THE EMBRYONIC CENTRAL-NERVOUS-SYSTEM LINEAGES OF DROSOPHILA-MELANOGASTER .1. NEUROBLAST LINEAGES DERIVED FROM THE VENTRAL HALF OF THE NEUROECTODERM

Central nervous system development in Drosophila starts with the delam ination from the neuroectoderm of about 30 neuroblasts (NBs) per hemis egment. Understanding the mechanisms leading to the specification of t he individual NBs and their progeny requires the identification of the ir lineages. Here we describe 17 embryonic NB lineages derived from th e ventral half of the neuroectoderm and rye assign these lineages to i dentified medial and intermediate NBs. The lineages are composed of in terneurons (NB 1-2, NB 2-1, MP2, NB 4-1, NB 5-1, NB 5-3, NB 6-1, NB 6- 2, and NB 7-2), interneurons and motoneurons (NB 3-1, NB 3-2, NB 4-2, NB 5-2, NB 7-1, and NB 7-3), or interneurons, motoneurons, and glial c ells (NB 1-1 and NB 2-2). NB 1-1, NB 2-2, and NB 3-1 form segment-spec ific lineages. Neuroectodermal progenitors forming NB 2-1, NB 5-1, and NB 7-3 divide while still in the ectoderm to give rise to an addition al epidermoblast. Expression of segmentation genes is not lineal in th e clones of NB 1-2 and NB 7-3 (engrailed), NB 1-1, NB 4-2, and NB 7-1 (even-skipped), and NB 7-1 (gooseberry-proximal). The timing of delami nation for individual NBs as well as the number of their progeny is no t strictly invariant. The 17 NBs produce about 200 neurons and only th ree glial cells, corresponding to about 70% of the estimated total num ber of neurons and 10% of the glial cells per thoracic and abdominal h emisegment. Previously identified neural cell types were linked to par ticular lineages and we introduce a systematic terminology for the ven tral nerve cord neurons. The wild-type clones provide a foundation for the analysis of mutants, expression patterns, and experimental manipu lations. (C) 1996 Academic Press, Inc.