Incomplete Ecdysis Is an Indicator of Ecdysteroid Exposure in Daphnia Magna

Abstract

Daphnia magna were exposed for 21 d to the ecdysteroids, 20-hydroxyecdysone (20-E), the accepted molting hormone, and ponasterone A (PoA), an ecdysteroid found in some crustaceans and many plants. Daphnids were monitored for alterations in molting, fecundity, and survival time. The 20-E elicited no significant effects on molting frequency, and its significant effects on reproduction were only at concentrations (260 nM) associated with premature death caused by incomplete ecdysis. We also examined PoA, which has been reported to have 10x higher affinity for the ecdysone receptor than 20-E. Ponasterone A elicited effects similar to those of 20-E at approximately 10x lower concentrations. This suggests that affinity for the receptor is the major parameter determining activity in vitro and in vivo and that differences in metabolism and elimination in vivo were not significant. The effects of PoA on daphnids mimicked those of 20-E except PoA reduced fecundity in the second generation and 20-E had no effect. Last, both exogenous 20-E and PoA show similar effects, including premature death associated with incomplete ecdysis, and the overall difference in toxicity is mostly likely due to receptor affinity.

Ecdysteroid levels in Daphnia magna during a molt cycle: Determination by radioimmunoassay (RIA) and liquid chromatography–mass spectrometry (LC–MS)

“…With regard to ecdysteroids, only a few studies are available focusing mainly on exposure-eVects of exogenously supplied ecdysteroids on the performance of D. magna. In D. magna, exposure to ecdysteroids tends to pre- vent successful exuviation in a dose dependent manner (Bodar et al, 1990;Baldwin et al, 2001). Bodar et al (1990) suggested that this molting impairment is due to persistently high ecdysteroid levels in the daphnids, which may override the typical drop of ecdysteroid levels just prior to exuviation as observed in other crustaceans, including D. magna (Mu and LeBlanc, 2004b, and the present study).…”

Cloning and characterization of the ecdysone receptor and ultraspiracle protein from the water flea Daphnia magna

“…In addition, Ponasterone A is known to cause molting in D. magna at a 10-fold lower concentration than Ec. These results support the suggestion that differences in the affinity of EcR for its ligands are responsible for the different concentrations of EC and Ponasterone A required to affect molting (Baldwin et al 2001). Recently, the structure of Heliothis virescens EcR/USP was elucidated using X-ray crystallography, and investigation of Ponasterone A and BYI06830 (a non-steroidal, lepidopteran-specific agonist) binding indicated the presence of different ligand-binding pockets for steroidal and nonsteroidal ligands (Billas et al 2003).…”

“…As in the case in other arthropods, Ec and JH play important roles in cladocerans (Baldwin et al 2001, Mu & LeBlanc 2002. The Ec affects male progeny production and chemicals related to JH increase male production in Daphnia magna and other daphnids (Peterson et al 2001, Tatarazako et al 2003.…”

Crustacean Molting: Regulation and Effects of Environmental Toxicants

“…Ponasterone A is a steroid that was first isolated from plants that has high-ecdysteroid activity in insects [65]. Exposure of D. magna to ponasterone A stimulated premature ecdysis [66]. RH 5849 accelerated molting; and, in the barnacle B. amphitrite, RH 5849 enhanced attachment and metamorphosis of the larvae [67].…”