INTRODUCTION Most sensory inputs to the central nervous system are topo- graphically arranged (reviewed by Udin and Fawcett, 1988), that is, the spatial order of neurons is reflected by the order of their axon terminals in the target organ. Such order is well illustrated in the extensively studied retinotectal projection of lower vertebrates, where axons of dorsal retinal ganglion cells (RGCs) project to the ventral (lateral) tectum while ventral axons project to the dorsal (medial) tectum. Likewise, nasal RGC axons project to posterior (caudal) tectum while temporal axons project to the anterior (rostral) tectum (Fig. 1). Thus a doubly inverted map of the retina is created on the tectum. Several mechanisms have been proposed to account for the establishment of this highly ordered projection. Roger Sperry suggested the existence of graded cues in the tectum and in the retina that could serve as positional markers (Sperry, 1963). Other models propose that interactions between fibers give rise to the observed topography (e.g. Hope et al., 1976; Horder and Martin, 1978). Still other models assume that synaptic activity is used to order an initially random or crudely sorted projec- tion (Willshaw and von der Malsburg, 1976; Whitelaw and Cowan, 1981). However, these mechanisms on their own are unable to explain certain features of regeneration and regulation shown by the projection (Hope et al., 1976; Fraser, 1980; Fraser and Perkel, 1990; Whitelaw and Cowan, 1981; Gierer, 1983). If, for example, the temporal retina is ablated, the anterior tectum will remain devoid of retinal innervation. The remaining nasal RGC axons will pass the anterior tectum and still project correctly to the posterior tectum, demonstrating that they can distinguish between both (Sperry, 1963). Given sufficient time, however, the nasal arbors will expand to cover the entire tectum (Schmidt, 1978; Schmidt et al., 1978; Yoon, 1972). If the tectum, or a piece of it, is rotated prior to innervation by retinal axons, the resulting projection is also rotated, demonstrating the existence of stable positional cues (Yoon, 1973, 1975). If the posterior tectum is ablated prior to innervation by the retinal axons, the nasal axons will innervate the posterior part of the remaining anterior tectum 439 Development 123, 439-450 Printed in Great Britain © The Company of Biologists Limited 1996 DEV3373 Retinal ganglion cells connect to their target organ, the tectum, in a highly ordered fashion. We performed a large- scale screen for mutations affecting the retinotectal pro- jection 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 estab- lishment 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 unaf- fected in who-cares. Two mutations in two complementa- tion 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. Key words: retinotectal projection, topographic mapping, zebrafish, Danio rerio, visual system SUMMARY Mutations disrupting the ordering and topographic mapping of axons in the retinotectal projection of the zebrafish, Danio rerio Torsten Trowe 1,§ , Stefan Klostermann 1, *, Herwig Baier 1,† , Michael Granato 2 , Alexander D. Crawford 1 , Barbara Grunewald 1 , Heike Hoffmann 1 , Rolf O. Karlstrom 1 , Stefan U. Meyer 1 , Bernhard Müller 1 , Sandra Richter 1,‡ , Christiane Nüsslein-Volhard 2 and Friedrich Bonhoeffer 1 1 Abteilung Physikalische Biologie and 2 Abteilung Genetik, Max-Planck-Institut für Entwicklungsbiologie, Spemannstrasse 35, 72076 Tübingen, Germany *Present address: Abteilung TB-D, Boehringer Mannheim, Nonnenwald 2, D-82377 Penzberg, Germany † Present address: Department of Biology 0366, University of California, San Diego, La Jolla, CA 92093, USA ‡ Present address: Zoologisches Institut der Universität Basel, Rheinsprung 9, CH-4051 Basel, Switzerland § Author for correspondence (e-mail: trowe@mpib-tuebingen.mpg.de)