Marco Lee, PhD Student Nelson and Moyes Lab Metabolic Phenotype of Daphnia under Hypoxia: Macroevolution, Microevolution, and Phenotypic Plasticity Hypoxia is a stressor that influences animal function from ecology to molecular biology. Animals cope with hypoxia through strategies that improve oxygen delivery and anaerobic energy metabolism. Daphnia is a freshwater crustacean that can upregulate hemoglobin (Hb) in response to hypoxia, imparting a red color. We combine multiple field surveys across season with a common garden experiment to evaluate changes in the metabolic phenotype of Daphnia in relation to environmental hypoxia. We observed seasonal changes in the metabolic phenotype that differed between red and pale animals. Hb was upregulated early in the season in Daphnia in hypoxic lakes, and a relationship between Hb and lactate dehydrogenase, a key enzyme in anaerobic metabolism, only emerged later in the season in a lake-specific temporal pattern. To evaluate whether these differences were due to specific lake environments or microevolutionary differences, we ran a common garden experiment using six isofemale lines from each of four lakes. We found a strong response to 18 h hypoxia exposure in the expression of both Hb and lactate dehydrogenase. Unexpectedly, other glycolytic enzymes, pyruvate kinase and enolase, were not upregulated under hypoxia. This contrasts observations in vertebrate models, as hypoxia often triggers a suite of responses controlled by the master transcriptional regulator hypoxia-inducible factor. Overall, the combination of lab and field studies suggest that the metabolic phenotype of the animal is dictated by both microevolutionary differences (within and between lakes) as well as the spatial and temporal environmental heterogeneity of the lakes. In addition, hypoxic responses in Daphnia may be regulated by serveral independent pathways, highlighting the importance of studying animal models with distinct ecology to understand the evolution of hypoxia-driven phenotypic remodeling.
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