Estimation of the propensity for sexual selection in a cyclical parthenogen

Cyclical parthenogenesis is a widespread reproductive strategy in which organisms go through one or multiple rounds of clonal reproduction before sexual reproduction. In populations of the planktonic cladoceran sexual reproduction is typically less common than parthenogenesis and therefore hardly studied. We studied the sexual process and its relation to sexual selection in rockpool populations, where sex is common throughout the summer, by observing natural mating in these shallow habitats. While microsatellite markers revealed no evidence for disassortative mating and thus, inbreeding avoidance, body length and infection status revealed assortative mating, suggesting sexual selection to act. In cases where two males mated with a single female, larger male remained longer, possibly giving them an advantage in sperm competition. Indirect evidence points at the brood pouch as the likely site of fertilization and thus, sperm competition. Sperm length was as variable within ejaculates as it was among males from different populations. Our data give firm evidence that sexual selection is present in this species and that it likely manifests itself by a combination of female choice and male - male competition.