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Stage-specific inductive signals in the Drosophila neuroectoderm control the temporal sequence of neuroblast specification

Institut für Genetik, Universität Mainz, Saarstraße 21, D-55122 Mainz, Germany

View larger version (46K): [in a new window]   Fig. 1. Spatial arrangement and temporal sequence (S1-S5) of segregating neuroblasts (Doe, 1992; Bossing et al., 1996). Each map represents the pattern of one hemisegment (thorax, abdomen) with those NBs highlighted that are added during the respective wave of segregation. Anterior is upwards; ventral midline is marked by broken line.

View larger version (46K): [in a new window]   Fig. 2. Transplantation experiments. Half cross sections through embryos at stage 7 (early gastrula) and stage 10 (elongated germ band). Dorsoventral extent of the neuroectoderm (NE) is marked in yellow. Owing to germ band extension, the respective anlagen appear twice in cross sections through abdominal segments at stage 10. Cells were transplanted isochronical isotopically (experiment I, stage 10 to stage 10), isochronical heterotopically (experiment III, stage 7 to stage 7), heterochronical heterotopically (experiment IV, stage 10 to stage 7) or heterochronical isotopically (experiment II, stage 10 to stage 7 or experiment V, stage 7 to stage 10). DE, dorsal ectoderm (green); MES, mesoderm, MG, midgut, ML, midline (gray for all three tissues); NE, neuroectoderm (yellow); S1 and S2 NBs, which are already delaminated at stage 10, are marked in orange.

View larger version (88K): [in a new window]   Fig. 3. Upon heterochronic transplantation from donors at stage 10 into the neuroectoderm of host embryos at stage 7 (see Fig. 2, experiment II) neuroectodermal cells can acquire an early NB fate. Horizontal views of HRP-labeled cell clones in the ventral nerve cord at stage 16/17 (A-C); anterior is towards the left; orientation of the CNS midline is marked by broken lines. Schematic drawings of wild-type lineages are shown for comparison (Bossing et al., 1996; Schmidt et al., 1997). (A) NB 5-6 thoracic clone consisting of a cluster of about six neurons with fibers projecting contralaterally through the anterior commissure (5-6Ica), and short ipsilateral projections (5-6 Iip; out of focus), three subperineural glia cells (V-, D-, L-SPG), and one cell body glia cell (CBG). (B) NB 3-5 clone consisting of about 22 interneurons with contralateral projection (3-5Ic) extending through the anterior commissure. Ipsilateral projections normally appear at later stages. (C) Cell clone derived from MP2 consisting of the two interneurons dMP2 and vMP2 with their typical ipsilateral projections posteriorly and anteriorly, respectively.

View larger version (73K): [in a new window]   Fig. 4. Neuroectodermal cells at stage 7 transplanted heterochronically into the neuroectoderm of host embryos at stage 10 (see Fig. 2, experiment V) can produce late NB lineages (B-D) or early NB lineages (A). Lateral (A, ventral to the top) and horizontal (B-D) views of HRP-labeled cell clones in the ventral nerve cord at stage 16/17; anterior is towards the left; orientation of the CNS midline is marked by broken lines. Schematic drawings of wild-type lineages are shown for comparison (Bossing et al., 1996; Schmidt et al., 1997). (A) NB 1-1 abdominal clone consisting of a cluster of about eight neurons with ipsilateral interneuronal projection (1-1I), the aCC with its ipsilateral motoneuronal projection and the pCC with its anteriorly extending interneuronal projection. Of the three subperneurial glia cells (LV-SPG, A-SPG and B-SPG) only LV-SPG is present. (B) NB 5-4 thoracic clone consisting of a cluster of about 6 neurons in the lateral cortex and a more medially located neuron. The typical ipsilateral motoneuronal projection (5-4M) exits through the segmental nerve. (C) NB 4-4 clone consisting of about 10 neurons in the ventrolateral cortex with interneuronal contralateral projections (4-4I) extending through the anterior commissure. (D) NB 7-3 clone consisting of four neurons. The motoneuron is lying posteriorly in the cluster of cells with its typical projection extending ipsilaterally (7-3M). The three interneurons form a fascicle (7-3I) that extends contralaterally across the posterior commissure.

View larger version (89K): [in a new window]   Fig. 5. Late NBs do not require a previous division in the neuroectoderm to be specified. (A1, B1) Lateral views of HRP-labeled cell clones in the ventral nerve cord at stage 16; anterior is towards the left, ventral on top. (A2,B2) Drawings show horizontal views of the same preparations (upon rotation). The clones derived from stage 7 neuroectodermal cells after heterochronical transplantation into the neuroectoderm of host embryos at stage 10. Both cells were determined to delaminate as NB 2-1 (S4), and produced about eight neurons with typical short ipsilateral fibers (2-1Ii) and a contralateral projection through the anterior commissure (2-1Ic). Normally, NB 2-1 is associated with an obligatory epidermal sister clone consisting of four cells, owing to a division of the neuroectodermal precursor (Bossing et al., 1996). The NB2-1 clone in A is associated with a sister clone of four epidermal cells (black arrow). The NB 2-1 clone in B has no epidermal sister clone.