Induction of dauer pupae by fenoxycarb in the silkworm, Bombyx mori

Abstract

Topical application of fenoxycarb (1 μg per animal) at 129 or 132 h of the fifth instar larvae of the silkworm, Bombyx mori, did not induce morphological abnormalities in the pupal stage, but these animals became dauer (permanent) pupae. This condition of B. mori and the endocrine events leading to permanent pupae are discussed in this work. Application of fenoxycarb at 132 h of the fifth instar elicited a high ecdysteroid titre in the pharate pupal stage and a steadily high ecdysteroid titre in the pupal stage. The fenoxycarb-induced permanent pupae had non-degenerating prothoracic glands that secreted low amounts of ecdysteroid and did not respond to recombinant prothoracicotropic hormone (rPTTH) late in the pupal stage. The Bombyx PTTH titre in the haemolymph, determined by a time-resolved fluoroimmunoassay, was lower than that of controls at the time of pupal ecdysis, but higher than controls later in the pupal stage in fenoxycarb-treated animals. After application of fenoxycarb, its haemolymph level, measured by ELISA, reached a peak at pupal ecdysis, then remained low. These results suggest that the fenoxycarb-mediated induction of permanent pupae is only partially a brain-centred phenomenon. It also involves alterations in the hormonal interplay that govern both the initiation of pupal–adult differentiation and changes in the steroidogenic pathway of the prothoracic glands of B. mori.