EGFR SIGNALING IS REQUIRED FOR THE DIFFERENTIATION AND MAINTENANCE OFNEURAL PROGENITORS ALONG THE DORSAL MIDLINE OF THE DROSOPHILA EMBRYONIC HEAD

EGFR signaling has been shown in recent years to be involved in the de termination, differentiation and maintenance of neural and epidermal c ells of the ventral midline (mesectoderm and ventromedial ectoderm), L ocalized activation of the TGF alpha homolog Spitz (Spi) in the mesect oderm is achieved by the products of the genes rhomboid and Star. Spi binds to its receptor, the Drosophila epidermal growth factor receptor homolog (Egfr), and triggers the Ras pathway which is needed for the survival and differentiation of ventral midline cells. The results rep orted here indicate that EGFR signaling is also required in a narrow m edial domain of the head ectoderm (called 'head midline' in the follow ing) that includes the anlagen of the medial brain, the visual system (optic lobe, larval eye) and the stomatogastric nervous system (SNS), We document that genes involved in EGFR signaling are expressed in the head midline, Loss of EGFR signaling results in an almost total absen ce of optic lobe and larval eye, as well as severe reduction of SNS an d medial brain. The cellular mechanism by which this phenotype arises is a failure of neurectodermal cells to differentiate combined with ap optotic cell death. Overactivity of EGFR signaling, as achieved by hea t-shock-driven activation of a wild-type rhomboid (rho) construct, or by loss of function of argos (aos) or yan, results in an hyperplasia a nd deformity of the head midline structures. We show that, beside thei r requirement for EGFR signaling, head and ventral midline structures share several morphogenetic and molecular properties.