Fig. 1

Retinotectal projections in the wild type and in mutants with ipsilateral retinotectal projections. Dorsal view, anterior to the left. Fish were stained with DAPI (blue fluorescence) to outline the tectal neuropil, which is surrounded by DNAcontaining cell bodies (see methods). Neuropilar regions are often difficult to see in these photographs and one neuropil is outlined in B. Black spots are melanophores in the skin (yellow arrowhead in B) (A) Diagram of an injected 5-day-old zebrafish. The left eye of each fish was injected with DiI (red) in the temporal/ventral quadrant and with DiO (green) in the nasal/dorsal quadrant except in D, E and F, where the dyes were reversed. Labeled retinal ganglion cell axons cross the brain and grow to the contralateral tectal lobe where they project topographically. (B) Wild-type projections, dorsal focal plane. On the contralateral tectal lobe, retinal ganglion cell axons project topographically with temporal/ventral RGC neurons sending axons to the anterior/dorsal tectum (arrowhead) and nasal/dorsal RGC neurons sending axons to the posterior/ventral tectum (arrow). (C) Wild-type projections, ventral focal plane; termination zones are now out of focus. Retinal ganglion cell axons leave the eye at the papilla and grow along the base of the diencephalon to the midline, where they cross completely at the optic chiasm. After the chiasm, axons sort into a dorsal (arrowhead) and a ventral (arrow) brachium and grow dorsoposteriorly to the contralateral tectal lobe. (D,E) Retinal ganglion cells project to the ipsilateral tectal lobe in belladonna mutants. Reverse dye injections. (D)Ventral focal plane, showing that the axons turn dorsal immediately after leaving the eye (arrow). (E) Dorsal focal plane showing normal topographic projections on the ipsilateral tectal lobe. (F) RGC axons also project to the ipsilateral tectal lobe in you-too mutants. (G,H) In detour mutants, RGC axons either project to the ipsilateral and contralateral tectal lobes in the same fish (G), or to the ipsilateral tectal lobe only (H). (G) Dorsal focal plane showing normal mapping of nasodorsal RGC axons (arrows) and ventral/temporal axons (arrowheads) from one eye on both tectal lobes. (H) In some dtr mutant fish, RGC axons grow to the midline (arrow) before turning back to the ipsilateral tectal lobe. (I) In blw mutants, RGC axons grow across the midline (arrow), then return to the ipsilateral tectal lobe. OP, olfactory placode; N, nasal; T, temporal. A, anterior; P, posterior; D, dorsal; V, ventral. Scale bars, 100 mm.