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The factors inducing sexual eggs, as well as the offspring sex ratio preceding sexual egg formation, were investigated in a cyclic parthenogen, Daphnia magna (Crustacea, Cladocera). Laboratory experiments were conducted on individual animals living in flow-through chambers, making it possible to separate the effects of two density-dependent factors: food limitation and a chemically mediated cue. These factors were studied under a short-day photoperiod and in permanent light. The simultaneous actions of an inductive photoperiod, food limitation and chemically mediated crowding, were all needed to induce sexual egg formation. When only two stimuli were present, the offspring sex ratio was 0.50 or lower, and no sexual eggs were produced. By contrast, environmental conditions inducing sexual eggs also effected strongly male-biased asexual offspring, with an average sex ratio up to 0.8 (even 1.0 in individual females).
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