In both vertebrates and insects, neurons typically arise from neural stem cells or terminally dividing intermediate progenitors. Here, we describe another mode of neurogenesis where neural stem cells generate secondary precursors that undergo multiple rounds of self-renewing transit-amplifying divisions. We identify the Posterior Asense-Negative (PAN) neuroblasts, which do not express the transcription factors Asense or Prospero. PAN neuroblasts rely on the segregating determinants Numb and Brat to generate smaller, secondary neuroblasts that in turn give rise to ganglion mother cells (GMCs) and neurons throughout larval development. In brat or numb mutants, misspecified secondary neuroblasts are unable to produce differentiated progeny and initiate tumor-like overgrowth. In prospero mutants, however, tumors arise from GMCs while secondary neuroblasts are correctly specified. Our data describe a transit-amplifying lineage in the Drosophila nervous system and suggest that different vulnerabilities in intermediate cell types can affect the outcome of tumor suppressor loss in stem cell lineages.

How stem cells generate both differentiating and self-renewing daughter cells is unclear. Here, we show that Drosophila larval neuroblasts-stem cell-like precursors of the adult brain-regulate proliferation by segregating the growth inhibitor Brat and the transcription factor Prospero into only one daughter cell. Like Prospero, Brat binds and cosegregates with the adaptor protein Miranda. In larval neuroblasts, both Brat and Prospero are required to inhibit self-renewal in one of the two daughter cells. While Prospero regulates cell-cycle gene transcription, Brat acts as a posttranscriptional inhibitor of dMyc. In brat or prospero mutants, both daughter cells grow and behave like neuroblasts leading to the formation of larval brain tumors. Similar defects are seen in lethal giant larvae (lgl) mutants where Brat and Prospero are not asymmetric. We have identified a molecular mechanism that may control self-renewal and prevent tumor formation in other stem cells as well.