Ultrastructural changes of neurosecretory cells in the pars intercerebralis during diapause development in Pieris rapae

Abstract

Ultrastructures of NS-II cells in the pars intercerebralis were observed during diapause development. Some other experiments (Measurement of O 2 consumption and decerebration) were simultaneously carried out in order to understand the endocrine regulation of diapause development in Pieris rapae. Pieris pupae needed 75 days of chilling to terminate diapause completely. Incubation of diapause pupae following shorter durations of chilling (32–35 days) gave less than 50% of pharate adult development. These pupae first showed a fall in O 2 uptake, which was never observed in the pupae chilled for 75 days. The ultrastructures of NS-II cells showed that secretory activity was already restored after 35 days of chilling although diapause did not terminate completely until this time. There was an abundant release of secretory material from NS-II cells when diapause pupae were incubated after 75 days of chilling. Decerebration experiments showed that a further brain hormone release was not necessary for diapause termination after 75 days of chilling. Decerebration after short durations of chilling (20 and 35 days) unexpectedly facilitated diapause termination. These results suggested other mechanism functioning through the early stages of chilling other than brain hormone shortage, which maintained the diapause.