Sex-specific gonadal and gene expression changes throughout development in fathead minnow.

Although fathead minnows (Pimephales promelas) are commonly used as a model fish in endocrine disruption studies, past studies have not characterized sex-specific baseline expression of genes involved in sex differentiation during development in this species. Using a sex-linked DNA marker to verify gender, we evaluated the expression over time of genes involved in sex differentiation (dmrt1, cyp19a, cyp17, star, esr1, ar) in developing fathead minnows (10-45 days post hatch). Evaluation of these molecular markers in combination with gender identification help us to better understand the mechanisms regulating sex differentiation in fathead minnows and how endocrine-disrupting chemicals may alter these processes.

Low exposure concentration effects of methoprene on endocrine-regulated processes in the crustacean Daphnia magna.

Methoprene is a growth-regulating insecticide that manifests its toxicity to target organisms by acting as a juvenile hormone agonist. Methoprene similarly may exert toxicity to crustaceans by mimicking or interfering with methyl farnesoate, a crustacean juvenoid. We hypothesized that methoprene interferes with endocrine-regulated processes in crustaceans by several mechanisms involving agonism or antagonism of juvenoid receptor complexes. In the present study, we evaluated this hypothesis, in part, by characterizing and comparing the concentration-response curves for methoprene and several endpoints related to development and reproduction of the crustacean Daphnia magna. Our results demonstrate that methoprene has multiple mechanisms of toxicity and low-exposure concentration effects. Methoprene reduced the growth rate of daphnids with evidence of only a single concentration-response line, having a threshold of 12.6 nM. Molt frequency was reduced by methoprene in a concentration-dependent manner, with a response curve corresponding to a 2-segmented line and thresholds at 4.2 and 0.21 nM. An endpoint related to reproductive maturation, the time of first brood deposition, was also affected by methoprene, with a clear concentration-dependent response and a NOEC of 32 nM. Methoprene reduced fecundity according to a 2-segmented line, with thresholds of 24 and < or =0.18 nM. These results demonstrate that methoprene elicits significant toxicity to endocrine-related processes in the 5-50 nM concentration range. Furthermore, molting and reproduction were impacted at significantly lower methoprene concentrations, with a distinct concentration response and a threshold of < or =0.2 nM. The different concentration-dependent response from that of methoprene could involve agonism or antagonism of various juvenoid receptor configurations.

Temporal and quantitative changes in sexual reproductive cycling of the cladoceran Daphnia magna by a juvenile hormone analog.

Cyclic parthenogens, such as the cladoceran, Daphnia magna, utilize both asexual (parthenogenetic) and sexual reproduction in order to maximize population fitness in variable environments. Parthenogenetic reproduction is the default strategy among D. magna, while various environmental cues trigger cycles of sexual reproduction. Experiments were conducted with the juvenile hormone analog methoprene to test the hypothesis that members of the insect juvenile hormone/vertebrate retinoic acid family of transcription factors are involved in the regulation of sexual reproduction in daphnids. Neither methoprene, food reduction, or crowding independently stimulated entry into the sexual reproductive phase of the daphnids. However, the combination of food deprivation and crowding stimulated entry into the sexual reproductive phase characterized by an initial high production of males and the subsequent intermittent production of haploid egg-containing ephippia. Exposure to 160 nM methoprene along with food deprivation and crowding caused a significant reduction in the percentage of males produced during the early phase of the sexual cycle and significantly increased the percentage of males produced during the later stages of the cycle. Methoprene concentrations as low as 6.4 nM significantly reduced the number of resting eggs produced and proportionately increased the production of parthenogenetically-produced neonates. These experiments demonstrate that methoprene uncouples the coordinate production of males and resting eggs during the sexual reproductive period of D. magna. Methoprene stimulates male offspring production and defers their production to latter stages of the sexual reproductive period, while inhibiting the production of resting eggs and promoting the continuance of parthenogenetic reproduction. J. Exp. Zool. 290:148-155, 2001.