Coexisting overwintering strategies in Daphnia pulex: A test of genetic differences and growth responses

We tested whether clones of Daphnia pulex in a mesotrophic lake differ with respect to overwintering strategies: either surviving the winter as parthenogenetic females with reduced metabolism in pelagic waters, or as sexually produced dormant embryos in ephippia. During winter 2007, we established two groups of clonal laboratory lines from females collected in the lake and neonates hatched from ephippia. Genetic relatedness and differentiation of the two clonal groups were studied with microsatellite markers, and compared to the genetic structure of a field reference population. Flow-through experiments designed to measure the specific juvenile growth rate (g(j)) of individual clones (a good proxy of fitness) at low and at high food concentrations show that differences in g(j) indicate physiological adaptation of the different starting populations to improving food conditions in spring. Although g(j) differed significantly among individual clones, both below and above the incipient limiting food concentration, we found no significant differences between the clonal groups. There was considerable clonal variation for reaction norms of g(j), but neither the group means of the slopes of reaction norms nor of the threshold food concentrations for growth differed significantly. The lack of differences in group means so far does not support the hypothesis that clones are specialized for either parthenogenetic or for dormant overwintering. It rather suggests a mixed strategy of individual females producing resting stages as insurance and then trying to survive the winter in an active state.