Many organisms can reproduce both asexually and sexually. For cyclical
parthenogens, periods of asexual reproduction are punctuated by bouts of
sexual reproduction, and the shift from asexual to sexual reproduction
has large impacts on fitness and population dynamics. We studied
populations of Daphnia dentifera to determine the amount of
investment in sexual reproduction as well as the factors associated with
variation in investment in sex. To do so, we tracked host density,
infections by nine different parasites, and sexual reproduction in 15
lake populations of D. dentifera for three years. Sexual
reproduction was seasonal, with male and ephippial female production
beginning as early as late September and generally increasing through
November. However, there was substantial variation in the prevalence of
sexual individuals across populations, with some populations remaining
entirely asexual throughout the study period and others shifting almost
entirely to sexual females and males. We found strong relationships
between density, prevalence of infection, parasite species richness, and
sexual reproduction in these populations. However, strong collinearity
between density, parasitism, and sexual reproduction means that further
work will be required to disentangle the causal mechanisms underlying
these relationships.