Slit1a: A third Slit factor involved in axon guidance at the optic chiasm in the zebrafish

Retinal ganglion cell (RGC) axons have become a paradigm to understand axon growth and guidance in vivo, with many known signals known to be involved along the trajectory followed by these axons to reach their target at the optic tectum. Among these, Slit-Robo signaling appears as one of the main regulators of axon crossing at the optic chiasm. In the zebrafish, a species with no stereoscopic vision and complete crossing of all optic fibers, Robo2 expressed by RGCs was shown to be essential in regulating correct axon midline crossing and bundling to form the optic tracts. We recently reported that both Slit2 and Slit3, which are expressed in the area closely surrounding the optic chiasm, take part in this process, and that the double mutation generates a phenotype very similar to that of the Robo2 mutant (astray). Surprisingly, we have now found evidence that a third Slit factor expressed around the chiasm, Slit1a, would also be involved. CRISPR-Cas9 F0 mutations directed to the slit1a gene caused an optic chiasm phenotype very similar to that seen in slit2 or slit3 mutants at 48 hpf, namely, a significant proportion of cases in which one of the optic nerves appeared bifurcated, with the two branches joining again to form an anatomically intact optic tract, and the contralateral optic nerve crossing between those branches at the midline. Our evidence thus indicates a very particular situation in which robustness is reached not by two, but three, independent genes expressed at the same area and time.