Control of Metamorphosis by PTTH and bombyxin in the silkworm Bombyx mori.

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We have developed a time-resolved fluoroimmunoassay for Bombyx prothoracicotropic hormone (PTTH) and determined the PTTH titers in the Bombyx hemolymph throughout development from the third larval ecdysis to the adult emergence. PTTH peaks were observed at specific stages in the molting cycles. During the early adult-development, a daily rhythm of the PTTH titer fluctuation was noticed. A small amount of PTTH was also present in the hemolymph over the first half of the fifth instar. We next measured the ecdysteroid titer in the same hemolymph samples as used for PTTH titer determination, to correlate the titer change patterns for PTTH and ecdysteroids. The release of ecdysteroids was closely related to that of PTTH both in timing and in amount, strongly suggesting a crucial role of PTTH in the control of the prothoracic gland activity. From the dynamics of both hormones, dual actions of PTTH on the prothoracic glands, acute and delayd, were suggested. The effects of PTTH antibody injection as assessed by the delay of wandering suggested that PTTH present at low titers in the hemolymph of the early fifth instar larvae also acts somehow on the prothoracic glands.The control mechanisms of PTTH secretion were also investigated. The phase-shift of the light-dark cycles resulted in the shift of the PTTH peaks in timing, suggesting that the PTTH release is regulated to occur at a specific phase in the light-dark cycles. When day-2 fifth instar larvae were starved, the hemolymph PTTH titer was decreased and, on refeeding, increased rapidly, suggesting that the PTTH release in the early fifth instar is affected by the nutritional conditions.