Genes affecting the midline Three groups of mutants elucidate the nature of the cell interactions that structure the CNS and the specific role of the midline. Group 1 mutants, consisting of single minded and slit , result in the complete absence of commissures and the collapse of the longitudinal tracts into a single, fused tract at the midline. Group 2 mutants, consisting of orthodenticle alone, results in a lack of the posterior commissure for each segment. In otd mutants, growth cones show no affinity for the midline and instead grow anteriorly and posteriorly on their own side. Group 3 mutants, consisting of spitz , star and rhomboid , lead to a fusion of the two commissures.
Subsequent analysis defines four steps involved in commissure development.
Next the segment polarity genes hedgehog, engrailed, patched and wingless are involved in the specification of midline cell number. It is possible that midline and ectodermal pattern formations occur at the same time. In addition to the segment polarity genes other signaling mechanisms appear important. Notch , for example, is required to specify the different midline lineages. The third step in commissure formation consists of the formation of commissures.