HOMEOTIC REGULATION OF SEGMENT-SPECIFIC DIFFERENCES IN NEUROBLAST NUMBERS AND PROLIFERATION IN THE DROSOPHILA CENTRAL-NERVOUS-SYSTEM

The number and pattern of neuroblasts that initially segregate from th e neuroectoderm in the early Drosophila embryo is identical in thoraci c and abdominal segments. However, during late embryogenesis differenc es in the numbers of neuroblasts and in the extent of neuroblast proli feration arise between these regions. We show that the homeotic genes Ultrabithorax and abdominal-A regulate these late differences, and tha t misexpression of either gene in thoracic neuroblasts after segregati on is sufficient to induce abdominal behaviour. However, in wild type embryos we only detect abdominal-A and Ultrabithorax proteins in early neuroblasts. Furthermore, transplantation experiments reveal that seg ment-specific behaviour is determined prior to neuroblast segregation Thus, the segment-specific differences in neuroblast behaviour seem to be determined in the early embryo, mediated through the expression of homeotic genes in early neuroblasts, and executed in later programmes controlling neuroblast numbers and proliferation. (C) 1998 Elsevier S cience Ireland Ltd. All rights reserved.