Zebrafish mutations affecting retinotectal axon pathfinding

We have isolated mutants in the zebrafish Danio rerio that have defects in axonal connectivity between the retina and tectum. 5-day-old fish larvae were screened by labeling retinal ganglion cells with DiI and DiO and observing their axonal projections to and on the tectum. 82 mutations, representing 13 complementation groups and 6 single allele loci, were found that have defects in retinal ganglion cell axon pathfinding to the tectum. These pathfinding genes fall into five classes, based on the location of pathfinding errors between eye and tectum. In Class I mutant larvae (belladonna, detour, you-too, iguana, umleitung, blowout) axons grow directly to the ipsilateral tectal lobe after leaving the eye. Class II mutant larvae (chameleon, bashful) have ipsilaterally projecting axons and, in addition, pathfinding mistakes are seen within the eye. In Class III mutant larvae (esrom, tilsit, tofu) fewer axons than normal cross the midline, but some axons do reach the contralateral tectal lobe. Class IV mutant larvae (boxer, dackel, pinscher) have defects in axon sorting after the midline and retinal axons occasionally make further pathfinding errors upon reaching the contralateral tectal lobe. Finally, Class V mutant larvae (bashful, grumpy, sleepy, cyclops, astray) have anterior-posterior axon trajectory defects at or after the midline. The analysis of these mutants supports several conclusions about the mechanisms of retinal axon pathfinding from eye to tectum. A series of sequential cues seems to guide retinal axons to the contralateral tectal lobe. Pre-existing axon tracts seem not to be necessary to guide axons across the midline. The midline itself seems to play a central role in guiding retinal axons. Axons in nearby regions of the brain seem to use different cues to cross the ventral midline. Mutant effects are not all-or-none, as misrouted axons may reach their target, and if they do, they project normally on the tectum. The retinotectal pathfinding mutants reveal important choice points encountered by neuronal growth cones as they navigate between eye and tectum.