Dopamine is a key factor for the induction of egg diapause of the silkworm, Bombyx mori.

Abstract

The silkworm, Bombyx mori, enters diapause in the early embryonic stage. Embryonic diapause is induced by incubating eggs of the maternal generation at high temperature (diapause type), whereas incubation at low temperature results in non-diapausing progeny (non-diapause type). Measurement of catecholamine concentrations in haemolymph and brain-subesophageal ganglia (Br-SGs) showed that only dopamine concentrations in both tissues are consistently higher in diapause-type than non-diapause-type larvae and pupae. In particular, the difference in dopamine concentrations in both tissues increases around pupal ecdysis. During the early pupal stage, Dopa decarboxylase activities and mRNA concentrations in Br-SGs were also much higher in diapause-type than non-diapause-type insects. Elevation of dopamine levels induced by feeding Dopa to penultimate-instar and last-instar larvae, and by injecting Dopa or dopamine into pupae 2 days after pupation made the non-diapause-destined insects lay diapause-destined eggs at 59% and approximately 70% frequencies, respectively. Furthermore, injection of Dopa or dopamine elevated mRNA levels of the diapause hormone in the Br-SGs of non-diapause-type pupae 1 day after injection. Incubation of Br-SGs isolated from non-diapause-type day-2 pupae with Dopa or dopamine also stimulated the expression of diapause hormone mRNA. These data indicate that environmental stimuli during embryonic development increase dopamine levels in both hemolymph and Br-SGs from the larval stage to early pupal stage, which results in laying of diapause-destined eggs by female adults through enhanced expression of the diapause hormone gene.