Steroid hormone fluctuations regulate ganglionic fusion during metamorphosis of the moth Manduca sexta

We examined the role of the steroid hormone, 20-hydroxyecdysone (20-E), in regulating ganglionic fusion during metamorphosis of the moth Manduca sexta. At the larval-pupal transition, the first and second abdominal ganglia begin to migrate anteriorly to coalesce eventually with the second and third thoracic ganglia, forming the pterothoracic ganglion. Near this time, three fluctuations of ecdysteroids are present: two prior to pupation and one after. Ligations posterior to the prothoracic segment isolated the posterior ganglia from the prothoracic glands, a primary source of ecdysteroids. When ligations were performed after the first ecdysteroid pulse (commitment pulse), neither pupal cuticle nor ganglionic fusion was observed. When these abdomens were subsequently given a 20-E infusion that mimicked the second pulse (pre-pupal peak), complete pupal abdomens were formed but ganglia did not migrate together and fuse. Only those isolated abdomens given two separate 20-E infusions (to mimic both the second pulse and third rise) produced fused ganglia. All three ecdysteroid fluctuations, as well as the drop in ecdysteroids between the second pulse and third rise (preadult rise), were required for normal ganglionic migration and fusion. Fusion-related events, and the appearance of other adult characteristics, sometimes occurred in abdomens that were isolated following pupation. The percentage of abdomens showing adult development was positively correlated with the time that elapsed between pupation and abdominal isolation. Using an ecdysteroid radioimmunoassay, we confirmed the presence of ecdysteroids in isolated abdomens, and demonstrated that isolated abdomens show a temporal pattern of ecdysteroid levels similar to that of intact insects.