CELL FATE SPECIFICATION BY EVEN-SKIPPED EXPRESSION IN THE DROSOPHILA NERVOUS-SYSTEM IS COUPLED TO CELL-CYCLE PROGRESSION

The correct specification of defined neurons in the Drosophila central nervous system is dependent on even-skipped. During CNS development, even-skipped expression starts in the ganglion mother cell resulting f rom the first asymmetric division of neuroblast NB 1-1. This first div ision of NB 1-1 (and of the other early neuroblasts as well) is tempor ally controlled by the transcriptional regulation of string expression , which we have manipulated experimentally. even-skipped expression st ill occurs if the first neuroblast division is delayed, but not if the division is prohibited. Moreover, even-skipped expression is also dep endent on progression through S phase which follows immediately after the first division. However, cytokinesis during the first NB division is not required for evert-skipped expression as revealed by observatio ns in pebble mutant embryos. Our results demonstrate therefore that ev en-skipped expression is coupled to cell cycle progression, presumably in order to prevent a premature activation of expression by a positiv e regulator which is produced already in the neuroblast during G(2) an d segregated asymmetrically into the ganglion mother cell during mitos is.