It might also depend on the function of single minded , the master regulatory gene of midline development. The single minded transcript accumulates in the midline glia and, depending on the context, can act either as a transcriptional activator or repressor. By the end of embryogenesis the final number of midline glial cells is about 3 to 4. This model suggests that activation of ras1 signaling in the entire midline glial equivalence group promotes survival of all cells in this cluster. Both Rhomboid and Argos control activation of the EGF-R during midline glia development.
Ectopic rhomboid leads to extra midline glial cells.
In mutants that lack commissural axons, the midline glial cells die. One can bypass the requirement of axonal contact for midline glia survival by the expression of activated Drosophila jun . Expression of activated Drosophila jun results in missing commissural axon tracts. In summary it appears that Egf-r signaling is required during at least two steps of midline development. The involvement of Htl in mesodermal founder cell fate specification was tested by reducing its activity under conditions where earlier cell migration is not compromised.