THE ENGRAILED AND HUCKEBEIN GENES ARE ESSENTIAL FOR DEVELOPMENT OF SEROTONIN NEURONS IN THE DROSOPHILA CNS

A number of the pattern formation genes expressed in the Drosophila ep idermis are also expressed in the developing CNS. It is uncertain how these genes contribute to neuronal differentiation. Here we show that the normal differentiation of the serotonin neurons of the Drosophila nerve cord is dependent on the expression of two pattern formation gen es, huckebein (hkb) and engrailed (en). hkb and en gene products coexp ress uniquely in the serotonin neurons and in neuroblast 7-3 (NB 7-3), indicating that as in the grasshopper, NE 7-3 is the progenitor for t hese neurons. Both genes are important for differentiation of this lin eage, since loss-of-function alleles for hkb or en eliminate expressio n of several markers that characterize these cells. hkb appears to act at a step subsequent to NE delamination, since NE 7-3 is detectable i n a strain containing a severe hkb allele.