The olfactory system in Drosophila melanogaster provides a unique opportunity to explore the mechanisms by which neurons form connections during development. Olfactory receptor neurons (ORNs) that express the same odorant receptors connect to the same neurons in the antennal lobe, while neurons that express different odorant receptors connect to different neurons. The studies proposed here are directed towards understanding the molecular basis of this specificity. Genetic studies have revealed that different neurons use different constellations of cell surface proteins to control specificity including Dscam, N-cadherin, Lar receptor tyrosine phosphatase and the novel cadherin, Flamingo. Of particular interest is the requirement for Dscam. Dscam is alternatively spliced and comes in many different forms (approximately 38,000) that may contribute to recognition specificity. In this proposal, the role of different forms of Dscam in controlling the specificity of neuronal connections in the olfactory system will be tested. Three sets of experiments are proposed to critically assess the importance of alternative forms of Dscam in ORN targeting. First, a set of deletions has been isolated that remove many different forms of Dscam. The effect of these mutations on a set of three different ORNs will be assessed in these mutants. Additional mutants lacking other forms will be isolated and their affects on ORN targeting will be explored. Second, homologous recombination will be used to introduce a single extracellular domain containing form of Dscam into the endogenous Dscam locus. This will reduce diversity from approximately 38,000 to 2 forms. The phenotypes in three sets of olfactory neurons will be assessed. And finally, single forms of Dscam will be expressed under heterologous promoters in both wild type and mutant backgrounds. Dominant phenotypes induced by these forms will be assessed. Forms that do not induce dominant phenotypes will be assessed for the ability to rescue the complete loss of function Dscam mutations. A better understanding of how neuronal connections are formed during development will provide insight into the mechanisms that may be disrupted in various neurological and psychiatric diseases.