The Role of Filippi’s Glands in the Silk Moths Cocoon Construction

Abstract

Filippi’s glands (FGs), formerly also called Lyonet’s glands, are accessory secretory structures of the labial (silk) glands of lepidopteran caterpillars, which were implicated to play an important role in the maturation of the silk material and the construction of the cocoon. In our previous study, we have identified several species of giant silk moths that completely lack the FGs. Interestingly, the absence of FGs in these species correlates with the construction of a loose cocoon architecture. We investigated the functions of FGs by their surgical extirpation in the last instar larvae of the silkworm, Bombyx mori. We found that the absence of FGs altered the structure of the resulting cocoon, in which the different layers of silk were separated. In further experiments, we found no effects of the absence of FGs on larval cocoon formation behavior or on changes in cocoon mass or lipid content. Differential proteomic analysis revealed no significant contribution of structural proteins from FGs to silk cocoon material, but we identified several low abundance proteins that may play a role in posttranslational modifications of some silk proteins. Proteomic analysis also revealed a difference in phosphorylation of the N-terminal sequence of fibroin-heavy chain molecule. Thus, FGs appear to affect silk stickiness during spinning by regulating posttranslational modifications. This could also explain the link that exists between the absence of these glands and the formation of loose cocoons in some giant silk moth species.

Bioselectivity of silk protein-based materials and their bio-inspired applications

“…Silkworm silk contains mainly three different proteins, namely a heavy chain fibroin, a light chain fibroin linked via disulfide bonds to the former one, and a P25 glycoprotein associated via noncovalent hydrophobic interactions as a putative stabilizer of the fibre complex integrity [115]. Recently, it has been reported that the accessory Filippi's glands of silkworms appear to regulate posttranslational modifications of fibroin heavy chain molecules, necessary for a tight cocoon architecture [116]. Silk fibroins make up to 75-83 wt % of raw silkworm silk, while sericins, hydrophilic proteins forming a connective coating, constitute 17-25% of the weight of the fibre.…”