Quantitative analysis of gene function in the Drosophila embryo

The specific functions of gene products frequently depend on the developmen tal context in which they are expressed. Thus, studies on gene function wil l benefit from systems that allow for- manipulation of gene expression with in model systems where the development context is well defined. Here we des cribe a system that allows for genetically controlled overexpression of any gene of interest under normal physiological conditions in the early Drosop hila embryo. This regulated expression is achieved through the use of Droso phila lines that express a maternal mRNA for the yeast transcription factor GAL4. Embryos derived from females that express GAL4 maternally activate G AL4-dependent UAS transgenes at uniform levels throughout the embryo during the blastoderm stage of embryogenesis. The expression levels can be quanti tatively manipulated through the use of lines that have different levels of maternal GAL4 activity. Specific phenotypes are produced by expression of a number of different developmental regulators with this system, including genes that normally do not function during Drosophila embryogenesis. Analys is of the response to over-expression of runt provides evidence that this p air-mle segmentation gene has a direct role in repressing transcription of the segment-polarity gene engrailed. The maternal GAL4 system will have app lications both for the measurement of gene activity in reverse genetic expe riments as well as for the identification of genetic factors that have quan titative effects on gene function in vivo.