ECDYSONE ET REPRODUCTION CHEZ LES FEMELLES ADULTES D'INSECTES

Abstract

The data in this paper describes the involvement of ovarian ecdysone during the development of Locusta migratoria and was gathered in our laboratory during the last 4 years. The cells of the follicular epithelium surrounding the terminal oocytes synthesize ecdysone at the end of oocyte maturation when vitellogenesis is nearly completed; this synthesis is triggered by a folliculotropic factor probably synthesized in the medial neurosecretory cells of the brain and released by the corpora cardiaca. Ecdysone is probably biosynthesized from cholesterol via several deoxyecdysteroids (2, 14, 22, 25-tetradeoxyecdysone; 2, 22, 25-trideoxyecdysone, 2, 22-dideoxyecdysone; 2-deoxyecdysone), whose presence can be ascertained by physico-chemical methods in this biological system. Ecdysone and 2-deoxyecdysone are transferred to the oocyte (there is no quantitative data on the other deoxyecdysteroids) where 95% accumulates as highly polar compounds presumed to be conjugated on the basis of hydrolysis experiments. If there is any, the secretion of ecdysone or ecdysone-immunoreactive molecules into the blood of the female is only minimal. The newly-laid egg contains practically all the ovarian ecdysteroids. The concentration of free ecdysone at that stage is 1 μM. As embryonic development proceeds, four distinct peaks of this concentration, reaching 4 to 8 μM, are observed. The first two are clearly ecdysone peaks with a low concentration of a compound presumed to be 20-hydroxyecdysone. In contrast, ecdysone peaks 3 and 4, found during postblastokinetic development, are followed by intense peaks of 20-hydroxyecdysone. Each peak of free ecdysone concentration is correlated on an exact time-scale to the onset of cuticle deposition by the embryonic cells of the serosa or the epidermis. This leaves little doubt that, as during postembryonic development, the function of ecdysone (and/or 20-hydroxyecdysone) in the embryo is to control cuticulogenesis. A crucial problem is the origin of embryonic ecdysone. In the absence of biochemical data and in vitro studies on the pathway of ecdysone biosynthesis and its site in the embryo, we are reduced to working hypotheses. Before the prothoracic glands in the preblastokinetic embryo differentiate, it is probable that ecdysone peaks 1 and 2 are attained either through hydrolysis of maternal conjugates or hydroxylation of ecdysone precursors, namely 2-deoxyecdysone. It is a theoretical possibility that the differentiated prothoracic glands in the postblastokinetic embryo control the de novo ecdysone synthesis, although maternal deoxyecdysteroids or conjugates might also be used to increase the titer of free ecdysone at this stage of development. Future studies must investigate this question.