Author: SLÁMA KAREL
Publisher: Entomological Society of America
ISSN: 1938-2901
Source: Annals of the Entomological Society of America, Vol.91, Iss.2, 1998-03, pp. : 168-174
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Abstract
Ecdysone and ecdysteroids have always been defined as insect hormones produced by the prothoracic glands to stimulate molts. I have recently reread some old papers about the prothoracic glands and found that the widely used brain‐prothoracic gland theory on the hormonal control of insect metamorphosis might be misleading. Removal of prothoracic glands has no effect on molt cycles. The quantitative responses induced by ecdysteroids are profoundly different from the qualitative, all‐or‐none responses which are induced by the centrally produced metamorphosis hormones. Ecdysteroids are produced in a number of peripheral target tissue and organs themselves, and this fact contradicts the exact definition of the term hormone. The true ecdysteroid status can be defined as peripheral feedback tissue factors, which synchronize tissue growth with someimportant developmental events like ecdysis or oviposition. It also has been concluded that the endogenous peaks of ecdysteroids, which always occur in a nonfeeding period with minimal metabolic intensity, have their origin in polar metabolites of sterol that is retrieved from the old disintegrating tissues; prothoracic gland contribution is very small. The partly hydrophilic, polyhydroxylated sterol (ecdysteroid) can be reutilized as an essential sterol for growth of the newlydeveloping adult tissues and organs. The transport and reutilization hypothesis of polyhydroxylatedsterol is consistent with a widespread distribution of ecdysteroids, not only in insects but in most if not in all living organisms—bacteria, fungi, ferns, higher plants, invertebrates, vertebrates, and also in the human body. Insects and other arthropods evolved special feedback mechanisms, in which ecdysteroid acquired a role of essential peripheral growth factors coordinating proliferation among various tissue and organs.
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