BIOLOGICAL OVERVIEW

Coming face to face with a highly magnified fruit fly's head can easily be confused with an extraterrestrial confrontation. Huge compound eyes protrude on either side of this alien head [Images]. Between them, a pair of swollen bulbous antenna lift and arch, covered with a multitude of short spines, as well as antler-like aristae and a sensory pouch, the sacculus. These non-human organs sit in a broad area on the frons, or fly's forehead. Lower down the "face," one finds the clypeus, analogous to a nose only because of anthropocentric attempts to cognitively absorb the strangeness of the fly. The clypeus neither breaths nor smells, nor serves as a perch for glasses. It is part of a segment that includes the labrum, a short tube used for sucking food that emerges to hang down from the bottom of the clypeus. On either side of the labrum are two small bulbs, the maxilla or maxillary palps. These organs are greatly reduced in the fruit fly, when compared to those of other insects. Below the maxilla lies an incredible structure, the labella or labial palps. These large, furrowed bulbs lie at the end of the proboscus, the fly's main feeding organ. The palps serve to gather liquid food, which moves up a series of collecting channels into the straw-like labrum, connecting to the muscular esophagus (Ferris, 1950).

Deformed (Dfd) is responsible for the normal development of the maxillary segment, consisting of those tiny bulbs in front of the labium. Deformed has other developmental responsibilities, as shown by three lethal effects associated with Deformed mutation that show up during metamorphosis: 1) failure to separate the subesophageal ganglion (SEG) from the thoracic ganglion; 2) structural and functional abnormalities of the proboscus and maxillary palps, and 3) failure of the adult salivary glands to extend into the thorax. The SEG is considered a ventral portion of the brain and also as part of the central nervous system (Restifo, 1994).

Broad-Complex is another transcription factor acting late in development. Deformed and Broad-Complex interact in the formation of these adult structures during the pupal stage. Deformed defects are in fact identical to those of Broad-Complex, a locus regulating metamorphosis. The homeotic gene labial is likewise important in the transition from pupal to adult structures (Restifo, 1994). This underscores the significance of homeotic genes in the final stages of pupal development, the transition from pupa to adult. Current interests in the analysis of pupal development and the intricacies of the final stages of wing, leg and head development have been largely side-tracked in this era of ground-breaking molecular genetics, but resurgent interest is only a matter of time.

The reduced size of the maxillary segment in adults, due no doubt to the Drosophila specialization of the proboscus for food gathering, is not as apparent in the larval and pupal stages. Important mouth parts are missing at these stages (Regulski, 1987), indicating that evolutionary change is much more conservative in the pre-adult morphology. The role of homeotics in neural differentiation has not yet been adequately explored. Dfd will very likely have a role in subesophageal ganglion differentiation and in the ventral nervous system, topics that have not yet received sufficient attention (Lou, 1995).

GENE STRUCTURE

labial is the most proximal member of the homeobox genes of the Antennapedia Complex. It is followed in the distal direction by proboscopedia, zerknüllt, bicoid, Deformed, Sex combs reduced, fushi tarazu and Antennapedia (Kaufman, 1990). Genomic length - 11 kb

cDNA clone length - 1758 bp

Bases in 5' UTR - 491

Exons - five

Bases in 3' UTR - 491

PROTEIN STRUCTURE

Amino Acids - 586

Structural Domains

The DFD protein has two N-terminal glycine rich regions with a histidine rich region between them. There is a central acidic domain, a homeodomain, and a C-terminal polyglutamine and polyasparagine domain (Regulksi, 1987).

See four paralogous Hox clusters of mammals for homologies of Deformed with mammalian Hox proteins.

date revised: 28 MAY 97  

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