1. Mean body size of female Daphnids
Exposure to 0.3 uM methoprene for 14 days elicits decreased change in body length in juvenile Daphnia mendotae Authors: Niko Angenant, Cori O’Brien, Manu Sunny University of Wisconsin-Madison
Abstract
Results
Methoprene accumulates in lake water systems as well as in its crustaceous inhabitants. Although methoprene is likely to be detrimental for lake crustaceans like the Daphnia mendotae in Madison’s Lake Mendota, the effects of this insecticide component have never been studied in these species. D mendotae health is a useful marker of lake pollution. Previous studies on other daphnids implicate methoprene as a growth inhibitor1. Here, we ask how methoprene alters body size in Daphnia mendotae. Methoprene is a known agonist of D. magna's juvenile growth hormone, Methyl Farnesoate (Mf) which is involved in the biochemical pathways that result in molting. We hypothesized that methoprene can likewise agonize Mf in its sister subspecies Daphnia mendotae and that exposed juvenile Daphnia mendotae will exhibit a decreased growth rate compared to unexposed controls. Samples of D. mendotae drawn from Lake Mendota were exposed to 0.3 μM methoprene and parental body size was measured over the course of 15 days. Our results revealed methoprene to be a biological stressor that decreased growth rates in the entire life cycle of exposed Daphnia mendotae and hinted that methoprene toxicity is prevalent within daphnids in lakes that are exposed to such insecticides.
Mean body size of female Daphnids
Introduction
Figure 8: Mean body size of experimental female Daphnia compared to females in the control group for 15 days. Body size in millimeters measured from tip of crown to tip of tail. n=150 for both groups and a two tailed t-test gave an output of p<0.05.
Figure 3: Change in body size in millimeters of the daphnids exposed to different conditions. Body size in millimeters measured from tip of crown to base of tail. Results were statistically significant as error bars depicted represent standard error.
Figure 1: Molting is regulated by ecdysteroids in D. magna and insects with ecdysteroid receptors (EcR) transcribing the molting genes and when this happens, proteins degrade the old cuticle & synthesize new one.2,3
Methods
Discussion
p=0.018
Support HA
p=0.057
TBME and methoprene
have an additive effect
Study Question: How does methoprene affect body size in D. mendotae?
The smaller size of daphnids in the experimental group indicates that methoprene has a negative effect on growth in D. mendotae. Thus, the results suggest that methoprene acts as an agonist of Mf in this pathway6. The resulting decreased levels of molting ensues from a large concentration of methoprene bound to EcR which may allosterically inhibit EcR from binding to the ecdysteroids. Thus, methoprene will inhibit the formation of and the binding of the transcription factor complex to the unknown gene's promoter8. This implies that there will be lower levels of the old cuticle's degradation and lower levels of a new cuticle's synthesis.
Juvenile Hormones control molting & reproduction by controlling ecdysteroid expressivity. Methoprene is a juvenile hormone agonist, and in D. magna it agonizes Mf due to structural similarities.4,5
What layer of the lake can they be found in?
Conclusion: Methoprene is a stressor on daphnid biochemistry and impairs growth. This chemical has been accumulating in lake water systems and thus, methoprene toxicity may be prevalent within daphnids, as displayed in this experiment.
Literature Cited
1. Chandra Nagaraju, Ganji Purna . "Is methyl farnesoate a crustacean hormone?." Aquaculture (2007):
Sciencedirect. Web. 16 Sept
2. Laufer, H, Borst, D, Baker, FC, Carasco, C, Sinkus, M, Reuter, CC, Tsai, LW, and Schooley, DA. (1987)
Identification of a juvenile hormone-like compound in a crustacean. Science 235:
3. Olmstead, Allen W. et al. "Low Exposure Concentration Effects of Methoprene on Endocrine-Regulated
Processes in the Crustacean Daphnia magna." Toxicological Sciences 62.2 (2001): toxsci.org. Web. 16 Sept
4. Tatarazako, N. "Juvenile Hormone Agonists Affect The Occurrence Of Male Daphnia." Chemosphere
53.8 (2003): DOI.ORG. Web. 16 Sept
5. Olmstead, Allen W., and Gerald A. LeBlanc. "Insecticidal Juvenile Hormone Analogs Stimulate The
Production Of Male Offspring In The Crustacean, Daphnia Magna." Environmental Health Perspectives
111.7 (2003): DOI.ORG. Web. 16 Sept
Future Studies: Compare effects of other common methoprene solvents on daphnid growth rate, such as THF or acetone.5
Compare different lakewater strata in Lake Mendota for methoprene levels and D. mendotae concentrations to find any correlations.
Hypothesis: Methoprene can likewise agonize Mf in its sister subspecies Daphnia
mendotae and exposed juvenile Daphnia mendotae will exhibit a decreased growth rate compared to unexposed controls.