Fig. 2. Lateral neuroblast lineages are absent in SoxNeuro mutants. Flat preparations of late stage 8 embryos stained with anti-Hunchback and anti-Engrailed. (A) Wild-type, Hunchback-expressing neuroblasts form an orthogonal array of four neuroblasts per hemisegment in each of the medial and lateral (large white arrowhead) columns, with a single cell in the intermediate column. (B) Lateral column neuroblasts are lost in between 20% and 70% of hemisegments in SoxNeuro^U6–35 mutant embryos (large white arrowhead), for example, NB3-5 (black arrowhead). Medial column neuroblasts are less frequently affected; NB1-1 (white arrow) and NB7-1 (white arrowhead) are present in over 88% of hemisegments. (C,D) Flat preparations of stage 16 embryos stained with anti-Even skipped. (C) Wild-type and (D) SoxNeuro^U6–35. The NB4-2 lineage RP2 motorneurones (arrows) and the NB3-3 lineage lateral cluster (arrowheads) are absent in SoxNeuro^U6–35; however, aCC/pCC and CQ cells are unaffected. (E) Stage 16 whole-mount SoxNeuro^U6–35 embryo stained with anti-Even skipped (black) and anti-Engrailed (brown). The spacing within the CNS is greatly increased between segments A3 and A4 and reduced between A2 and A3; notice that both Eve- and En-expressing neurones are no longer aligned with Engrailed expression in the epidermis (arrow). (F) Stage 16 whole-mount SoxN^U6–35 /SoxN^U6–35; KrGAL4/UASSoxN embryo. Within the domain of KrGAL4 expression, RP2 motorneurones (arrow) and ELC cells (arrowhead) are present in 67% and 33% of hemisegments, respectively.

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