MUTATIONS DISRUPTING THE ORDERING AND TOPOGRAPHIC MAPPING OF AXONS INTHE RETINOTECTAL PROJECTION OF THE ZEBRAFISH, DANIO-RERIO

Retinal ganglion cells connect to their target organ, the tectum, in a highly ordered fashion, We performed a large-scale screen for mutatio ns affecting the retinotectal projection of the zebrafish, which resul ted in the identification of 114 mutations, 44 of these mutations dist urb either the order of RGC axons in the optic nerve and tract, the es tablishment of a topographic map on the tectum, or the formation of pr oper termination fields, Mutations in three genes, boxer, dackel and p inscher, disrupt the sorting of axons in the optic tract but do not af fect mapping on the tectum, In these mutants, axons from the dorsal re tina grow along both the ventral and the dorsal branch of the optic tr act, Mutations in two genes, nevermind and who-cares, affect the dorso ventral patterning of the projection, In embryos homozygous for either of these mutations, axons from dorsal retinal ganglion cells terminat e ventrally and dorsally in the tectum, In nevermind, the retinotopic order of axons along the optic nerve and tract is changed in a charact eristic way as well, while it appears to be unaffected in who-car-es, Two mutations in two complementation groups, gnarled and macho, affect the anteroposterior patterning of the projection, In these mutants, n asodorsal axons branch and terminate too soon in the anterior tectum, In 27 mutants belonging to six complementation groups, retinal axons d o not form normal termination fields, Some implications for models con cerning the formation of topographic projections are discussed.