Enhancer detector analysis of the extent of genomic involvement in nervous system development in Drosophila melanogaster

We conducted a survey of the patterns of gene expression in the central nervous system (CNS) of larvae of the fruitfly Drosophila melanogaster to identify genes that may be important in the development of the CNS, aid in the recognition of basic organizing features that might underlie CNS development, and estimate the extent of the use of information encoded in the genome in the construction of the nervous system. A so-called enhancer detector strategy was used to generate many thousands of lines containing a beta-galactosidase reporter gene. These lines were screened as third-instar larvae for patterns of expression in the developing optic lobes and other portions of the CNS. Most of the lines recovered which evidence staining within the CNS could be included in one of a relatively small number of patterns. A random sample of 594 lines from the larger population screened was selected to quantify the relative frequencies of these patterns, and a more careful analysis of the changes in the patterns of expression with developmental time was done for representative lines of nine of the patterns. These studies demonstrated great variability in the pattern of gene expression as a function of developmental stage. Few, if any, lines showed beta-galactosidase activity limited to the optic lobes; similarly, few lines were identified in which staining was limited to only a small number of cells. Together with the limited number of patterns of gene expression seen, this suggests that in the larval CNS developmental pathways may be controlled by a combinatorial process of gene activity that involves the majority of the genome rather than by having a specific gene specify the fate of only a few neuronal precursors.