Characterization of differentially expressed genes in purified Drosophila follicle cells: Toward a general strategy for cell type-specific developmental analysis

Axis formation in Drosophila depends on correct patterning of the follicula r epithelium and on signaling between the germ line and soma during oogenes is, We describe a method for identifying genes expressed in the follicle ce lls with potential roles in axis formation. Follicle cells are purified fro m whole ovaries by enzymatic digestion, filtration, and fluorescence-activa ted cell sorting (FACS). Two strategies are used to obtain complementary ce ll groups. In the first strategy, spatially restricted subpopulations are m arked for FACS selection using a green fluorescent protein (GFP) reporter. In the second, cells are purified from animals mutant for the epidermal gro wth factor receptor ligand gurken (grk) and from their wild-type siblings, cDNA from these samples of spatially restricted or genetically mutant folli cle cells is used in differential expression screens employing PCR-based di fferential display or hybridization to a cDNA microarray. Positives are con firmed by in situ hybridization to whole mounts. These methods are found to be capable of identifying both spatially restricted and grk-dependent tran scripts. Results from our pilot screens include (i) the identification of a homologue of the immunophilin FKBP-12 with dorsal anterior expression in e gg chambers, (ii) the discovery that the ecdysone-inducible nuclear hormone receptor gene E78 is regulated by grk during oogenesis and is required for proper dorsal appendage formation, and (iii) the identification of a Droso phila homologue of the human SET-binding factor gene SBF1 with elevated tra nscription in grk mutant egg chambers.