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Fig. 4. Nkx6 is necessary for axon growth. Three hemisegments of Stage 16 dissected (A) wild-type (B) Nkx6^D25/Nkx6^D25 (C) Nkx6^P[JG]/Nkx6^D25, and (D) UAS-Nkx6/+; Nkx6^Gal4/Nkx6^D25 embryos labeled with -Fas2 to mark motor axon projections. Schematics summarizing the observed phenotype are shown below each panel. (A) In wild type, ISNb (arrowheads) and ISNd innervate the ventral muscle field. (B) In Nkx6^D25 homozygous embryos, ISNb/d do not innervate their muscle targets. Arrowheads indicate ventral muscle field. (C) In Nkx6^P[JG]/Nkx6^D25 mutants the ventral muscle field is not innervated, though presumptive ISNb axons extend along ISN (arrowheads). (D) In Nkx6 rescue embryos ISNb axons usually contact their appropriate target muscles. (E) Penetrance of mutant phenotypes for different Nkx6 allelic combinations. See text for details. (F) Three hemisegments of wild-type and (G) UAS-GAPGFP/+; hb9^GAL4 Nkx6^D25/Nkx6^D25 mutants labeled with -GFP (green) to mark Hb9-positive axons and -MHC (red) to mark muscles. Hb9-positive axons extend along ISNb to contact ventral muscles in wild-type (F), but not mutant (G) embryos. (H) Three segments of wild-type and (I) UAS-GAPGFP/+; hb9^GAL4 Nkx6^D25/Nkx6^D25 mutant nerve cords labeled with -GFP to mark Hb9-positive axons. In wild type (H), numerous Hb9-positive longitudinal fascicles are visible, while in I very few Hb9-positive axons are present. Anterior is up in all panels.

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