Characterization of enhancer fragments in .

Receptor proteins of the Roundabout (Robo) family regulate axon guidance decisions during nervous system development. Among the three family genes ( and displays a dynamic expression pattern and regulates multiple axon guidance outcomes, including preventing midline crossing in some axons, promoting midline crossing in others, forming lateral longitudinal axon pathways, and regulating motor axon guidance. The identity and location of enhancer elements regulating complex and dynamic expression pattern in different neural cell types are unknown. Here, we characterize a set of 17 transgenic lines expressing GAL4 under the control of DNA sequences derived from noncoding regions in and around , to identify enhancers controlling specific aspects of expression in the embryonic ventral nerve cord. We identify individual fragments that confer expression in specific cell types where is known to function, including early pioneer neurons, midline glia and lateral longitudinal neurons. Our results indicate that s dynamic expression pattern is specified by a combination of enhancer elements that are active in different subsets of cells. We show that expression in lateral longitudinal axons represents two genetically separable subsets of neurons, and compare their axon projections with each other and with Fasciclin II (FasII), a commonly used marker of longitudinal axon pathways. In addition, we provide a general description of each fragment's expression in embryonic tissues outside of the nervous system, to serve as a resource for other researchers interested in expression and its functional roles outside the central nervous system.