Intraspecific Variation of Three Phenotypic Morphs of Daphnia pulicaria in the presence of a Strong Environmental Gradient
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Freshwater lake ecosystems often exhibit strong oxygen, and temperature gradients across which many zooplankton species live. Daphnia sp. vary in their ability to up-regulate hemoglobin in response to low oxygen environments. However; the role that hemoglobin up-regulation plays in diel vertical migration, and how it might mediate coexistence of Daphnia within lakes is still unclear. Using an oligotrophic lake in Ontario, I studied three distinct phenotypes of Daphnia pulicaria, which differed in the ability to up-regulate hemoglobin (classified as red, pink, and pale). Twenty-four hour surveys were conducted during the fall of 2012 and samples were drawn at 1m intervals to monitor changes in diel vertical migration. At each 1m interval Daphnia were color indexed, photographed, and preserved for genetic analysis using cellulose acetate electrophoresis. Red and pink Daphnia showed little change in distribution over the water column through time, suggesting individuals experienced little vertical migration. Pale individuals showed strong changes in vertical distribution through time suggesting vertical migration. The phenotypes are strongly correlated with multi-locus genotypes, suggesting genetic differences in migration behavior. Mesocosm experiments were used to manipulate migration over heterogeneous environments to test the hypothesis that vertical migration impacts genetic and phenotypic diversity in Daphnia pulicaria. The first mesocosm experiment contained two treatment groups; a migrating and non-migrating treatment containing the three phenotypes. The migrating treatment permitted unrestricted movement throughout the water column, and the non-migrating treatment restricted Daphnia to discrete 1m intervals. The second mesocosm experiment comprised two non-migrating treatments; red non-migrating and pale non-migrating. Results from the first set of mesocosm experiments indicate decreased genetic and phenotypic diversity in the migrating treatment. Shifts in hemoglobin up-regulation between pales and reds in the second mesocosm experiments suggest hemoglobin up-regulation is plastic, whereby pale, pink, and red individuals have the ability to up and down regulate hemoglobin. The differences in Daphnia migration patterns and the plastic response in hemoglobin up-regulation permits migrating genotypes to withstand low oxygen conditions. Overall implications of this study suggest that migration over a strong environmental gradient plays a key role in fostering phenotypic plasticity and genetic diversity in organisms living in heterogeneous environments.