The antennapedia complex (ANT-C) is a group of tightly linked genes that is found in the proximal portion of the right arm of the third chromosome of Drosophila melanogaster. The most prominent members of the complex produce striking homeotic transformations in adult flies carrying mutations in these genes. Thus mutations in the Antennapedia (Antp) locus cause a transformation of the antenna of the adult fly into a leg, while lesions in the proboscipedia (pb) gene cause the adult mouth parts to develop into legs rather than the normal palps used in feeding. The existence of the homeotic ANT-C was originally proposed based on the tight linkage of the proboscipedia (pb), Sex combs reduced (Scr), and Antennapedia (Antp) loci. Subsequent genetic analyses have shown that two other homeotic loci, labial (lab) and Deformed (Dfd), are also members of the complex. The homeotic loci of the ANT-C are involved in the specification of segmental identity in the posterior head (gnathocephalic) and anterior thoracic regions of the embryo and adult. Moreover, the linear order of the homeotic loci in the complex, lab, pb, Dfd, Scr, and Antp, corresponds to the anterior-posterior order of altered segments (intercalary, mandibular, maxillary, labial, and thoracic) found in animals bearing mutations in each of the resident loci. Taken together, the results of mutational analyses indicate that members of the complex are necessary to repress head development in the thorax (Antp) and elicit normal segmental identity in the anterior thorax (Scr) and posterior head (Scr, Dfd, pb, and lab).
A similar group of homeotic genes called the Bithorax Complex (BX-C) is found more distally on the third chromosome. This set of three homeotic genes (Ultrabithorax, abdominal-A, and Abdominal-B) acts in a similar fashion to the ANT-C, but in the posterior of the thorax and in the abdomen. The ANT-C is distinguished from the BX-C not only by virtue of the domain of action of its homeotic loci (anterior versus posterior) but also by the presence of loci that are not overtly homeotic in character. Two of these, fushi tarazu (ftz) and zerknullt (zen), have been shown to affect segment enumeration (ftz) and the formation of dorsal structures (zen) in the early embryo. A third nonhomeotic gene is bicoid (bcd). Mutations in this locus result in female sterility and maternal effect lethality. Eggs laid by bcd females fail to develop normal anterior ends and instead produce mirror-image duplications of structures normally produced at the posterior terminus of the embryo.
In addition to these genetically defined loci, several other genes have been found in the ANT-C by molecular mapping. The first of these is a cluster of cuticle-protein-related genes that map between the lab and pb loci. Eight small (about 1 kbp) transcription units make up the cluster, and all have sequence similarities to known cuticle protein genes. These genes (cc1 through cc8) are also apparently regulated by ecdysone in imaginal discs. Deletion of the entire cluster has no apparent effect on the development or cuticle morphology of embryos, larvae, or adults. The second molecularly identified gene is the Amalgam (Ama) transcription unit. The encoded protein places the gene in the immunoglobulin superfamily and, like the cuticle cluster, the locus can be deleted from the genome with no discernible effect on the organism. Finally, there is the zen2 or z2 transcription unit that resides immediately adjacent to the zen gene. This locus is similar in structure and sequence to zen, but like Ama and the cc genes has no discernible function.
The entire complex has been cloned and sequenced and shown to cover 335 kbp of genomic DNA. The most distal transcription unit is Antp, which covers the distalmost 100 kbp of the complex and is made up of eight exons. Proximally, the next 75 kbp contain the Scr and/tz loci. The distal 50 kbp of this interval contain sequences necessary for Scr expression, as well as the two exons of the /tz locus and its associated regulatory elements. The proximal 25 kbp contain the three identified exons of the Scr transcription unit. The five exons of the D/d gene are found in the central portion of the next-most-proximal 55-kbp interval. The D/d transcription unit covers only 11 kbp of this region, and the 20-kbp interval flanking the gene proximally is the location of cis-acting regulatory elements for the locus. The next 25-kbp interval contains four of the nonhomeotic transcription units that help distinguish the ANT-C and BX-C. The distalmost is Ama, next bcd, and finally zen and z2. The z2, zen, and Ama transcription units are all relatively small (1 to 2 kbp) and comprise two exons each. The bcd gene is somewhat larger (3.6 kbp) and is made up of four exons. Immediately proximal to the z2 transcription unit (about 1 kbp from its 3′ end) is the 5′ end ofpb, which extends over the next 35 kbp of genomic DNA and contains nine exons. The next 25 kbp of the complex contain the cuticle cluster and its eight identified transcription units. The final 25 kbp are the sites of the lab gene, which is made up of three exons. Despite the nonhomeotic nature of three of the smaller transcription units (zen, bcd, and /tz) resident in the complex, these loci are tied to the larger homeotic genes of the region by the nature of their protein products. All five of the large homeotics (Antp, Scr, D/d, pb, and lab), and the three small genes, have a homeobox motif, and their protein products are found in the nuclei of the cells in which they are expressed. Thus eight of the genes in the ANT-C encode regulatory proteins that act as transcription factors. The z2 gene also contains a homeobox; however, the biological significance of the gene is not known, as deletions of this transcription unit have no discernible effect. The cuticle-like genes and Ama do not contain a homeobox.
The reasons for the clustering of these developmentally significant loci of similar function are not known. The existence of common or overlapping regulatory elements, the need to insulate regulatory sequences from chromosomal position effect, and the possibility of higher-order chromatin structures for proper expression have all been proposed. Whatever the reason, the homeotic complex structure has a long evolutionary standing. Similar clusters are found in vertebrates, an observation consistent with a very early origin of these genes, probably predating the separation of protostomes and deuterostomes.
