Mutations disrupting the ordering and topographic mapping of axons in the retinotectal projection of the zebrafish, Danio rerio.

Retinal ganglion cells connect to their target organ, the rectum, in a highly ordered fashion. We performed a large-scale screen for mutations affecting the retinotectal projection of the zebrafish, which resulted in the identification of 114 mutations. 44 of these mutations disturb either the order of RGC axons in the optic nerve and tract, the establishment of a topographic map on the tectum, or the formation of proper termination fields. Mutations in three genes, boxer, dackel and pinscher, disrupt the sorting of axons in the optic tract but do not affect mapping on the tectum. In these mutants, axons from the dorsal retina grow along both the ventral and the dorsal branch of the optic tract. Mutations in two genes, nevermind and who-cares, affect the dorsoventral patterning of the projection. In embryos homozygous for either of these mutations, axons from dorsal retinal ganglion cells terminate ventrally and dorsally in the tectum. In nevermind, the retinotopic order of axons along the optic nerve and tract is changed in a characteristic way as well, while it appears to be unaffected in who-cares. Two mutations in two complementation groups, gnarled and macho, affect the anteroposterior patterning of the projection. In these mutants, nasodorsal axons branch and terminate too soon in the anterior tectum. In 27 mutants belonging to six complementation groups, retinal axons do not form normal termination fields. Some implications for models concerning the formation of topographic projections are discussed.