The role of the cell cycle and cytokinesis in regulating neuroblast sublineage gene expression in the Drosophila CNS.

The precise temporal control of gene expression is critical for specifying neuronal identity in the Drosophila central nervous system (CNS). A particularly interesting class of genes are those expressed at stereotyped times during the cell lineage of identified neural precursors (neuroblasts): these are termed 'sublineage' genes. Although sublineage gene function is vital for CNS development, the temporal regulation of this class of genes has not been studied. Here we show that four genes (ming, even-skipped, unplugged and achaete) are expressed in specific neuroblast sublineages. We show that these neuroblasts can be identified in embryos lacking both neuroblast cytokinesis and cell cycle progression (string mutants) and in embryos lacking only neuroblast cytokinesis (pebble mutants). We find that the unplugged and achaete genes are expressed normally in string and pebble mutant embryos, indicating that temporal control is independent of neuroblast cytokinesis or counting cell cycles. In contrast, neuroblasts require cytokinesis to activate sublineage ming expression, while a single, identified neuroblast requires cell cycle progression to activate even-skipped expression. These results suggest that neuroblasts have an intrinsic gene regulatory hierarchy controlling unplugged and achaete expression, but that cell cycle- or cytokinesis-dependent mechanisms are required for ming and eve CNS expression.