Gene expression analysis in the larval silk gland of the eri silkworm Samia ricini

Insects produce silk for a range of purposes. In the Lepidoptera, silk is utilized as a material for cocoon production and serves to protect larvae from adverse environmental conditions or predators. Species in the Saturniidae family produce an especially wide variety of cocoons, for example, large, golden colored cocoons and those with many small holes. Although gene expression in the silk gland of the domestic silkworm （Bombyx mori L.） has been extensively studied, considerably fewer investigations have focused on members of the saturniid family. Here, we established expression sequence tags from the silk gland of the eri silkworm （Samia ricini）, a saturniid species, and used these to analyze gene expression. Although we identified thefibroin heavy chain gene in the established library, genes for other major silk proteins, such asfibroin light chain andfibrohexamerin, were absent. This finding is consistent with previous reports that these latter proteins are lacking in saturniid silk. Recently, a series offibrohexamerin-like genes were identified in the Bombyx genome. We used this information to conduct a detailed analysis of the library established here. This analysis identified putative homologues of these genes. We also found several genes encoding small silk protein molecules that are also present in the silk of other Lepidoptera. Gene expression patterns were compared between eri and domestic silkworm, and both conserved and nonconserved expression patterns were identified for the tested genes. Such differential gene expression might be one of the major causes of the differences in silk properties between these species. We believe that our study can be of value as a basic catalogue for silk gland gene expression, which will yield to the further understanding of silk evolution.

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.