Cyclical parthenogenesis presents an interesting challenge for the study of sex allocation, as individuals’ allocation decisions involve both the choice between sexual and asexual reproduction, and the choice between sons and daughters. Male production is therefore expected to depend on ecological and evolutionary drivers of overall investment in sex, and those influencing male reproductive value during sexual periods. We manipulated experimental populations, and made repeated observations of natural populations over their growing season, to disentangle effects of population density and the timing of sex from effects of adult sex ratio on sex allocation in cyclically parthenogenetic Daphnia magna. Male production increased with population density, the major ecological driver of sexual reproduction; however, this response was dampened when the population sex ratio was more male-biased. Thus, in line with sex ratio theory, we show that D. magna adjust offspring sex allocation in response to the current population sex ratio.

Density, sex ratio, clutch size and offspring sex in natural D. magna populationsRepeated density and population structure samples from 11 natural rock pool populations of Daphnia magna; clutch sizes and offspring sex from individual females from each sample. Each row represents a single female, giving information on the population sample in which she occurred and the details of her clutch. Variables defined in excel worksheet.DaphniaSexAlloc.xlsxOffspring sex ratios produced in experimental populations of D. magnaOffspring sex ratios produced in experimental populations of D. magna. Initial density and sex ratio of populations was manipulated. Each row represents a single experimental population. Variables described in excel worksheet.DaphniaSexAllocExp.xlsx