GENETIC DIVERSITY AND BREEDING SYSTEM VARIATION IN DAPHNIA-PULICARIA FROM NORTH-AMERICAN LAKES

Comparative distributional studies of closely related taxa suggest tha t sexuals dominate in stable habitats, while asexuals are most abundan t in disturbed environments. For freshwater zooplankton, temporary pon ds represent unstable systems in which populations must be re-establis hed from diapausing eggs, while lakes are relatively stable habitats p ermitting populations to persist all year round. This study involves t he comparison of breeding systems in populations of the zooplankter Da phnia pulicaria from lake and pond habitats. In common with other memb ers of the genus, some populations of this species have made the trans ition from cyclic parthenogenesis, in which sexual reproduction is a r egular part of the life cycle, to obligate parthenogenesis. Prior work has shown that populations of D. pulicaria from ponds in central Cana da largely reproduce by obligate parthenogenesis. However, this study shows that cyclic parthenogenesis is the dominant mode of reproduction in lake populations of D. pulicaria in two regions of the USA. Obliga tely asexual lake populations were more frequent in the western than c entral states, a pattern which may be due to regional variation in the extent of interspecific gene flow from the closely related species D. pulex.