Environmental toxicant effects on sexual reproduction in Daphnia magna

The freshwater zooplankton, Daphnia magna alternates between asexual and sexual reproductive strategies. Asexual reproduction is characterized by the clonal production of all-female offspring. The shift to sexual reproduction begins with the induction of male offspring production, which is followed by the production of resting (diapause) eggs. While asexual reproduction has been widely studied in ecotoxicological studies, sexual reproduction has been largely ignored and the extent to which xenobiotics can interfere with this process is not known. I hypothesized that certain environmental toxicants can interfere with sexual reproduction in Daphnia magna by disrupting the endocrine regulation of this process. This hypothesis was tested in three parts. (1) The susceptibility of multiple aspects of daphnid sexual reproduction to chemical insult was evaluated. The development of both female and male secondary sex characteristics, the induction of male offspring, and the production of resting eggs were altered by exposure to various chemicals. (2) An endocrine process responsible for regulating the production of males, an early component of the sexual reproductive phase was elucidated. The crustacean terpenoid hormone, methyl farnesoate, was found to program eggs in the ovaries to develop into males during neonatal development. This effect only occurred when eggs were exposed during a specific period of ovarian maturation just prior to their release into the brood chamber. (3) The ability of environmental chemicals to disrupt this process of sex determination was determined. Insecticides with the ability to mimic juvenile hormones, specifically pyriproxyfen and methoprene, induced male production in daphnids when exposure occurred during the critical window for sex determination. Results from binary mixture studies of methyl farnesoate:pyriproxyfen and methyl farnesoate:methoprene supported a mechanism of action of disruption of methyl farnesoate-regulated pathways for these insecticides. This research demonstrates that sexual reproduction in daphnids can be targeted by toxicant action, characterizes the role of methyl farnesoate in daphnid sex determination, and generates a novel model for endocrine disruption in crustaceans.