Fig. 1. DISCO and DFD are required for maxillary identity. (A) In otherwise normal embryos, ectopic DFD, which is driven by prd-Gal4 in this example, induced maxillary structures in the trunk segments. Cirri (ci) and, occasionally, sclerotized rod-like structures (see Fig. 2B) appear in the trunk cuticle. (B) Without disco and disco-r, no ectopic gnathal structures appeared in Df(1)XR14/Y embryos ectopically expressing Dfd. (C-F) Together, ectopic DFD and DISCO activate 1.28 transcription. Segment abbreviations in white letters indicate those with ectopic 1.28 RNA. (C) In wild-type embryos 1.28 transcripts accumulate along the posterior edge of the maxillary epidermis, as well as in the gut and anterior spiracles (out of the plane of focus) and in the posterior spiracles (not shown). (D) 1.28 transcript distribution was normal in prddisco. (E) In prdDfd embryos, in addition to normal transcript accumulation, 1.28 transcripts were detected in the labial segment, which has been transformed toward a maxillary identity, and weakly in the posterior-lateral edge of the expressing segments (not visible in figure). (F) In embryos co-expressing disco and Dfd, 1.28 transcripts significantly accumulate in the posterior portion of each affected trunk segment, in addition to the maxillary and labial segments. (G) disco expression in an early stage 12 wild-type Drosophila embryo. The regions of disco mRNA accumulation relevant to this study are the gnathal segments (mn, mandibular; mx, maxillary; lb, labial) which make up the visibly segmented region of the head, and the bilaterally symmetric spots along the ventrolateral region of each trunk segment (t2, second thoracic; a, abdominal segments). (H) disco mRNA distribution in an early stage 12 prddisco embryo. prd-Gal4 activates disco expression in cells forming the posterior portion of alternating segments, beginning at about stage 10 and continuing through early stage 13. In addition to the regions described in G, disco mRNA accumulates in stripes encompassing the posterior half of the mn, lb, t2, a1, a3, a5, a7 and a9/10 segments. (I) prddisco expression in segments a1-a3. Red circles represent nuclei expressing the EN protein. prddisco expression includes the five posterior-most cells (green cells) of every other segment. [Staging is according to Campos-Ortega and Hartenstein (Campos-Ortega and Hartenstein, 1997).]