Temporal and quantitative changes in sexual reproductive cycling of the cladoceran Daphnia magna by a juvenile hormone analog

Cyclic parthenogens, such as the cladoceran, Daphnia magna, utilize both as exual (parthenogenetic) and sexual reproduction in order to maximize popula tion fitness in variable environments. Parthenogenetic reproduction is the default strategy among D. magna, while various environmental cues trigger c ycles of sexual reproduction. Experiments were conducted with the juvenile hormone analog methoprene to test the hypothesis that members of the insect juvenile hormone/vertebrate retinoic acid family of transcription factors are involved in the regulation of sexual reproduction in daphnids. Neither methoprene, food reduction, or crowding independently stimulated entry into the sexual reproductive phase of the daphnids. However, the combination of food deprivation and crowding stimulated entry into the sexual reproductiv e phase characterized by an initial high production of males and the subseq uent intermittent production of haploid egg-containing ephippia. Exposure t o 160 nM methoprene along with food deprivation and crowding caused a signi ficant reduction in the percentage of males produced during the early phase of the sexual cycle and significantly increased the percentage of males pr oduced during the later stages of the cycle. Methoprene concentrations as l ow as 6.4 nM significantly reduced the number of resting eggs produced and proportionately increased the production of parthenogenetically-produced ne onates. These experiments demonstrate that methoprene uncouples the coordin ate production of males and resting eggs during the sexual reproductive per iod of D. magna. Methoprene stimulates male offspring production and defers their production to latter stages of the sexual reproductive period, while inhibiting the production of resting eggs and promoting the continuance of parthenogenetic reproduction. (C) 2001 Wiley-Liss, Inc.