Figure 6.

Loss of Robo1 function closely mimics the RabGDI phenotype. In ovo RNAi with dsRNA derived from Robo1 (A), Robo2 (B), and Robo3 (C) was used to silence these genes and compare their effect on commissural axon guidance at the midline. In the absence of Robo1, commissural axons stalled in the floor plate (dashed lines) and failed to reach the contralateral floor-plate border, as seen after loss of RabGDI function (arrowheads in A, compare to Figure 1C and D). Silencing Robo2 resulted in a distinct phenotype (B). While axons still stalled in the floor plate following loss of Robo2 (arrowheads in B), many injection sites additionally showed axons with abnormal ipsilateral turns (arrow in B, and D). Similarly, loss of Robo3 function resulted in ipsilateral turns (arrows in C). In contrast to the phenotypes seen after downregulation of Robo1 and Robo2, postcommissural axons were strongly defasciculated in the absence of Robo3 and many failed to grow along the contralateral floor-plate border. Phenotype quantifications are shown in (D). Significance levels: n.s. not significant, * p<0.05, ** p<0.01, *** p<0.001. Bar: 50 μm.