The optic lobes of the adult brain originate from a population of cells derived from invaginating ectoderm that attach to the surface of the developing brain hemispheres during stages 12 and 13. The outer anlagen gives rise to the lamina and medulla, the target neuropiles for rhabdomere axons. The inner optic anlagen generates the remaining neuropiles, lobula, and lobula plates. Argos plays a role in photoreceptor determination, and helps regulate optic lobe development. Expression of repo in glial cells is impaired in argos mutants.
Glial development is likely to be disturbed in argos mutants prior to the retinal innervation.
Thus the misrouting of R-axons is likely to be due to a failure in glial development. What is most important here is the interaction between argos and two other genes, Star and rhomboid . The signal sequence is potentially cleaved during protein secretion.