Ecdysis Ecdysone

Ecdysone stimulates the synthesis of RNA in tissues. Visual demonstration of the effect is provided by its action on polytene chromosomes of fly larvae (Fig. 26-14).361 Fifteen minutes after the application of ecdysone, a puff is induced on one band of the chromosome a second puff forms at a later time while a preexisting puff diminishes. Thus, like steroid hormones in mammals, ecdysone appears to have a direct controlling effect on transcription. The cuticle-shedding process (ecdysis) is initiated by the brain peptide eclosian. However, the brain may be responding to the ecdysis-triggeiing hormone, a peptide that is secreted by a series of epitracheal glands located in various segments of the body.362... 

One hypothesis on themecKanlsm of action of JH In the Inhibition of ecdysis Implicates the active role of JH In the regulation of ecdysone tltres In the Insect body (17), possibly a consequence of an effect on the prothoradc glands (22). Another hypothesis Implicates an effect of JH on ecloslon hormone or Its action (20, 18). 

Our present meagre understanding of normal endocrine processes in invertebrates makes the assessment of chemical endocrine disruption in the field difficult (LeBlanc 1999). Steroid roles differ between species and sometimes sexes, and their influence may vary at different developmental stages. In most studies of invertebrates, endocrine disruption appears to involve androgenization rather than oestrogenic effects (see Box 7.2). Arthropods (crustaceans and insects), annelids and molluscs use ecdysteroids, terpenoids and vertebratelike sex steroids for endocrine control. For example, the ecdysteroid ecdysone is naturally converted to 20-hydroxyecdysone (Fig. 7.10), which induces moulting (ecdysis) in both insect larvae and crustaceans. 

Ecdysone induces the larval-pupal metamorphosis in the absence of JH or a JH-active insecticide, but the presence of active compounds leads to a new larval stage at ecdysis, or to the development of larval-nymphal, larval-pupal, or larval-adult intermediates that are unable to give rise to normal adults. Treated pupae (e.g., tobacco cutworms treated with pyriproxyfen) may develop into normal adults. However, the females are unable to deposit eggs because a substance that induces oviposition behavior after mating is not released in the hemolymph. Other physiological and behavioral effects of JH-active insecticides are also observed. 

Rao KR, Fingerman M, Hays C (1972) Comparison of the abilities of a-ecdysone and 20-hydroxyecdysone to induce precocious proecdysis and ecdysis in the fiddler crab Uca pug-ilator. Z Vergl Physiol 76 270-284... 

Ecdysone has proved to be active in all the orders of insects investigated. Of major interest is the influence on symbionts in certain cockroaches. The symbionts go through a sexual cycle while the host casts its skin. The sexual cycles can be induced by means of ecdysone without the host undergoing ecdysis . 

---