Stage-specific inductive signals in the Drosophila neuroectoderm control the temporal sequence of neuroblast specification

One of the initial steps of neurogenesis in the Drosophila embryo is the de lamination of a stereotype set of neural progenitor cells (neuroblasts) fro m the neuroectoderm. The time window of neuroblast segregation has been div ided into five successive waves (S1-S5) in which subsets of neuroblasts wit h specific identities are formed. To test when identity specification of th e various neuroblasts takes place and whether extrinsic signals are involve d, we have performed heterochronic transplantation experiments. Single neur oectodermal cells from stage 10 donor embryos (after S2) were transplanted into the neuroectoderm of host embryos at stage 7 (before S1) and vice vers a. The fate of these cells was uncovered by their lineages at stage 16/17. Transplanted cells adjusted their fate to the new temporal situation. Late neuroectodermal cells were able to take over the fate of early (S1/S2) neur oblasts. The early neuroectodermal cells preferentially generated late (S4/ S5) neuroblasts, despite their reduced time of exposure to the neuroectoder m. Furthermore, neuroblast fates are independent from divisions of neuroect odermal progenitor cells. We conclude from these experiments that neuroblas t specification occurs sequentially under the control of non-cell-autonomou s and stage-specific inductive signals that act in the neuroectoderm.