Dynamics of diapause hormone and prothoracicotropic hormone transcript expression at diapause termination in pupae of the corn earworm, Helicoverpa zea

Abstract

Both diapause hormone (DH) and ecdysone (E) are capable of terminating pupal diapause in members of the Helicoverpa/Heliothis complex. In this study we examine how the transcript encoding prothoracicotropic hormone (PTTH), the neuropeptide that stimulates the prothoracic gland to produce E, and the transcript encoding DH respond to developmental changes, as well as environmental and hormonal cues that can trigger the termination of diapause. In nondiapausing individuals PTTH and DH transcripts are abundant from pupation until adult eclosion, while in pupae that enter diapause PTTH transcripts are undetectable and abundant DH transcripts are present only briefly after pupation. Injection of E can break diapause at either 18 or 21°C, but DH is effective in breaking diapause only at the higher temperature. Transfer of pupae to a diapause-terminating temperature of 25°C, injections of 1nmol DH or 75ng E at 21°C, and injections of 500ng E at 18°C, are all accompanied by a simultaneous elevation of mRNAs encoding both PTTH and DH, although the rate of PTTH mRNA increase is consistently more rapid than that of DH. Subthreshold doses of E or injections of distilled water elicit a temporary rise in PTTH and DH transcripts but do not lead to diapause termination. The results suggest that these two hormonal systems work together in the cascade of events leading to diapause termination, producing a sophisticated control system that is finely tuned and responsive to subtle temperature changes in the overwintering environment.