RETINAL PATTERNING IN THE ZEBRAFISH MUTANT CYCLOPS

Determination of cell fate in the vertebrate retina has been shown to be largely independent of lineage. After cell fates are determined, re tinal neurons become organized in a precise laminar pattern. The mecha nisms for this patterning could involve morphogens distributed in grad ients or, alternatively, direct cell-cell interactions. In the zebrafi sh mutant cyclops (cyc(b16)), most embryos have two partial retinas jo ined in the ventral midline. This presents developing retinal cells ne ar the midline with abnormal cellular environments, whereas laterally the pattern of developing cells is normal. We examined the consequence s of this for patterning in the mutant's retina. We found that the ret inas are joined in the midline at the apical surfaces of the photorece ptor layers. A laminar pattern emerges in the midline that preserves n ormal positional relationships between retinal cell types locally but is abnormal with respect to patterning over the entire retina. Lateral to the midline, retinal patterning appears normal. Metabolic labeling experiments showed that late rounds of DNA synthesis precede the emer gence of the novel pattern in this midline region. We conclude that th ese observations in the cyclops mutant are compatible with mechanisms of pattern formation in the retina involving local cell interactions. (C) 1997 Wiley-Liss, Inc.