The goal of the research proposed in this application is to elucidate the genetic and molecular mechanisms by which nearly identical cells express distinct cell fates. This research has broad relevance in developmental biology, especially neural development where expression of distinct neural identities is crucial in establishing proper synaptic connections. These studies will be conducted in the soil nematode, Caenorhabditis elegans, which is amenable to genetic and molecular analyses. The model to be studied is the development of nine bilateral pairs of peripheral sense organs in the male tail of C. elegans that are know as the sensory rays. The sensory rays are similar in morphology and cellular composition, but have been shown to express distinct identities. Mutations in several genes that appear to affect these identities have been isolated and characterized. The specific aims of this proposal are to: 1) more thoroughly characterize sensory ray identities, focusing primarily on neurotransmitter usage; 2) characterize the cellular interactions required to specify distinct ray identities by laser ablation studies; 3) further characterize mutations affecting ray identity specification (including genetic mapping and mosaic analyses); 4) isolate and characterized recombinant DNA clones of the genes identified by ray identity specification mutations (characterizations will include DNA sequence analyses and northern analyses to determine temporal patterns of gene expression); and, 5) dissect the functions of these genes in vivo by introducing altered copies of them into wild-type and mutant strains of C. elegans.