Ecdysteroids represent a family of more than 250 members present in animals and plants. In addition, many more or less related molecules belonging to a very large group of "polyhydroxysterols" are present in both phyla. Of the several fundamental questions that remain unanswered at the moment, three major ones will be considered here:
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The biosynthetic pathway is only partially understood; several steps remain to be elucidated in arthropods, and almost nothing is known in plants. Is it the same in plants as in animals? Some recent developments using cell cultures and new labeled precursors appear very promising to answer this fundamental question. The exact nature and the origin of ecdysteroids in non-arthropod invertebrates is also a matter of debate, and this problem should take advantage of a better knowledge of the biosynthetic pathway in arthropods.
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The identification of "active hormone(s)" in arthropods is a too-neglected field. However, the physiological significance of the multiple secretory products of molting glands, and of the peripheral metabolism which takes place in target cells, is not so clear. Whether there are specific roles for specific molecules, or if all receptor isoforms display the same relative affinity toward various ecdysteroids, remains unanswered, although this question was addressed more than 25 years ago. We hope that new bioassays using several different species/tissues will be developed for extensive structure/activity relationships studies, and also that this problem will appear more attractive for those "ecdysonists" working on the regulation of gene expression.
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Up to now, the studies on the non-genomic effects of ecdysteroids have been underdeveloped. Although many articles on vertebrate neurosteroids have appeared during the last few years, we are still waiting for "neuroecdysteroids." Several membrane actions of ecdysteroids on nerve cells and salivary glands Acknowledgments: This paper is the written version of the Karlson lecture given at the XIIth Ecdysone Workshop. I wish to thank Professor Peter Karlson and the Karlson Trust for this privilege. It is also a pleasure to thank my colleagues, Drs.