Institut für Entwicklungsbiologie, Universität zu Köln, D-50923 Köln,
During the development of any complex central nervous system (CNS) the cells located at the CNS midline perform a variety of important functions. In Drosophila as well as in vertebrates, midline cells (or mesectodermal or floor plate cells) have been implicated in axon pattern formation, in particular in guiding commissural growth cones towards and across the ventral midline (1). Furthermore, vertebrate CNS midline cells are required for the correct differentation of flanking CNS neurons (2). In the present study we analysed whether the CNS midline performs similar inductive functions in Drosophila.
single-minded (sim) is specifically expressed in the CNS midline cells and acts as a master regulator of midline development (3). Embryos lacking sim subsequently lack the CNS midline. In addition, the cell number in the lateral CNS of these embryos is reduced by 20%. This is exemplified by several enhancer trap lines, which allow labelling of discrete subsets of neuronal cells. To assay whether the loss of lateral CNS cells depends on the absence of midline cells, we ectopically expressed sim. Ectopic expression of sim results in the formation of ectopic midline cells, which are in part sufficient to rescue the single-minded mutant phenotype in the lateral CNS.
(1) Seeger, M., Tear, G., Ferres-Marca, D. Goodman, C.S. Neuron 10, 409-426 (1993) (2) Yamada, T., Pfaff, S., Edlund, T. Jessell, T.M. Cell 73, 673-686 (1993) (3) Nambu, J.R., Lewis, J.O., Wharton, K.A. Crews, S.T. Cell 67, 1157-1167 (1991)