The rôle of ecdysone in pupation of Manduca sexta

Abstract

Slow infusions of β-ecdysone are more effective in eliciting a normal physiological response than are discrete injections of the hormone. Infusion of β-ecdysone into final instar larvae in the presence of juvenile hormone (JH) induces apolysis and the deposition of a normal larval cuticle. In the absence of JH larvae display the prodromal symptoms of pupation (exposure of the heart, purging of the gut, etc.) in response to a β-ecdysone infusion. The occurrence of certain covert physiological events that accompany the exposure of the heart are evidently necessary to prepare a larva for pupation. An infusion of β-ecdysone can induce apolysis and pupal cuticle deposition only after the prodromal signs of pupation have become evident. Of the two pulses of ecdysone that normally precede pupation in Manduca , the first is apparently responsible for the genetic switchover from larval to pupal development whereas the second one triggers apolysis and the subsequent events that lead to pupation. Results obtained from infusion experiments in which the dose and exposure time were varied independently are consistent with the idea that ecdysone has to be present for a certain minimum time above a threshold concentration to induce a physiological response. The requisite exposure time is apparently not dose-dependent.