Photoperiodic response of sexual reproduction in the Daphnia pulex group is reversed in two distinct habitats

Abstract

Two traditional species in the Daphnia pulex group (D. pulex and D. pulicaria) are not completely reproductively isolated. The microhabitats they dominate are distinct but overlap geographically, and migration between the microhabitats is not uncommon. Their species identity is not fully supported by a recent molecular study. A laboratory experiment shows that Daphnia clones isolated from two populations of each species switch to sexual reproduction in response to totally different photoperiodic cues, so that their mating seasons are expected to differ. This phenomenon is most likely due to local adaptation of the two species to their respective distinct microhabitats. The phenomenon, together with the peculiarity of the study system, is interpreted as an ongoing allochronic speciation process in aquatic systems.