The CNS midline cells coordinate proper cell cycle progression and identity determination of the Drosophila ventral neuroectoderm

The CNS midline cells, specified by the single-minded (sim) gene, are requi red for the proper patterning of the ventral CNS and epidermis, which are d erived from the Drosophila ventral neuroectoderm. Defects in the sim mutant are characterized by the loss of the gene expression, which is required fo r the proper formation of the ventral neurons and epidermis, and by a decre ase in the spacing of longitudinal and commissural axon tracks. Molecular a nd cellular mechanisms for these defects were analyzed to elucidate the pre cise role of the CNS midline cells in proper patterning of the ventral neur oectoderm during embryonic neurogenesis. These analyses showed that the ven tral neuroectoderm in the sim mutant fails to carry out its proper formatio n and characteristic cell division cycle. This resulted in the loss of the dividing neuroectodermal cells that are located ventral to the CNS midline. The CNS midline cells are also required for the cell cycle-independent exp ression of the neural and epidermal markers. This indicates that the CNS mi dline cells are essential for the establishment and maintenance of the vent ral epidermal and neuronal cell lineage by cell-cell interaction. On the ot her hand, the CNS midline cells do not cause extensive cell death in the ve ntral neuroectoderm. This study indicates that the CNS midline cells play i mportant roles in the coordination of the proper cell cycle progression and the correct identity determination of the adjacent ventral neuroectoderm a long the dorsoventral axis, (C) 2000 Academic Press.