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 Drosophila melanogaster, which suggests species differences may exist in the mechanisms delaying development.