Effect And Mechanism Analysis Of Different Rearing Methods On Silkworm Cocoon Quality And Silk Properties

In recent years,artificial feed rearing of silkworms in China has achieved rapid development and became an important way of raising silkworms either in basic research or production applications.At present,there are three rearing methods in sericulture production: rearing with mulberry leaf in all instars,rearing with artificial feed in all instars,and young silkworm rearing with artificial feed +grown silkworm rearing with mulberry leaf.There are obvious differences in growth of silkworms and cocoons quality among the three rearing methods.The property of cocoon filament is an important index to evaluate the production efficiency of sericulture,but there is no systematic study on the difference of cocoon filament property among the three feeding methods.Therefore,this study focuses on "Youshi No.1",which is a commonly used artificial feed silkworm varieties in production,as the research object.The silkworms were reared with three different method: rearing with mulberry leaf in all instars(ML),rearing with artificial feed in all instars(AF),and 1st-3rd instars of silkworm were reared with artificial feed+4th-5th instars of silkworm were reared with mulberry leaf(TF).Then,the effects of three feeding methods on the quality and silk properties of silkworm cocoons were investigated and analyzed.After that,the reasons for the differences in cocoon quality and silk mechanical properties among different feeding methods were explored by analyzing the differences in amino acid composition of mulberry leaves and artificial feed,the differences in spinneret tube structure among the three different feeding methods,and the differences in silk gland development among the three different feeding methods.The main results are as follows:1)There were significant differences in cocoon quality among the three rearing methods.In terms of cocoon weight,cocoon shell weight and cocoon shell rate,the performance of silkworms in AF group was significantly lower than that of TF and ML group.In terms of cocoon filament fineness,reelability percentage and rushing upon cocoons per 10000 m,the cocoons in AF group is superior to which in ML group.In terms of cleanness and neatness,there is no difference between the cocoons in AF group and ML group.2)There were significant differences in the mechanical properties of silkworm cocoon filament among the three rearing methods.The relative strength of silk in TF group was the strongest,followed by which in ML group and AF group.Besides,the tensile rate of silk in ML group and TF group were significantly better than which in AF group,but there was no significant difference between which in ML group and TF group;3)There were significant differences in amino acid composition and protein structure of silkworm cocoon filaments among the three feeding methods.Though there was no significant difference in the content of glycine,serine and alanine which is the three main amino acids that make up cocoon silk,there was significant difference in the content of cysteine,phenylalanine and isoleucine.In the secondary structure of protein,the proportion of β-sheet in cocoon silk of AF group was significantly higher than that of TF group and ML group,the proportion of α-helix and β-turn was less than that of TF group and ML group,and the crystallinity of silk protein of AF group was higher than that of ML group and lower than that of TF group.4)There were significant differences in the structure of silk pressing area and the expression of related genes among the three rearing methods.The silk pressing area has an important influence on the size of cocoon filament size.Compared with AF group,the tube wall of silkworm pressing area of silkworm in ML group and TF group was thicker and the caliber was thicker.The gene expression analysis indicates that the expression level of LOC693032,which was related to chitin degradation,was significantly higher than that of the other two feeding methods.In addition,the expression levels of voltage-gated potassium channel,sodium potassium transport ATPase subunit α protein and muscle calcium channel subunit in the muscle of the pressing area of silkworm in TF group were significantly higher than those of the other two feeding methods,whose expression are highly correlated with cocoon filament size and mechanical properties and also consistent with the mechanical strength of the three group.5)Electron microscopic observation and transcriptome analysis of posterior silk gland of silkworm reared by three feeding methods.The electron microscopy results indicate that the number of ribosomes,mitochondria and silk material synthesis in the posterior silk gland of silkworm in ML group was significantly more than that of TF group,and the which in TF group was also significantly more than that in AF group,while the number of autophagosome vesicle was significantly more than that of TF group and ML group,which indicated that the protein synthesis of the posterior silk gland of silkworm reared by artificial feed was obviously insufficient.Transcriptome analysis showed that in the ribosome metabolism pathway and amino acid metabolism pathway of the posterior silk gland,the silkworms in ML group were more vigorous than that in TF group,and that in TF group was more vigorous than which in AF group.The differences in signaling pathways such as nutrient absorption and metabolism may lead to insufficient nutrient reserves in the artificial feed rearing silkworm,and the silk gland does not have enough material and energy to synthesize and secrete silk substances.In summary,this paper systematically studied the differences in cocoon quality and cocoon silk properties of silkworm under different feeding methods,and clarified the reasons for the d ifferences in cocoon quality and cocoon silk properties of silkworm under different feeding methods by deeply analyzing the spinning tube,the ultrastructure and transcriptome differences of spinneret tube and posterior silk gland cells of silkworm,which would provide a theoretical reference for improving the quality of cocoon silk of silkworm reared by artificial feed in production.