The building blocks of the nervous system and the neuronal circuits that process information are not genes, but cells. And so the genetic methods that were so powerful in elucidating embryonic development and other processes will not be adequate to probe the function of the nervous system. Instead, we will need to be able to assay and manipulate the function of individual cells with the same facility as we can now manipulate and assay the function of individual genes. The ability to label single cells, or small subsets of cells, is also a longstanding and critical tool in neuroanatomy.

A variety of genetically encoded probes have been developed that allow experimenters to visualize individual cells to study anatomy as well as to monitor and modulate their activity to study physiology and behavior. The utility of these probes is highly dependent on the precision with which their expression can be directed to small subsets of cells in reproducible, controllable, and convenient ways. The primary objective of our current work is to provide the tools required to accomplish such precise, controlled expression in the nervous system of Drosophila melanogaster.

Drosophila researchers have known for more than 20 years how to identify and, to some extent, manipulate the promoters and enhancers that control the temporal and spatial expression of individual genes. This work, and similar studies in other animals, has revealed that the complex spatial and temporal expression pattern of a gene usually results from the combined action of a set of individual enhancer elements that act, in a largely autonomous manner, to dictate aspects of a gene’s expression. The number of enhancers per gene varies widely, but is generally thought to be in the range of two to 10 in Drosophila.

As individual enhancers appear to represent the fundamental cis-acting modules through which gene expressions patterns are generated, our objective is to identify a large set of enhancers that each can confer on a reporter gene expression in distinct, small, reproducible subset of cells in the adult brain when placed in a stereotyped genomic configuration in a transgenic animal.

The feasibility of this approach depends on: (1) expression patterns being robust when enhancers are placed in a new environment that is held constant for all enhancers; (2) the expression pattern driven by a given enhancer being reproducible from animal to animal; (3) the expression patterns driven by individual enhancers containing an appropriate fraction of the cells in the brain to make them useful tools for neuroanatomy and behavioral genetics, and (4) our methods to identify suitable enhancers being efficient. I will describe the development of a strategy that meets all four of these criteria.