Third instar larvae homozygous for sbb4 or transheterozygous for sbb03432/sbb4 display a significant increase in turning rate during crawling, as compared to controls.
In contrast to the wild-type, in mutant larvae, the lamina plexus is discontinuous. Thick bundles of axons are frequently observed between the lamina and the medulla, in striking contrast to the thin bundles in wild-type. In the most severe cases, the lamina plexus is largely missing and the medulla neuropil is hyperinnervated. In addition, disruptions in the normally smooth topographic map in then medulla are observed. However R-cell projections do not extend through the medulla into deeper layers of the optic lobe, suggesting that R8 neurons terminate, as in wild-type, within the medulla neuropil. R-cell fate and neuronal differentiation occurs normally in sbb4 mutants. Also photoreceptor arrangement remains normal, in almost all ommatidia.