Secondary sex ratio of the cyclic parthenogen Daphnia magna (Crustacea: Cladocera) in the Canadian Arctic

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Abstract
Secondary sex ratios of the cyclic parthenogen Daphnia magna were studied in habitats near Churchill, Manitoba. Daphnia magna is believed to possess an environmentally mediated sex-determining system. Throughout the season females produced broods that were predominantly unisexual. The proportion of male offspring was low early in the season but rose to a value near 50% in each of four populations. It is hypothesized that after they have an initial series of female broods, individual females begin to alternate the sexes of their broods in response to an environmental cue. Such an alternation of brood sexes would explain the population sex-ratio pattern observed and would satisfy theoretical requirements for population and individual secondary sex ratios of 1:1. In studies on cladocerans, environmental cues inducing the sex-ratio response must be distinguished from cues with a sex-determining effect.
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