We previously found Xenopus slit mRNA in the amacrine cells of the developing retina, at the appropriate stages to influence retinal ganglion cell (RGC) dendrite outgrowth and branching (Hocking et al., 2004 SFN abstract). Here, we examined the mRNA expression of the Slit receptors, robo1, robo2 and robo3 in the Xenopus retina by in situ hybridization. Xenopus robo1 (cDNA obtained from Dr. B. Key) showed no expression in the retina. Fragments of the Xenopus laevis robo2 gene were amplified by PCR using primers based on the robo2 sequence in the Xenopus tropicalis genome assembly. Expression of robo3 was examined using two EST sequences obtained from the IMAGE consortium. Starting at stage 32, robo2 is strongly, and robo3 weakly, expressed in RGCs. Since RGCs express Robo receptors, and an adjacent cell population expresses the ligand Slit, we hypothesized that Slit signaling enhances outgrowth of RGC dendritic arbors. To test this, we expressed a rat dominant negative Robo1 (dnRobo1) (Stein and Tessier-Lav...