HEDGEHOG SIGNALING INDEPENDENT OF ENGRAILED AND WINGLESS REQUIRED FORPOST-S1 NEUROBLAST FORMATION IN DROSOPHILA CNS

The hedgehog gene product, secreted from engrailed-expressing neuroect oderm, is required for the formation of post-S1 neuroblasts in rows 2, 5 and 6. The hedgehog protein functions not only as a paracrine but a lso as an autocrine factor and its transient action on the neuroectode rm 1-2 hours (at 18 degrees C) prior to neuroblast delamination is nec essary and sufficient to form normal neuroblasts. In contrast to epide rmal development, hedgehog expression required for neuroblast formatio n is regulated by neither engrailed nor wingless, hedgehog and wingles s bestow composite positional cues on the neuroectodermal regions for S2-S4 neuroblasts at virtually the same time and, consequently, post-S 1 neuroblasts in different rows can acquire different positional value s along the anterior-posterior axis. The average number of proneural c ells for each of three eagle-positive S4-S5 neuroblasts was found to b e 5-9, the same for S1 NBs. As with wingless (Chu-LaGraff et al., Neur on 15, 1041-1051, 1995), huckebein expression in putative proneural re gions for certain post-S1 neuroblasts is under the control of hedgehog , hedgehog and wingless are involved in separate, parallel pathways an d loss of either is compensated for by the other in NB 7-3 formation, NBs 6-4 and 7-3, arising from the engrailed domain, were also found to be specified by the differential expression of two homeobox genes, go oseberry-distal and engrailed.