A rapid and efficient approach to vital enhancer trap screening in Drosophila embryos. S.S. Gisselbrecht ¹, J. Bayes ¹, J. Etchin ¹, B. Dell'Orfano ², A. Ferrante ², A.M. Michelson ¹. 1) Dept. of Medicine, Howard Hughes Medical Institute, Brigham and Women's Hospital and Harvard Medical School, Boston, MA; 2) Union Biometrica, Inc., Somerville, MA.

   Enhancer trap lines offer a wide variety of tissue- and temporal-specific markers for developmental studies. A major drawback of prior enhancer trap screens employing LacZ or Gal4 P element reporters is the necessity of generating and analyzing large numbers of uninformative stable insertion lines due to the relatively low proportion of novel insertions that yield detectable reporter expression. We now describe a modified enhancer trap screen employing yellow fluorescent protein (YFP) as a reporter combined with high throughput mechanical sorting of fluorescent embryos. Since the sorted embryos are largely viable, stable stocks can be established from single founder embryos. Thus, only insertions that yield an expression pattern need be isolated and followed up. This approach facilitates the direct selection of informative insertions from the progeny of the dysgenic cross, thereby enabling the rapid generation of large numbers of expression lines with very little effort. We also have used inverse PCR to rapidly map each of the P element insertions obtained to a specific genomic site. In this way, we have uncovered several previously unidentified cell types and at least one lethal insertion in a previously annotated but uncharacterized gene. The availability of YFP markers for particular subsets of living cells will facilitate real-time analysis of various morphogenetic processes in wild type and genetically manipulated backgrounds, as well as the isolation of unique cell populations for expression profiling studies.