In Vitro Studies on the Role of the Brain and Prothoracic Glands in the Pupal Diapause of Manduca Sexta

Pupal diapause in the tobacco horn worm, Manduca sexta (Johannson), is characterized by the absence of an increased ecdysteroid titre in the haemolymph during the first week of pupal life as measured by radioimmunoassay. This virtual absence of the steroid moulting hormone is thought to be responsible for the diapause state and it is apparently a consequence of the failure of the diapausing pupal prothoracic glands (PG) to synthesize ecdysone at an increased rate. In Manduca, this apparent failure of PG activation during diapause may be in response to two endocrinological circumstances: the curtailment of PTTH release as opposed to its synthesis and transport, and the development of refractoriness of the gland to stimulation by PTTH. The former was examined by measuring PTTH activity present in brains and brain-retrocerebral complexes of diapausing and non-diapausing pupae and the latter by assessing PG biosynthetic activity in vitro in the presence and absence of PTTH.