EcR expression in the prothoracicotropic hormone‐producing neurosecretory cells of the Bombyx mori brain

The steroid hormone 20‐hydroxyecdysone (20E) initiates insect molting and metamorphosis through binding with a heterodimer of two nuclear receptors, the ecdysone receptor (EcR) and ultraspiracle (USP). Expression of the specific isoforms EcR‐A and EcR‐B1 governs steroid‐induced responses in the developing cells of the silkworm Bombyx mori . Here, analysis of EcR‐A and EcR‐B1 expression during larval‐pupal development showed that both genes were up‐regulated by 20E in the B. mori brain. Whole‐mount in situ hybridization and immunohistochemistry revealed that EcR‐A and EcR‐B1 mRNAs and proteins were exclusively located in two pairs of lateral neurosecretory cells in the larval brain known as the prothoracicotropic hormone (PTTH)‐ producing cells (PTPCs). In the pupal brain, EcR‐A and EcR‐B1 expression was detected in tritocerebral cells and optic lobe cells in addition to PTPCs. As PTTH controls ecdysone secretion by the prothoracic gland, these results indicate that 20E‐responsive PTPCs are the master cells of insect metamorphosis.