Divergent mechanisms for regulating growth and development after imaginal disc damage in the tobacco hornworm, Manduca sexta.

Holometabolous insects have been able to radiate to vast ecological niches as adults through the evolution of adult-specific structures such as wings, antennae and eyes. These structures arise from imaginal discs that show regenerative capacity when damaged. During imaginal disc regeneration, development has been shown to be delayed in the fruit fly Drosophila melanogaster, but how conserved the delay-inducing mechanisms are across holometabolous insects has not been assessed. The goal of this research was to develop the hornworm Manduca sexta as an alternative model organism to study such damage-induced mechanisms, with the advantage of a larger hemolymph volume enabling access to the hormonal responses to imaginal disc damage. Upon whole-body X-ray exposure, we noted that the imaginal discs were selectively damaged, as assessed by TUNEL and Acridine Orange stains. Moreover, development was delayed, predominantly at the pupal-to-adult transition, with a concomitant delay in the prepupal ecdysteroid peak. The delays to eclosion were dose dependent, with some ability for repair of damaged tissues. We noted a shift in critical weight, as assessed by the point at which starvation no longer impacted developmental timing, without a change in growth rate, which was uncoupled from juvenile hormone clearance in the body. The developmental profile was different from that of D. melanogaster, which suggests species differences may exist in the mechanisms delaying development.

Perfusion with cAMP analogue affects pheromone-sensitive trichoid sensilla of the hawkmoth Manduca sexta in a time-dependent manner.

Octopamine causes time-dependent disadaptation of pheromone-sensitive olfactory receptor neurons (ORNs) of Manduca sexta. Because the majority of insect octopamine receptors are positively coupled to adenylyl cyclases we examined whether cyclic adenosine monophosphate (cAMP) mimics octopamine-dependent modulation of pheromone transduction in a time-dependent manner. Long-term tip recordings of single trichoid sensilla of Manduca sexta were performed during three zeitgeber times (ZTs, ZT 0=lights on), while stimulating the sensilla with two doses of the main pheromone component bombykal in a non-adapting protocol. The membrane-permeable cAMP analogue 8bcAMP increased the normalized sensillar potential amplitude in a time- and bombykal dose-dependent way. At the higher bombykal dose only, the applied 8bcAMP antagonized an endogenous decrease in the mean sensillar potential amplitude at ZT 1-4 and ZT 8-11 when ORNs were adapted but not at ZT 22-1, when ORNs were sensitized. In contrast to octopamine, 8bcAMP did not consistently affect the initial pheromone-dependent action potential frequency, the phasic/tonic response pattern, or the time-dependent shift to lower mean action potential frequencies at ZT 8-11. Furthermore, 8bcAMP increased the spontaneous action potential frequency time dependently, but differently from octopamine. In conclusion, our results show that cAMP only partly mimics the octopamine-dependent disadaptation of olfactory receptor neurons during photophase, apparently due to another missing octopamine-dependent synergistic factor such as defined intracellular calcium levels.