Atrazine Stimulates Hemoglobin Accumulation in Daphnia magna: Is it Hormonal or Hypoxic?

Hemoglobin accumulation in daphnids is an important adap-tive response that is regulated by at least two distinct molecular pathways: an endocrine pathway stimulated by terpenoid hor-mones and an oxygen-sensing pathway involving the hypoxia-inducible factor. We found that the herbicide atrazine elevated hemoglobin levels in Daphnia magna and hypothesized that atrazine induced hemoglobin in daphnids through the hormonal regulatory pathway. This hypothesis was tested by modeling the combined effects of atrazine and the terpenoid hormone mimic pyriproxyfen on hemoglobin mRNA levels assuming the same mechanism of action (concentration addition model) and alterna-tively, assuming different mechanisms of action (response addi-tion model). Model predictions were then compared to experimental assessments of the combined action of these two chemicals on hemoglobin mRNA levels. Changes in hemoglobi