ARGOS AND SPITZ GROUP GENES FUNCTION TO REGULATE MIDLINE GLIAL-CELL NUMBER IN DROSOPHILA EMBRYOS

The midline glia of the Drosophila embryonic nerve cord undergo a redu ction in cell number after facilitating commissural tract morphogenesi s. The numbers of midline glia entering apoptosis at this stage can be increased by a loss or reduction of function in genes of the spitz gr oup or Drosophila EGF receptor (DER) pathway, Argos, a secreted molecu le with an atypical EGF motif, is postulated to function as a DER anta gonist, In this work, we assess the role of argos in the determination of midline glia cell number. Although all midline glia express DER, a rgos expression is restricted to the midline glia which do not enter a poptosis, Fewer midline glia enter apoptosis in embryos lacking argos function, Ectopic expression of argos is sufficient to remove al DER-e xpressing midline glia from the nerve cord, even those that already ex press argos, DER expression is not terminated in the midline glia afte r spitz group signaling triggers changes in gene expression, It is the refore likely that an attenuation of DER signaling by Argos is integra ted with the augmentation of DER signaling by Spitz throughout the per iod of reduction of midline glia number, We suggest that signaling by Spitz but not Argos is restricted to adhesive junctions, In this manne r, midline glia not forming signaling junctions remain sensitive to ju xtacrine Argos signaling, while an autocrine argos signal is excluded by the adhesive junction.