Methoprene and 20-OH-ecdysone affect male production in Daphnia pulex

Exposure of Daphnia pulex to the insecticide and juvenile hormone-mimic met hoprene resulted in a decrease in the incidence of all-male broods and an i ncrease in the incidence of all-female broods compared with controls. These effects were observed at nominal concentrations of 10 and 100 mug/L, withi n the upper range of concentrations at which methoprene is applied in the e nvironment. Because methoprene has been found to bind to the mammalian reti noid X receptor, we also tested the effects of retinoic acid on Daphnia rep roduction. Neither 9-cis-retinoic acid nor all-traps-retinoic acid had any observable effect. Because juvenile hormone and ecdysteroids interact in ma ny insect systems, we also exposed Daphnia to 20-OH-ecdysone. Exposure to t he crustacean hormone 20-OH-ecdysone at levels of 1 and 10 mug/L resulted i n an increase in all-male broods and a decrease in all-female broods, but 1 00 mug/L 20-OH-ecdysone resulted in a decrease in all-male broods and an in crease in all-female broods. Our results suggest that juvenile hormone and ecdysteroids might play a role in the Daphnia sex determination system.