20-Hydroxyecdysone Regulates the Expression of 30 Genes Specifically Expressed in Larval Digestive Tube of the Silkworm, Bombyx mori.

Nutrients needed to fuel the processes of growth, development, and reproduction, and to form the proteins in the cocoon, are derived from the larval stage. The amount of leaf ingested in the fifth instar silkworm larvae accounted for about 85% of the whole instars. Nutrients were absorbed following digestion in the digestive tube. We previously reported the successful identification of 227 silkworm larval digestive juice proteins, wherein most of the genes of the identified proteins had high expression features in the midgut, and 30 of them were midgut-specific genes. We chose 30 genes and detected their expression characteristics in the digestive tube, and investigated the developmental expression profiles in the midgut of silkworm larvae from the day-3 fourth instar to the wandering period, as well as the expression dynamics after the 20-hydroxyecdysone (20E) treatment. The genes were specifically expressed in the larval digestive tube. The expression of BMSK0013805 was activated, and that of the other 29 genes were suppressed by 20E. The ecdysteroid ingestion dramatically suppressed the protease activity in the larval midgut, which revealed that 20E may also directly affect digestive activity. The present study is the first to demonstrate that the expression control mechanism of these genes is primarily hormonal regulation, which contributes to furthering our understanding of the mechanism of the genes expressed in the larval digestive tube during development.