Ecdysone haemolymph levels and in vivo development of imaginal wing discs have been studied during the last larval instar of Pieris brassicae.During this period, β-ecdysone variations show two successive peaks, the first one related to the induction of wandering stage, and the second (main) one to pupal cuticle synthesis. The observed situation is very similar to that of Manduca sexta. Imaginal wing disc growth is composed of several genetically programmed steps that need the presence of ecdysone, but do not appear very closely linked to circulating hormone levels. It seems that ecdysone haemolymph peaks should be considered as periods where ecdysone levels are above a threshold value.
"In the last-larval instar, ahead of the molt peak, there is an additional pulse of ecdysteroids secreted in the absence of the juvenile hormone (JH) which commits the larva to metamorphose into the pupal stage. Previous studies have shown that altering the hormone titer by administering either ecdysteroids or JH or their analogs resulted in the phenotypic manifestation of developmental deformities (Riddiford and Ajami, 1973; Kurushima and Ohtaki, 1975; Lafont et al., 1977; Truman et al., 1980). Although more than 4500 synthetic analogs of JH are known and some of them were commercialized as insecticides (Slama, 1985; Dhadialla et al., 1998; Cusson and Palli, 2000), very few analogs of ecdysone have been reported. "
[Show abstract][Hide abstract] ABSTRACT: The effect of RH-5992 (tebufenozide), a non-steroidal ecdysone agonist, on adult development of the spruce budworm, Choristoneura fumiferana, was investigated by administering the compound intrahemocoelically to pupae on days 1–6 after pupal ecdysis. At concentrations of 200 ng/pupa there was significant mortality but at doses of 50–100 ng/pupa, the emerging adults displayed wing deformities which reduced their ability to mate and oviposit. Light microscopy of the pupal wings revealed that there was degeneration of the epithelial cells, reduction in the number of veins, precocious cuticle formation and inhibition of growth of normal wing scales. Injection of RH-5992 into pupae resulted in a dose dependent induction of mRNA for ecdysone-induced transcription factor, Choristoneura hormone receptor 3 (CHR3). These results suggest that the pupae respond to RH-5992 in a manner similar to larvae. However, the effects are not expressed overtly and are camouflaged by the pharmacological effects.
"depends upon a decline in the high titre of juvenile hormone present during the last larval-larval ecdysis to undetectable levels. This drop permits a biphasic secretion of ecdysone in the last stadium, which completes the transformation (Lafont et al., 1977; Maroy and Tarnoy, 1978; Dean et af., 1980; Bollenbacher et al., 1981). The first ecdysteroid peak is responsible for a change in the commitment of the epidermal cells and coincides with the onset of wandering, while the second peak promotes apolysis and pupal cuticle formation (Riddiford , 1980). "
[Show abstract][Hide abstract] ABSTRACT: The haemolymph ecdysteroid titre during the late last stadium of the cabbage looper moth, Trichoplusia ni, was monitored in the presence of experimentally altered juvenile hormone levels to test the hypothesis that the appearance of a prepupal burst of juvenile hormone enhances prothoracic gland activity. Two peaks of ecdysteroid activity were detected in the haemolymph of last-stadium larvae (55 ng/ml and 950 ng/ml) with 20-hydroxyecdysone the most prevalent ecdysteroid. Treatment of post-feeding late last-stadium larvae with juvenile hormone I or the juvenoids, methoprene or epofenonane, had no significant effect on the timing or character of pupation. Application of juvenile hormone I to neck-ligated late last-stadium larvae failed to restore peak ecdysteroid levels to those of unligated controls. Application of the anti-juvenile hormone compounds, fluoromevalonolactone or 3,3-dichloro-2-propenyl hexanoate, disrupted metamorphosis causing delays in tanning and the formation of abnormal pupae. Yet, corresponding deviations in the timing or magnitude of ecdysteroid levels or in the conversion of ecdysone to 20-hydroxyecdysone were not observed. When early last-stadium larvae were treated with the juvenile hormone esterase inhibitor, 3-octylthio-1,1,1-trifluoro-2-propanone, shifts in the timing of peak ecdysteroid levels, coincident with delayed pupation, were observed. These results suggest that the prepupal prothoracic glands of T. ni may be considerably less sensitive to changes in the endogenous juvenile hormone titre by the time larvae have begun to wander, even though treatment at this time may still produce toxic effects that influence the success of ecdysis and/or resultant pupal morphology.
[Show abstract][Hide abstract] ABSTRACT: Summary Ecdysone haemolymph levels have been analyzed in neck- or thorax-ligatedPieris larvae in order to explain the physiological significance of critical periods. It appeared that head critical period corresponds to an incomplete activation of prothoracic glands, while the thoracic critical period is related to the secretion of a minimal amount of ecdysone necessary for moulting. DuringPieris' last larval instar, there is no evidence for any noticeable synthesis of ecdysone in isolated abdomens.
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