ECDYSTEROIDS AND RELATED MOLECULES IN ANIMALS AND PLANTS

Ecdysteroids represent a family of more than 250 members present in an imals and plants. In addition, many more or less related molecules bel onging to a very large group of ''polyhydroxysterols'' are present in both phyla. Of the several fundamental questions that remain unanswere d al the moment, three major ones will be considered here: 1) The bios ynthetic pathway is only partially understood; several steps remain to be elucidated in arthropods, and almost nothing is known in plants. I s it the same in plants as in animals? Some recent developments using cell cultures and new labeled precursors appear very promising to answ er this fundamental question. The exact nature and the origin of ecdys teroids in non-arthropod invertebrates is also a matter of debate, and this problem should take advantage of a better knowledge of the biosy nthetic pathway in arthropods. 2) The identification of ''active hormo ne(s)'' in arthropods is a too-neglected field. However, the physiolog ical 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 molecul es, or if all receptor isoforms display the same relative affinity tow ard various ecdysteroids, remains unanswered, although this question w as addressed more than 25 years ago. We hope that new bioassays using several different species/tissues will be developed for extensive stru cture/activity relationships studies, and also that this problem will appear more attractive for those ''ecdysonists'' working on the regula tion of gene expression. 3) Up to now, the studies on the non-genomic effects of ecdysteroids have been underdeveloped. Although many articl es on vertebrate neurosteroids have appeared during the last few years , we are sri II waiting for ''neuroecdysteroids.'' Several membrane ac tions of ecdysteroids on nerve cells and salivary glands have been des cribed, and there is growing evidence for their perception by taste re ceptors in both insects and crustaceans. It may well be that spectacul ar advances will occur in the near future. (C) 1997 Wiley-Liss, Inc.