened, and obvious vacuolation (the gap between the membrane and the protein aggregation) was observed in the lumen of the silk gland (Fig. 3. f). After 72 h of exposure to azadirachtin, a significant reduction in protein was observed, and the glandular membrane ruptured (Fig. 3. i).
In addition, TEM of the silk gland ultrastructure showed that the cell structure of the control was normal, the cell membrane was intact, the endoplasmic reticulum was abundant and distributed, and a large amount of rough endoplasmic reticulum was present (Fig. 4. a), whereas in the group treated with azadirachtin, after 48 h, the silk gland epithelial cells were mildly to moderately oedematous, the chromatin was unevenly distributed, intracellular matrix electron density was reduced, vacuolization appeared (shown by the red arrow), and the silk gland cell membrane was locally damaged. The organelles in the cytoplasm were swollen, and the nucleus was irregularr (Fig. 4. b). These data indicate that azadirachtin caused damage to the silk gland, which may further affect spinning behaviour.
3.4. Metabolomics analysis
The metabolites in the silk glands of S.frugiperda after feeding on azadirachtin were analysed. XCMS software was used for peak extraction and quality control, and CAMERA was used for additive ion annotation of the extracted substances (Fig. S2). MetaX software was used for metabolite identification (database matching identification for primary mass spectrometry information and secondary mass spectrometry information for matching identification with an in-house standard product database). The annotated metabolites were mapped to the Kyoto Encyclopedia of Genes and Genomes (KEGG) ID and the Human Metabolome Database (HMDB) to examine the physicochemical properties and biological functions of the metabolites (Fig. S3). MetaX software was used to quantify metabolites and screen differential metabolites. Pathways with P <0.05 were considered to be significant.
After treatment of the 3rd instar S.frugiperda with 0.3 mg/kg azadirachtin for 48 h, the metabolome data of the silk glands revealed a total of 5038 differential metabolites, with 1687 metabolites upregulated and 3351 metabolites downregulated (Fig. 5. b, Table S1). Among them were 154 differential secondary metabolites (Fig. S4, Table S2), 87 of which were enriched in metabolic pathways, indicating that the metabolic rate in the silk gland was very high, 17 were enriched in the biosynthesis of amino acids, and 13 were involved in the ABC transport pathway, which indicates that transport processes in the silk gland were also very active (Fig. 5. a).
KEGG pathway enrichment analysis showed that 30 differential metabolites were identified, of which 12 were upregulated and 18 were downregulated, mainly in four categories: lipids and lipid-like molecules, organoheterocyclic compounds, phenylpropanoids and polyketide, and organic acids and derivatives. The metabolites enriched in the silk gland were L-citrulline, genistin, PC(20:4(8Z,11Z,14Z,17Z)/16:1(9Z)), urocanic acid, linoleic acid, ricinolic acid, alpha-dimorphecolic acid, 9,10-epoxyoctadecenoic acid, riboflavin, testosterone propionate, coumarin, and (9S,10S)-9,10-dihydroxyoctadecanoate(9S,10S)-9,10 (Fig. 6. a). The decreased metabolites were His-Leu, guanine, lysoPE 16:1, suberic acid, pergolide, Gly-Leu, dodecanedioic acid, jasmonic acid, aflatoxin M1, adipic acid, sporol, azelaic acid, quinidine, lysoPI 20:5, sebacic acid, trimethoprim, phlorizin, and strychnine (Fig. 6. b). The results showed enrichment in 15 different pathways, including choline metabolism in cancer, arginine biosynthesis, histidine metabolism, phenylalanine, tyrosine and tryptophan biosynthesis, biosynthesis of amino acids, and aminoacyl-tRNA biosynthesis (Fig. 5. c).