The role of 20-hydroxyecdysone and juvenile hormone in pupal commitment of the epidermis of the silkworm, Bombyx mori.

During the pupal metamorphosis in insects, cellular commitment for pupal differentiation must precede before its differentiation. The pupal commitment of Bombyx mori epidermis occurred from day 3 to day 6 last (5th) instar larvae in response to the gradual increase in ecdysteroid titer in the presence of a small amount of juvenile hormone (JH). Yet the concealed preparatory process of the commitment had begun in the newly synthesized 5th instar larval epidermis (approximately 6 h before the ecdysis) as a competence phase, in which pupal commitment in vitro was induced by 20-hydroxyecdysone (20E) but inhibited by JH. This competence phase continued until day 2 5th instar, and the decrease and increase in cellular sensitivity to JH and 20E, respectively, occurred gradually during this period. In early day 3, autonomous pupal commitment began in vitro and 20E stimulated the commitment, but JH could only partially prevent the commitment in both cases. This apparent reversible to irreversible transition ended in early day 6 by the completion of pupal commitment, when the cells completely lost their sensitivity to JH and no longer expressed the larval cuticle protein gene 30. The expression of the transcription factor, broad, closely followed the commitment, so that we could use this gene expression as a molecular marker for pupal commitment. These results indicate that exposure to 20E and loss of the sensitivity of the epidermal cells to JH are required for the completion of pupal commitment, and suggest that the unusually long process over 3 days could be due to the presence of the detectable JH during the commitment.