Daphnia pulex and Daphnia pulicaria are cyclically parthenogenetic crustaceans that are distributed widely in North American freshwaters. Hybrids of D. pulex and D. pulicaria are common in the wild, where population genetic analyses have indicated that they are invariably obligately asexual. In this study, we characterize three sexual aspects (sexual egg production, male production, and sexual offspring production) of life history in experimentally bred hybrids to determine the degree of reproductive isolation. In striking contrast to natural hybrids, out of the 53 F-1 hybrids we bred, none were obligately asexual. Neither male production nor sexual egg production was limited by hybridization. Our experimental crosses showed that there was little difficulty in generating or backcrossing the F-1 generation. In addition, we were able to intercross F(1)s successfully to produce F-2 progeny. We found no evidence that these taxa are reproductively incompatible, despite other reports of apparent adaptive genetic divergence. We conclude that reproductive isolation between D. pulex and D. pulicaria is due to ecological separation, not genetic incompatibility. In light of our results and published data on the biogeography and breeding system of these taxa, neither sterility due to hybridization nor direct inheritance of a sex-limited meiosis suppressor appears to provide a satisfactory explanation for the asexuality of wild hybrids. We propose an alternate hypothesis in which one parental taxon invades a population of the other, the resulting F-1 hybrids cross back to a carrier of a meiosis suppressor, and out of the resulting progeny, a high-fitness asexual clone eventually displaces the rest of the population.
