characteristics in common with vertebrate nervous systems, although it is much less complex and is more accessible to inspection, particularly during embryonic development. For example, the fruitfly has a well developed sensory system including relatively advanced visual, olfactory and gustatory systems. In addition, flies exhibit complex behavior including diurnal rhythmicity and intricate courtship behavior, and they also have the capacity to learn and memorize. A combination of these features has allowed the identification by genetic analysis of genes that play key roles in the development of the central nervous system (CNS)('-*) and the eripheral nervous system (PNS)(3), eye devel-~pment$-~), membrane excitabilityc6), learning and memory(7), sensory tranduction, circadian rhythm and courtship behavior@).
Despite considerable progress, probably only a small number of Drosophila genes required in the nervous system have yet been identified in mutagenesis screens. Indeed, many genes that play a significant role may be difficult to identify by genetic approaches. For example, a large number of genes may be essential for the normal development and function of the nervous system, but mutations in these genes may cause only subtle morphological or behavioral defects. Mutations in other genes may be lethal at a relatively early developmental stage, obscuring additional roles during later neural development. Furthermore, mutations in a number of genes may not cause a defect because other genes encode products with related and complementing functions. Recent results suggest that P-element-mediated enhancer detection can be combined with existing genetic approaches to identify and study these more elusive genes.
Summary
Enhancer detectors are DNA constructs which, when introduced into a eukaryotic genome, respond to nearby genomic transcriptional regulatory elements by means of a reporter gene, revealing the expression pattern of genes in their vicinity. Recent experiments in Drosophila suggest that enhancer detection is a powerful method to identify genes that are expressed in the nervous system. Since enhancer detectors allow a rapid molecular and genetic characterization of genes in their vicinity, the method will greatly facilitate the study of neural development and behavior.