Monday, August 6, 2007

PS 2-27: Production of male neonates and resting eggs in an experimental population of a cyclical parthenogen, Daphnia magna, exposed to a juvenoid insect growth regulator and an herbicide

Shigeto Oda1, Norihisa Tatarazako1, and Taisen Iguchi2. (1) National Institute for Environmental Studies, (2) National Institutes of Natural Science

Effects of the juvenoid insect growth regulator fenoxycarb and the herbicide atrazine to the production of both male neonates and resting eggs in the population of a cyclical parthenogen, Daphnia magna, were studied. Experiments were conducted by spiking test chemicals once a week to genetically identical populations in a 2-L glass beaker. The results of the first experiment conducted for ten months showed that population in the control was maintained near the carrying capacity with almost all the animals being females. Fenoxycarb induced the production of male neonates at 1µg/L, resulting in the population sex ratio of more than 50% toward male. The sex ratio increased to 90% as the fenoxycarb concentration was increased to 4µg/L. This highly skewed sex ratio caused a temporary decline in the population density. Scarcely any resting egg was produced in both groups. In the second experiment using another strain of D. magna, males in the control were kept at 10% of the total population. The population sex ratio was elevated again as high as about 50% at a concentration of 2µg/L of fenoxycarb. Exposure to 200µg/L of atrazine also increased population sex ratio, resulting in 20-30% of total population being male. The production of resting eggs was observed in either of the experimental groups. It was kept in lower number in the fenoxycarb group than in the control, while atrazine exposure increased the resting egg production. The results in the present study showed that juvenoid IGRs can cause a populational decline to non-target species, cladocerans, by skewing population sex ratio toward male. It should be noted that atrazine increased production of both male neonates and resting eggs. We need to pay more attention to chemicals which affect the shift in reproductive mode in cladocerans through population dynamics, as well as to juvenoid IGRs.