Expression of Drosophila epidermal growth factor receptor homologue in mitotic cell populations

Expression of the Drosophila homologue (DER) of the human epidermal growth factor (EGF) receptor has been studied during development by RNA blot hybridizations and in situ hybridizations. One of the hypothetical functions of the protein encoded by this gene is mitotic signal transduction and, therefore, we have searched for evidence of its expression in mitotic cell populations. Increased DER transcript levels were detected in virtually all cells in cellular blastoderm embryos, indicative of the onset of transcription of the gene at this stage of development. These transcripts persist at least until the formation of the ventral furrow at the beginning of gastrulation. Expression of DER has been investigated in populations of nondividing cells, imaginal cells and nervous tissue in third instar larvae. By RNA blot hybridizations, we have shown that imaginal discs contain the bulk of the DER transcripts present in the whole third instar larvae. This result has been confirmed by in situ hybridizations. All imaginal discs, representing dividing cells, contain DER transcripts while salivary glands, representing nondividing cells, do not. Within the central nervous system, hybridization of the DER probe is confined to small clusters of cells in the brain cortex. During oogenesis, cell division programs are highly ordered temporally and spatially. Follicle cells express high levels of DER RNA during their mitotic phase of growth but lose these transcripts as they cease division. Nurse cells also express DER transcripts at lower levels even though they are not dividing. However, nurse cell transcripts represent stored maternal RNA species destined for use during preblastoderm embryonic development. These results indicate that the DER gene is expressed in mitotic cell populations during several stages of development and is not expressed in certain populations of nonmitotic cells.