The embryonic central nervous system lineages of Drosophila melanogaster. I. Neuroblast lineages derived from the ventral half of the neuroectoderm

Central nervous system development in Drosophila starts with the delamination from the neuroectoderm of about 30 neuroblasts (NBs) per hemisegment. Understanding the mechanisms leading to the specification of the individual NBs and their progeny requires the identification of their lineages. Here we describe 17 embryonic NB lineages derived from the ventral half of the neuroectoderm and we assign these lineages to identified medial and intermediate NBs. The lineages are composed of interneurons (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 cells (NB 1-1 and NB 2-2). NB 1-1, NB 2-2, and NB 3-1 form segment-specific 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 additional epidermoblast. Expression of segmentation genes is not lineal in the 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 delamination for individual NBs as well as the number of their progeny is not strictly invariant. The 17 NBs produce about 200 neurons and only three glial cells, corresponding to about 70% of the estimated total number of neurons and 10% of the glial cells per thoracic and abdominal hemisegment. Previously identified neural cell types were linked to particular lineages and we introduce a systematic terminology for the ventral nerve cord neurons. The wild-type clones provide a foundation for the analysis of mutants, expression patterns, and experimental manipulations.