Comparative transcriptome analysis of Bombyx mori spinnerets and Filippi's glands suggests their role in silk fiber formation.

The spinneret is located at the end of silk glands in the silkworm, Bombyx mori. The Filippi's gland (FG), which communicates with the silk gland, is considered an accessory to the latter. Although these two organs have been known for centuries, only their morphology has been studied. Their gene expression profiles and physiological roles are still unknown. Aided by next-generation sequencing, we profiled more than 11000 transcripts from the spinneret and FG of silkworm larvae on day 3 of the fifth instar (L5D3) and wandering stage (W1) in this study. A total of 59 ion-transporting protein genes and 106 cuticle protein genes were identified in the spinneret. To analyze the dynamic changes in the expression of spinneret genes, differential expression analysis was performed, and 1452 differentially expressed genes (DEGs) were identified in spinneret tissue harvested on L5D3 and W1 of the silkworm larvae. Gene Ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment of the DEGs revealed that the spinneret had active ion-transporting, chitin binding, and energy metabolism processes at W1. Based on these data, we hypothesized that the role of the spinneret is to provide a favorable physiological environment for the silk fiber formation. Furthermore, differential expression analysis and GO enrichment of the DEGs in the FG suggested a possible role of this gland in transporting small solutes such as ions, sugars and amino acids to the silk gland. Our findings pave a way for further functional explanation of the spinneret and FG.