Gene Dmel\Dfd

Null mutations act as recessive lethals. Homozygous or hemizygous animals die at the end of embryogenesis and show a spectrum of defects in the head. There are no discernible defects in the trunk. The head defects are associated with missing structures normally derived from the mandibular and maxillary segments, the dorsal lateral papillae of the maxillary sense organ, the mouth hooks, and the maxillary cirri. The remaining gnathal structures are present albeit disarranged likely due to abnormalities in the movements associated with head involution. A weak homeotic transformation (30-50% penetrance) has also been noted in animals hemizygous for a breakpoint-associated revertant of the single dominant gain-of-function allele (Dfdrv1). The phenotype is an apparent transformation of the H piece and lateral-graten which appear to be replaced by cephalopharyngeal plates. This phenotype has not been observed in any other mutant genotype and the reason for its low-penetrance production by this particular allele is not known. X-ray-induced somatic clones of Dfd- cells have shown that the locus is also required for adult head development. These cells develop normally in the thorax and abdomen but do not form structures in the ventral anterior aspect of the head; specifically the vibrissae and maxillary palps. Clones in the dorsal posterior part of the head form ectopic bristles which have been interpreted as a head to thoracic transformation. A temperature-conditional allele has been used to define two temperature-critical periods for Dfd+ activity. The first is during embryogenesis during segmentation and head involution, while the second occurs in the late third instar larval through mid pupal stages. These times correlate nicely with the observed cuticular defects in mutant animals and the times of peak gene product accumulation. There is a single dominant gain-of-function allele which causes defects in the ventral aspects of the adult head similar to those seen in the Dfd- head clones mentioned above. There are no defects seen in the posterior of the head nor does this allele cause any embryonic or larval defects as a heterozygote, homozygote, or hemizygote. This allele is associated with a group of B104 (roo) insertion elements (ca. 50 kb of inserted DNA) as well as a duplication of the 3 exons of the Dfd transcription unit (see below). The mutant causes an extended spatial domain of expression of the locus into the eye portion of the eye-antennal disc as compared to the pattern seen in normal animals. The precise cause-effect relationship between the observed molecular defect and the mutant phenotype is not known except that partial deletion of the B104 elements but not the 3 end duplication causes a reversion of the dominant phenotype and has no apparent effect on the wild type function of the resident Dfd gene. This dominant allele has been reverted and these revertants act as a simple recessive loss-of-function alleles with the one exception noted above. The Dfd transcript is initially detected at the blastoderm stage in a band of cells at the position of the future cephalic furrow. This RNA shows maximal accumulation from 6-12 hours of embryogenesis when it is found in the mandibular and maxillary lobes, as well as in the subesophageal reigon of the CNS. The amount of Dfd RNA diminishes through the first and second larval instars and peaks again during the third instar. At this point, it is found in the peripodial membrane cells of the eye-antennal discs. The cells which accumulate the RNA are those which have been fate mapped to give rise to the adult-head-capsule structures which are defective in Dfd- clones. Antibodies raised to Dfd protein have shown a similar pattern of accumulation to that seen for the RNA. The protein is first detected in cellular blastoderm stage in a stripe of six cells which circumscribes the embryo. As germ-band elongation proceeds and segmentation becomes evident Dfd protein is detected in the mandibular and maxillary lobes and a portion of the dorsal ridge. During germ-band shortening protein is no longer detectable in the mandibular lobe or in the anterior lateral aspect of the maxillary lobe. The process of head involution carries the Dfd-expressing cells interiorly where they are found in portions of the pharynx at the end of embryogenesis. Dfd-positive cells are also found in the subesophageal region of the CNS in the maxillary ganglion. This expression pattern has been shown to be dependent on the prior expression of the gap and pair-rule segmentation genes for its inception and on an autogenous regulatory element upstream of the Dfd transcription initiation site for the maintenance of Dfd expression into the later stages of embryogenesis. Immunostaining of imaginal discs shows Dfd- positive cells in the peripodial membrane of the eye-antennal discs with no detectable accumulation in the disc proper. There are also a few cells in the stalk of the labial discs which appear to accumulate Dfd protein. The Dfd cDNA driven by a heat shock promotor has been returned to flies and used to ectopically express Dfd protein. Animals carrying this construct subjected to heat shock produce ectopic mouth hooks and maxillary cirri in the ventral aspect of their thoracic segments, two structures missing in Dfd- animals. There is no phenotypic affect on abdominal pattern; however, head development is severely disrupted in heat-pulsed animals.