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Fig. 1. Anterior dMP2 motor neurons degenerate in late embryogenesis. dMP2-GAL4/UAS-nls-myc-EGFP expression in the VNC. (A) At stage 16, a pair of dMP2 neurons is present in every segment. (B) At stage 17, most anterior dMP2s undergo apoptosis, as apparent from their pyknotic cell bodies (C,D, arrow) and fragmented axons (D, arrowhead, E). (F) In first instar larvae, only three pairs of dMP2s (A6 to A8) remain. These posterior dMP2s persist throughout larval stages (not shown). (G,H) dMP2 neurons express the transcription factor Odd-skipped (arrowheads). Expression of dMP2-GAL4 is not always robust in A8 dMP2. However, Odd staining confirms the presence of two dMP2 neurons in this segment (asterisk). The smaller, more dorsal MP1 neuron (arrows) also expresses Odd. (I) At late embryonic stages, dMP2s mature into peptidergic neurons that express the neuropeptide Proctolin. (J) dMP2 neurons project their axons posteriorly, fasciculating with their homologues in the next segment to form a discrete axonal tract that runs the length of the nerve cord (dMP2-GAL4/UAS-myc-EGFP^F). This tract exits in the posterior nerve (arrows) and innervates the hindgut (K, arrow). (L) Lipophilic dye (DiI) injections from the hindgut backfills dMP2 neurons in segments A6-8, but not in more anterior segments (n=11 VNCs). (M) dMP2 pioneer neurons are generated at embryonic stage 10 in segments S3 to A8. They extend their axons at stage 12, mature and then degenerate anteriorly at stage 17. Posterior dMP2 become peptidergic motor neurons that express Proctolin and pMad (Allan et al., 2003). dMP2-GAL4 expression in dMP2 neurons is not apparent until stage 14 (not shown) and is never detected in the vMP2 neuron, the dMP2 sibling. dMP2-GAL4 is, therefore, a postmitotic driver.

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