Fat body stimulation of ecdysone synthesis in Heliothis zea

Prothoracic glands of Heliothis zea pupae require both a humoral factor and prothoracicotropic hormone (PTTH) to synthesize ecdysone. The humoral factor is absent when pupae are maintained at diapause-sustaining temperatures. Thus, pupae remain in diapause despite the release of PTTH at or before larval-pupal ecdysis. Tissue implantation experiments revealed that a diapause-terminating factor is present in the fat body of non-diapausing pupae. Other tissue implantation experiments showed that, when diapausing pupae were transferred from 19 to 27°C, diapause-terminating activity appeared first in the fat body and then the fat body into the haemolymph. HPLC separation of the haemolymph and fat body fractions followed by bioassay demonstrated that fractions containing diapause-terminating activity eluted from both tissues within 28–30 min. These results suggest that the factors found in the fat body and haemolymph may be the same compound. Evidence from ecdysone radioimmunoassay experiments ruled out the possibility that the diapauseterminating activity was due to either free or conjugated ecdysteroids. Corresponding in vitro experiments in which the prothoracic glands were cultured with brain extracts versus fat body and haemolymph fractions also indicated that the haemolymph/fat body factor was not PTTH.