Trong robot, các sợi trục thần kinh SP1 có thể vượt qua đường giữa hơn một lần, trong khi các sợi trục thần kinh vMP2 và PCC qua đường giữa bất thường. (B) điều chế các cấp protein Robo điều khiển sợi trục đi qua. Kể từ khi các sợi trục thần kinh cùng bên thể hiện mức độ cao của Robo | Guidance of Axons and Dendrites Chapter 9 255 A C WT slit B B II s I comm robo robo comm FIGURE 10. Crossing the midline in the Drosophila CNS. Diagrams showing axon pathways in the ventral nerve cord of wild type WT and mutant Drosophila embryos. A The normal nerve cord is a ladder-like structure composed of longitudinal and commissural axon bundles. Each repeated segment has an anterior and a posterior commissure. B In commissureless comm mutants both commissures fail to form. In roundabout robo mutants the longitudinals are greatly reduced and the commissures are much thicker. robo comm double mutants have the same phenotype as robo. C In slit mutants all of the axons collapse onto the midline. axons which would be hidden amongst all the other axons but was rather a way to find defects in the overall pattern of the ventral nerve cord. This screen found some mutants in which the longitudinal fascicles were disrupted and others in which the commissural axons were disrupted. The longitudinal mutants included longitudinals gone logo which has been little studied and longitudinals lacking lola which proved to be a mutation in a transcription factor Giniger et al. 1994 and thus does not affect axon guidance directly. The commissural mutants included roundabout robo and commissureless comm which have proved to be key genes in the control of midline crossing. robo comm and slit As seen by BP 102 staining the robo and comm mutant phenotypes are opposites Fig. 10B . In robo the commissures are thickened and the longitudinals are somewhat reduced so that the normal ventral nerve cord ladder now resembles a chain of traffic roundabouts. In comm the commissures are completely absent. These phenotypes predict that normal comm gene function promotes midline crossing while robo function discourages crossing. Furthermore the double robo comm phenotype looks like robo showing that they act in the same pathway. In understanding axon guidance phenotypes in mutants it is critical to
