Interaction Between Ecdysteroid, Eclosion Hormone, and Bursicon Titers inManduca sexta

Abstract

SYNOPSIS. The end of the molting process in the tobacco hornworm includes the rapid digestion of the old cuticle, molting fluid resorption, ecdysis of the old cuticle, and expansion and hardening of the new cuticle. The coordination of these processes is accomplished by three hormones. Each ecdysis during the life of Manduca appears to be triggered by eclosion hormone. Depending on developmental stage, the hormone comes either from the brain-corpora cardiaca complex or from the chain of ventral ganglia. The neural programs triggered by eclosion hormone include a neuroendocrine event, the release of the tanning hormone, bursicon, thereby ensuring that tanning of the new cuticle must follow ecdysis. Ecdysis, itself, appears to be controlled by the ecdysteroid levels since ecdysteroid injections delay ecdysis at physiological concentrations and in a dose dependent fashion. This delay is due to inhibition of eclosion hormone secretion and to the retardation of the terminal phases of the molt including the digestion of the old cuticle and the onset of sensitivity to eclosion hormone. Thus, eclosion hormone secretion and the ecdysis it triggers are coordinated with the end of development because both are influenced by the same endocrine signal—the decline in the ecdysteroid titer.