Segment-specific modulation of sequential transcription factor expression in neuroblasts. C. Cenci , P. Fichelson , B. Bello , A.P. Gould. MRC National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA UK.

   Individual neuroblasts sequentially express a series of transcription factors (Hunchback>Kruppel>Pdm>Castor) that impart temporal identity to their daughter cells (Brody and Odenwald, Dev. Biol. 2000, Isshiki, Pearson, Holbrook and Doe, Cell 2001). As the expression of these factors persists in neurons, to a first approximation, this results in a laminar organization of the final CNS. Late-born neurons expressing Castor are typically located in a superficial layer while those that are born earlier express Hunchback and reside in a much deeper layer. However, in each of the thoracic segments, we noticed an atypical Hunchback pattern, whereby clusters of approximately 12 neurons (termed THB neurons) express this factor in both deep and superficial regions. We have shown that this segment-specific THB pattern requires a positive input from homothorax (but not the thoracic Hox genes) and is redundantly suppressed in the abdomen by early activities of Ultrabithorax and abdominal A. To label all cells that share a lineage with the THB neurons, we made a GAL4 construct containing a neural hunchback enhancer (identified by J. Margolis and J. Posakony) and used this to drive a stable UAS-nlslacZ reporter. Using this embryonic lineage-labelling method, we show that the neuroblasts producing the THB lineages do appear to undergo a sub-lineage transition from Hunchback-positive to negative status but this is delayed relative to the majority of neuroblasts. In addition, evidence will be presented that, in these particular segment-specific lineages, there is gene skipping within the canonical Kruppel>Pdm>Castor sequence.