Differences in hemoglobin up-regulation in zooplankton are strongly correlated with differences in diel vertical migration patterns of zooplankton genotypes

Background/Question/Methods

Freshwater lake ecosystems exhibit strong temperature and environmental gradients across which many zooplankton species live and migrate. Daphnia sp. vary in their ability to up-regulate hemoglobin in response to low oxygen environments. It is unclear what the relationship is between the role that hemoglobin up-regulation plays in migration , and how it might mediate coexistence of Daphnia within lakes. Using a deep meso-oligotrophic lake in Ontario, we studied three distinct phenotypes of Daphnia pulicaria, that coexist and differ in their ability to up-regulate hemoglobin (classified as reds, pinks, and pales). A series of twelve hour surveys were conducted; sampling at one meter intervals to monitor changes in diel vertical migration patterns of the morphs. At each one meter interval D. pulicaria were color indexed, photographed, and preserved for genetic analysis using cellulose acetate electrophoresis. In addition, thirty individuals of each phenotypic morph were analyzed using a spectrophotometer to quantify hemoglobin content of individual D. pulicaria. Photographs were analyzed using the red green blue (RGB) color model, to help compare hemoglobin content between individuals collected in the twelve hour samplings and individuals used in the spectrophotometer analyses. Cellulose acetate electrophoresis was performed on individuals looking for polymorphism at five loci: malate dehydrogenase (me), lactate dehydrogenase (ldh), phosphoglucomutase (pgm), glucose-6-phostphate isomerase (gpi), and aldehyde oxidase (ao). 

Results/Conclusions

Daphnia pulicaria that were classified as red and pink individuals showed little to no change in their distribution over the water column through time, suggesting that these individual remained low in the water column with little vertical migration. In contrast, the individuals classified as pales showed strong changes in vertical distribution through time suggesting vertical migration.  Differences in color are strongly correlated with differences in hemoglobin content. Cellulose acetate electrophoresis reveals that these phenotypes are strongly correlated with multi-locus genotypes, suggesting that there are genetic differences controlling the phenotype. Since all phenotypes can up-regulate, this suggests genetic differences in vertical migration behavior. This underlying difference in genetics may allow for coexistence of the three phenotypes, adding to a growing body of work that explores intraspecific variation in maintaining species diversity.