As a model for understanding how neurotransmitter identity is specified, we are studying the genetic basis for dopaminergic (DA) cell fate in the male rays. In wild type males, dopamine is synthesized in the A-type neurons of rays 5, 7 and 9, as well as in 8 neurons common to both sexes. We have found that tyrosine hydroxylase, which is essential for dopamine biosynthesis, is encoded by the gene
cat-2 , and that
cat-2 ::gfp reporters are specifically expressed in all dopaminergic cells. We are using these reporters, together with staining for dopamine, to understand how DA fate is specified in the rays. The TGF b -family ligand DBL-1 plays a critical role in DA fate specification. In the absence of the signal, DA expression in rays 5, 7 and 9 is dramatically reduced, while, unexpectedly, rays 4 and 6 display low-frequency ectopic expression of DA fate. Conversely, heat shock-induced expression of DBL-1 causes the normally non-dopaminergic rays 3, 4, 6 and 8 to express DA with a frequency of 5 to 70%. Heat shock, however, does not induce expression in rays 1 and 2. Temperature shift experiments using a ts allele of
daf-4 , the type II DBL-1 receptor, indicate that the DBL-1 pathway functions in the ray neuroblast (Rn.a), its daughter, and its granddaughter, the A-type neuron itself. Taken together, these results suggest that the DBL-1 pathway acts to refine a prepattern that biases rays 5, 7 and 9 toward DA fate. In wild type, we propose that regulated expression of DBL-1 ensures that rays 5, 7 and 9 receive appropriate levels of ligand to enhance the prepatterned expression of DA fate; other competent rays are exposed to levels that act to suppress expression of this fate or are possibly inhibited by a secondary signal. The prepattern requires the Hox gene
egl-5 . EGL-5 is present in V rays 3 to 6 and is necessary for these rays to respond to the DBL-1 signal. However,
egl-5 alone is not sufficient to explain the prepattern, as it is not expressed in the T rays (rays 7, 8 and 9). Taken together, these studies suggest that in wild-type males, the robust pattern of DA fate results from the combined activities of the prepattern, established in part by
egl-5 , and the DBL-1 signal.