Time course of hormonal independence for developmental events in neurons and other cell types during insect metamorphosis.

The ecdysteroid dependence of developmental events during the larval-pupal transformation of the tobacco hornworm, Manduca sexta, was studied using the technique of abdomen ligation to eliminate the ecdysteroid-secreting prothoracic glands (PTG) in the thorax. The time at which a particular developmental event no longer required the presence of the PTG for its successful completion, i.e., when the event became ecdysteroid independent, was determined for the dendritic regression and death of proleg motor neuron PPR, the degeneration of its target muscle PPRM, and the secretion of pupal cuticle by the abdominal epidermis. Groups of abdomens were ligated from synchronously developing larvae at 12-hr intervals over the 4-day period preceding entry into the pupal stage. PPR's dendritic regression became ecdysteroid independent over the first 24 hr of this period. When abdomens were ligated midway through this period PPR became arrested in a partially regressed state. Twelve hours after PPR's regression became independent, the degeneration of PPRM and the secretion of pupal cuticle by the epidermis acquired independence. Another 12 hr elapsed before PPR's programmed death became independent. Thus, different developmental events acquired ecdysteroid independence in a stereotyped temporal sequence, and even different events within a single cell, i.e., PPR's regression and death, became independent at different times.