Conversion of different putative ecdysteroid precursors in Lithobius forficatus L. (Myriapoda: Chilopoda)

Abstract

The biosynthetic pathway of ecdysteroids has been investigated in the centipede Lithobius forficatus. By using various labeled putative precursors already known to be efficiently converted into ecdysone and 20-hydroxyecdysone by insects and crustaceans, it was found that the reactions always stopped at 2-deoxyecdysone, and that this compound was only epimerized and conjugated at the C-3 position. Ecdysone was efficiently converted into 20-hydroxyecdysone, whereas 20-hydroxylation was not effective with 2-deoxyecdysteroids. None of the tested precursors was hydroxylated at the C-2 position. In addition, in vivo labeling experiments with [ 3H]cholesterol led only the formation of unidentified products. Therefore, the origin of the already reported ecdysone and 20-hydroxyecdysone in these animals remains still conjectural. The “ecdysial glands” (i.e. the putative molting glands) and gonads proved the most effective tissues to convert 2,22,25-trideoxyecdysone into 2-deoxyecdysone in vitro, although at rather low levels. By contrast, this compound was very efficiently converted in vivo. The present data raise many unanswered questions, and they point out that classical inserct schemes are not applicable in the present case. It might thus well be that divergent pathways have evolved in arthropods, and this problem awaits further investigations.