Abstract
The trace biogenic amine tyramine is present in the nervous systems of animals ranging in complexity from nematodes to mammals. Tyramine is synthesized from tyrosine by the enzyme tyrosine decarboxylase (TDC), a member of the aromatic amino acid family, but this enzyme has not been identified in Drosophila or in higher animals. To further clarify the roles of tyramine and its metabolite octopamine, we have cloned two TDC genes from Drosophila melanogaster, dTdc1 and dTdc2. Although both gene products have TDC activity in vivo, dTdc1 is expressed nonneurally, whereas dTdc2 is expressed neurally. Flies with a mutation in dTdc2 lack neural tyramine and octopamine and are female sterile due to egg retention. Although other Drosophila mutants that lack octopamine retain eggs completely within the ovaries, dTdc2 mutants release eggs into the oviducts but are unable to deposit them. This specific sterility phenotype can be partially rescued by driving the expression of dTdc2 in a dTdc2-specific pattern, whereas driving the expression of dTdc1 in the same pattern results in a complete rescue. The disparity in rescue efficiencies between the ectopically expressed Tdc genes may reflect the differential activities of these gene products. The egg retention phenotype of the dTdc2 mutant and the phenotypes associated with ectopic dTdc expression contribute to a model in which octopamine and tyramine have distinct and separable neural activities.
Highlights
The trace biogenic amine tyramine is present in the nervous systems of animals ranging in complexity from nematodes to mammals
Two Candidate Tyrosine Decarboxylase Genes in the Drosophila Genome—Based on sequence similarity to plant tyrosine decarboxylase (TDC), we identified two potential TDC genes in the D. melanogaster genome, CG30445 and CG30446
Our results show that Drosophila contain two functional and highly related TDC genes, dTdc1 and dTdc2. dTdc1 is primarily expressed in nonneural abdominal organs, whereas dTdc2 is expressed in the CNS and innervates the female reproductive tract. dTdc2RO54, a point mutation in dTdc2, results in a loss of neural tyramine and octopamine and leads to female sterility
Summary
Tyramine, dopamine, and serotonin (Sigma) were injected at a concentration of 10 ng/ml, and a calibration curve was generated based on injections containing 1, 5, 10, 50, and 100 pg Both gene products have TDC activity in vivo, dTdc is expressed nonneurally, whereas dTdc is expressed neurally. Flies with a mutation in dTdc lack neural tyramine and octopamine and are female sterile due to egg retention; unlike Th mutants, the sterility of dTdc mutant females is not due to a defect in ovulation, since eggs are often found in the oviducts Transgenic rescue of this phenotype can be accomplished by expression of either dTdc or dTdc with a dTdc2-specific driver. The specific egg retention phenotype of the dTdc mutant and the phenotypes associated with ectopic dTdc expression contribute to a model in which neurally derived octopamine and tyramine have distinct functions
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