Reduction of histamine and enhanced spinning behavior of Daphnia magna caused by scarlet mutant

The ABC transporter, Scarlet, and its binding partner, White are involved in pigment synthesis in the insect eye and mutations in these genes are used as genetic markers. Recent studies have suggested that these transporters also have additional functions in the neuronal system. In our previous study, we generated scarlet mutant in the small crustacean, Daphnia magna and showed that the mutant lacked the eye pigment in the mutant. Here, we show that the scarlet mutant exhibits spinning behavior. This phenotype is partly associated with the presence of light. Metabolomic analysis of a juvenile mutant revealed that the scarlet mutant has approximately one-tenth of the histamine content of the wild type. Application of histamine to the scarlet mutant rescued the spinning behavior in juveniles, suggesting that the spinning behavior of the mutant is caused by the reduction of histamine. However, the altered behavior was not rescued in the adult mutant by the addition of histamine, suggesting that Scarlet plays an irreversible role in the development of histaminergic neurons. These results suggest that Scarlet plays an important role in histaminergic signaling, which might be related to control the spinning behavior, in addition to its role in eye pigmentation.