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Fig. 6. Abd-B prevents grim-mediated apoptosis of MP1 pioneer neurons. (A) Odd expression in late embryos (air-filled trachea stage). In addition to the larger, more lateral dMP2 neurons in A6-A8, Odd is expressed in two MP1 neurons (smaller, more medial and more dorsal) up to A5. Odd is also expressed in an unidentified posterior cluster of cells in the posterior tip of the VNC. (G) At early stage 17 (when anterior dMP2 neurons have already died), rpr^GAL4 is occasionally observed in MP1 neurons (arrowhead). (B) In XR38/H99 mutant embryos (dMP2-GAL4, H99/XR38; UAS-nls-myc-EGFP) both anterior dMP2 and MP1 neurons survive, and four Odd-positive cells are typically observed per segment. Two of these cells are dMP2 neurons, as revealed by dMP2-GAL4 expression (H, arrows), and the other two are surviving MP1 neurons (H, arrowheads). (C) In X25/X25 mutant embryos, loss of grim results in the survival of most anterior MP1 neurons. However, no extra dMP2 neurons are rescued, and only A6-A8 dMP2s are present and express Odd. (D) In Abd-Bm mutants (Abd-B^M5/Abd-B^M2), both posterior dMP2 and MP1 neurons are lost, and only the posterior cluster of Odd-expressing cells remains. For each genotype in A to D, n>9 VNCs (see Fig. 7B for numbers). (E) Postmitotic rescue of anterior dMP2 and MP1 neurons (elav^GAL4/+; UAS-Abd-B/+). Both cell types are rescued in every anterior segment (100% rescue, n=9 VNCs). Hence, four Odd-positive cells are invariably observed per segment (I): two dMP2 neurons (arrow) and two MP1 neurons (arrowhead). (F) Expression of Abd-B in A5 to A7 dMP2 (arrows) and MP1 (arrowheads) pioneer neurons at stage 16. In segments A6 and A7, both dMP2 and MP1 neurons express Abd-B. However, only MP1 neurons express Abd-B in segment A5.

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