Transcriptomic analysis of the bagworm moth silk gland reveals a number of silk genes conserved within Lepidoptera

Lepidopteran insects produce cocoons with unique properties. The cocoons are made of silk produced in the larval tissue silk gland and our understanding of the silk genes is still very limited. Here, we investigated silk genes in the bagworm moth Eumeta variegata, a species that has recently been found to produce extraordinarily strong and tough silk. Using short-read transcriptomic analysis, we identified a partial sequence of the fibroin heavy chain gene and its product was found to have a C-terminal structure that is conserved within nonsaturniid species. This is in accordance with the presence of fibroin light chain/fibrohexamerin genes and it is suggested that the bagworm moth is producing silk composed of fibroin ternary complex. This indicates that the fibroin structure has been evolutionarily conserved longer than previously thought. Other than fibroins we identified candidates for sericin genes, expressed strongly in the middle region of the silk gland and encoding serine-rich proteins, and other silk genes, that are structurally conserved with other lepidopteran homologues. The bagworm moth is thus considered to be producing conventional lepidopteran type of silk. We further found a number of genes expressed in a specific region of the silk gland and some genes showed conserved expression with Bombyx mori counterparts. This is the first study allowing comprehensive silk gene identification and expression analysis in the lepidopteran Psychidae family and should contribute to the understanding of silk gene evolution as well as to the development of novel types of silk.