Reproduction and development in Daphnia: The role of hormones, pesticides and detoxification

Daphnia are frequently used in ecotoxicology to establish human and environmental health standards. Daphnia are model organisms due to their small size, short life cycle, ubiquitous distribution in surface waters, and key ecological role in aquatic food chains. Although Daphnia are routinely used in pesticide testing, standard Daphnia toxicity tests were developed before the knowledge or concern existed regarding chemicals that disrupt normal development and endocrine function of wildlife and humans. Thus, it is important to determine whether Daphnia can be used as efficient screens for these chemicals, and to establish their sensitivity to these compounds. Therefore, I modified the standard Daphnia bioassay to include sublethal endpoints likely to reflect endocrine related impairments (i.e. sex determination, growth rate, fecundity), and investigated the role of detoxification enzymes in Daphnia. First, vertebrate hormones were screened to compare developmental responses of Daphnia to similar responses observed in vertebrate systems. Natural and synthetic vertebrate hormones assayed included beta-estradiol, gonadotropin, hydrocortisone, insulin, melatonin, progesterone, somatostatin, testosterone, thyroxine, ICI-182,780 (antiestrogen), R-1881 (androgen) and diethylstilbestrol (estrogenic). Second, I assayed several pesticides reported as having endocrine disrupting activity in vertebrate systems: o' p'-DDT, Di-n-butyl phthalate, toxaphene (estrogenic); p'p-DDE, linuron (androgenic); acetochlor, alachlor, metribuzin (thyroid activity); 2,4-D (lutenizing hormone activity). I also evaluated pesticides with unknown effects on vertebrate endocrine systems: chlorosulfuran, cyanazine, diflubenzuron, metolachlor and diquat. Finally, I established the potential for Daphnia to acclimate to a model chemical (the insecticide toxaphene) via P-450 detoxification enzymes.;This modified assay was more sensitive than those classically used for Daphnia in ecotoxicology, detecting sublethal but ecologically relevant effects. The use of Daphnia as a model system in both toxicological and ecological studies is therefore warranted; however, the inconsistencies observed in the response of Daphnia to vertebrate hormones suggest that care must be taken in extrapolating results in which daphnid biology is not comparable to that of vertebrates. This research demonstrates that the standard approach of pesticides screening using Daphnia can be augmented to include additional parameters that reflect endocrine related processes and account for the potential of acclimation.