Identification of the critical timing of sex determination in <Emphasis Type="Italic">Daphnia magna</Emphasis> (Crustacea,Branchiopoda) for use in toxicological studies |
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Authors: | Danielle D Ignace Stanley I Dodson Donna R Kashian |
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Institution: | (1) Department of Ecology & Evolutionary Biology, The University of Arizona, BioSciences West room 310, 1041 E. Lowell St., Tucson, AZ 85721, USA;(2) Department of Zoology, University of Wisconsin, Birge Hall, 430 Lincoln Drive, Madison, WI 53706-1381, USA;(3) Department of Biological Sciences, Wayne State University, 5047 Gullen Mall, Rm. 3115, Detroit, MI 48202, USA |
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Abstract: | Daphnia are often used as test organisms to determine toxicity of chemicals found in the environment. Fecundity and mortality are
the classic endpoints in such tests; however, there is increased interest in other sub-lethal endpoints. Most Daphnia reproduce by cyclic parthenogenesis, typically reproducing asexually to produce genetically identical females. Environmental
cues trigger the production of males allowing for sexual reproduction. This ability to switch reproductive strategies has
been increasingly used as a bioassay endpoint in screens for sub-lethal effects of contaminants. In order to optimize such
test, it is necessary to know when sex is determined during Daphnia development. A previous study using the insect juvenile hormone methyl farnesoate has shown that sex determination occurred
during the egg maturation period in Daphnia. Our study exposed female Daphnia magna carrying eggs and embryos at different stages of maturation, to the insecticide toxaphene to determine if the timing of sex
determination is similarly influenced by a pesticide. Our results suggest that in response to toxaphene exposure sex is determined
in the immature oocyte, before the final maturation cleavage, and before the developing egg is extruded into the brood chamber.
Thus, sex determination is transgenerational, being determined while the egg is still immature and in the mother’s ovary. |
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