ZEBRAFISH MUTATIONS AFFECTING RETINOTECTAL AXON PATHFINDING

We have isolated mutants in the zebrafish Danio rerio that have defect s 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 mu tations, representing 13 complementation groups and 6 single allele lo ci, were found that have defects in retinal ganglion cell axon pathfin ding to the tectum. These pathfinding genes fall into five classes, ba sed on the location of pathfinding errors between eye and tectum. In C lass I mutant larvae (belladonna, detour, you-too, iguana, umleitung, blowout) axons grow directly to the ipsilateral rectal lobe after leav ing the eye, Class II mutant larvae (chameleon, bashful) have ipsilate rally projecting axons and, in addition, pathfinding mistakes are seen within the eye. In Class III mutant larvae (esrom, tilsit, tofu) fewe r axons than normal cross the midline, but some axons do reach the con tralateral rectal lobe. Class IV mutant larvae (boxer, dackel, pinsche r) have defects in axon sorting after the midline and retinal axons oc casionally make further pathfinding errors upon reaching the contralat eral tectal lobe. Finally, Class V mutant larvae (bashful, grumpy, sle epy, cyclops, astray) have anterior-posterior axon trajectory defects at or after the midline. The analysis of these mutants supports severa l conclusions about the mechanisms of retinal axon pathfinding from ey e 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 se ems to play a central role in guiding retinal axons. Axons in nearby r egions of the brain seem to use different cues to cross the ventral mi dline. Mutant effects are not all-or-none, as misrouted axons may reac h their target, and if they do, they project normally on the tectum. T he retinotectal pathfinding mutants reveal important choice points enc ountered by neuronal growth cones as they navigate between eye and tec tum.