The segmentation gene, giant, is located in 3A1 within a cloned chromosome region surrounding the zeste locus. Rearrangement breakpoints associated with giant mutations were localized on the genomic clone map, and nearby transcription units were identified. One transcription unit is active during early embryogenesis and its transcripts are spatially localized from blastoderm into extended germband stages, consistent with expected expression patterns predicted by the 'gap' phenotype of giant mutants. Germ line transformation experinents using a 10-kb DNA fragment containing this transcription unit gave complete rescue of the abdominal giant defect but only partial correction of the head defect. The effect of mutations in three other gap loci, Kr, kni and hb, were also analyzed. Key words: gap gene/head segmentation/in situ hybridization/ germ line transformation
IntroductionThe segmental pattern of the larva of Drosophila melanogaster is under the control of several zygotically active genes. These genes can be divided into three classes, depending on the nature of the defects associated with mutations in a given gene (Niisslein-Volhard and Wieschaus, 1980 Baumgartner et al., 1987; wg, Baker, 1987) is expressed in 14 or more stripes during early gastrulation at intervals corresponding to every segment. In addition, analysis of larvae mosaic for one of various X-linked mutations (gt, runt, arm) revealed that these gene loci functioned primarily autonomously (although some local non-autonomy was observed; Gergen and Wieschaus, 1985), indicating that the gene function is required in those tissues fated to be affected by mutations in that locus. It was, therefore, expected that the giant gene would also be expressed only in those tissues that are affected by mutations of the giant locus.The giant locus is located in polytene band 3A1,2 and is included in a set of overlapping genomic clones isolated by Mariani et al. (1985) ventral portion of the cephalopharyngeal skeleton is extruded out of the anterior end of the larva. Larvae mutant for strong alleles (e.g. gtYA82, Figure lb and c) lack the labrum, epistomal sclerite, H-piece, hypostomal sclerite and dorsal bridge and retain in the pseudocephalon: cirri, ventral organ, antennal sense organ, maxillary sense organ (including the dorso-lateral and dorso-medial papillae), mouthhooks, the dorsal and ventral arms of the cephalopharyngeal skeleton, the ventral T-ribs and the hypopharyngeal organ. The lateralgraten are present but reduced and disorganized. Small, additional ectopic patches of naked cuticle, or cuticle covered with structures similar to dorsal hairs, can be found both dorsally and ventrally between the prothorax and the maxillary structures. Strong hypomorphic giant larvae also possess salivary glands, but these are attached to the exoskeleton at the anterior edge of the prothorax. Placement of the affected structures on the blastoderm fate map of Jurgens et al. (1986) reveals two regions of the anterior blastoderm fated to give rise to defective structu...
