delaminates from abdominal and thoracic segmental domains of the neuroectoderm and gives rise to the aCC and pCC

The ventral nerve cord (vNC) of Drosophila derives from neural stem cells, the neuroblasts (NB) (Poulson, 1950), which delaminate from the ventral neurogenic region of the ectoderm (vNR) in a reproducible spatiotemporal segmental pattern (Hartenstein and Campos-Ortega, 1984; Hartenstein et al., 1987; Doe, 1992b). In the trunk, this pattern of segregating NB does not show any obvious differences between segments (Doe, 1992b). However, significant segmental-specificities in the vNC arise gradually during further development: in the course of embryogenesis segment-specific expression of several markers and genes arises (e.g. Jiménez and CamposOrtega, 1981; Gould et al., 1990; Bourgoin et al., 1992; Graba et al., 1992; Mellerick et al., 1992; Hoshino et al., 1993), and proliferation patterns in the thorax and abdomen diverge gradually (Prokop and Technau, 1991) and become even more distinct during postembryonic stages (Truman and Bate, 1988). Eventually, in the imago, the volume of the thoracic neuromeres clearly predominates over that of the abdominal ones (Hertweck, 1931). In the early embryo a cascade of patterning genes provides the blastoderm with positional cues along the anterior-posterior as well as the dorsal-ventral axis. These coordinates finally define polarized metameric units, the segments (for review see e.g. Pankratz and Jäckle, 1993). Specification of these segments according to their location in the anterior-posterior axis is mediated by the homeotic selector genes (for reviews see e.g. Lewis, 1978; Duncan, 1987; Peifer et al., 1987; Beachy, 1990; McGinnis and Krumlauf, 1992; Morata, 1993). For the vNC of Drosophila it has been demonstrated that mutations in the bithorax-complex alter segment-specific patterns of markers and gene expression (Jiménez and Campos-Ortega, 1981; Graba et al., 1992; Gould et al., 1990) and eventually interfere with neuromere-specific morphology (reviewed by Doe and Scott, 1988). However, the mechanisms of segment-specific regulation in the CNS are not yet understood. In order to address such mechanisms at the cellular level we made use of the previously described embryonic lineage of Drosophila neuroblast NB1-1 (Udolph et al., 1993). NB1-1 delaminates from abdominal and thoracic segmental domains of the neuroectoderm and gives rise to the aCC and pCC neurons in all of these segments (Doe et al., 1988a and b; Doe, 1992a). However, further components of the NB1-1 lineage express specific differences between thoracic segments (referred to as thoracic tagma) and abdominal ones (abdominal tagma) (Udolph et al., 1993). We show that tagma specificity of this lineage is determined at the early gastrula stage and is autonomously expressed by the neuroectodermal progenitors upon heterotopic transplantation. Activity of the homeotic genes Ubx or abd-A is autonomously required for the abdominal pathway of the NB1-1 lineage. By ectopic induction of Ubxor abd-A expression until several hours after gastrulation or by homeotic derepression in Polycomb mutants, thoracic determination of NB1-1 can be overridden. About the same time, during normal development, UBX and ABD-A are detectable in NB1-1. We suggest that tagma-specific commit2567 Development 120, 2567-2578 (1994) Printed in Great Britain © The Company of Biologists Limited 1994