Fig. 1. A third chromosome deletion uncovers an aberration on the TM3 balancer that is responsible for loss of the HRP epitope. (A) In balanced wild-type embryos (+/TM3-lacZ), anti-HRP antibodies stain the central nervous system (vnc, ventral nerve cord) and the peripheral nervous system (out of the plane of focus). Three non-neural tissues also express the HRP epitope: the garland gland (gg), anal pads (ap) and the posterior hindgut (phg). Blue staining at the anterior end of the embryo (towards the left in all panels) indicates the relative dose of the TM3-lacZ chromosome. (B) In TM3 homozygotes, the HRP epitope is absent from the nervous system but still detected in non-neural tissues. (C) Brd¹⁵ fails to complement the TM3 defect (Brd¹⁵/TM3-lacZ) and the epitope is still detected in non-neural tissues. (D-G) Whole-mount embryos were double stained: first for lacZ activity to confirm genotype; and then with mAb BP102 to provide landmarks for comparison of embryonic gross morphology. Dorsal views of stage 15 and 16 embryo pairs are shown for the genotypes (D) +/+, (E) TM3-lacZ/TM3-lacZ, (F) Brd¹⁵/TM3-lacZ and (G) Brd¹⁵/Brd¹⁵. Brd¹⁵ homozygotes display a characteristic defect in head involution (arrowhead in G). Scale bar: 350 µm in A-C; 500 µm in D-G.