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

Comparative distributional studies of closely related taxa suggest that sexuals dominate in stable habitats, while asexuals are most abundant in disturbed environments. For freshwater zooplankton, temporary ponds represent unstable systems in which populations must be re-established from diapausing eggs, while lakes are relatively stable habitats permitting populations to persist all year round. This study involves the comparison of breeding systems in populations of the zooplankter Daphnia pulicaria from lake and pond habitats. In common with other members of the genus, some populations of this species have made the transition from cyclic parthenogenesis, in which sexual reproduction is a regular part of the life cycle, to obligate parthenogenesis. Prior work has shown that populations of D. pulicaria from ponds in central Canada 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. Obligately asexual lake populations were more frequent in the western than central states, a pattern which may be due to regional variation in the extent of interspecific gene flow from the closely related species D. pulex.