Using the even-skipped homeo box as a probe to identify diverged homeo box genes in the Drosophila genome, we isolated the empty spiracles {ems) gene. Structural and functional comparisons between ems and other embryonic patterning genes of Drosophila suggest that ems acts, in part, as a homeotic selector gene, specifying the identity of some of the most anterior head segments. Mutant embryos lacking ems protein have severe patterning defects in the anterior head and are missing tracheal structures, including the filzkorper, which are normally developed by the eighth abdominal segment, ems has two different spatial patterns of expression during embryogenesis. The early, head-specific pattern consists of a single anterior stripe at the syncytial and cellular blastoderm stages. The later, metameric pattern consists of bilateral patches of ems expression in neural and ectodermal cells of every head and body segment. Variations of the ems expression pattern in bicoid mutants suggests that the morphogen protein produced by bicoid has a concentration-dependent regulatory role in the establishment of head-specific ems expression. In contrast, the metameric ems pattern is initiated independently of bicoid protein, and ems becomes expressed at high levels in the primordia of the duplicated filzkorper that develop in the anterior half of bicoid mutant embryos.

The characteristic morphology of many elements of the Drosophila body plan is crucially dependent upon the proper spatial expression of homeotic selector genes. The Deformed locus, which we isolated by virtue of its homology to the homeo box, is a candidate for a homeotic selector in the head region of the developing embryo. Here we show that null mutants of Deformed result in a loss of pattern elements derived from the maxillary and mandibular segments, and a duplication of a cuticular element of the larval head skeleton. Molecular analysis of the locus shows that Dfd transcripts are encoded in five exons distributed over 11 kb. The major transcript of 2.8 kb contains a 1758‐bp open reading frame that would translate to yield a 63.5‐kd protein containing a homeo domain and conspicuous regions of monotonic amino acid sequences. The Dfd protein exhibits extensive homology to a protein encoded by a Xenopus homeo box gene, Xhox 1A, suggesting that the Xenopus gene is the frog homologue of Dfd.

The Deformed gene of Drosophila is necessary for the proper development of epidermal pattern elements arising from the maxillary and mandibular segments of the head. We find one major transcript (2.8 kb) homologous to Deformed (Dfd) probes which is expressed continuously from 3 h of embryogenesis into adulthood. Localized transcript accumulation is first detected just prior to the formation of the cellular blastoderm in a single circumferential band at about 65‐75% egg length. The zone of Dfd expression is approximately two segment primordia in width. At later stages of embryogenesis, Dfd transcripts accumulate in the posterior ectoderm of the mandibular segment, and in the ventro‐lateral ectoderm of the maxillary segment. Transcripts are also detected in the mesoderm and neuromeres of the mandibular and maxillary segments. The distribution of Dfd transcripts supports the hypothesis that Dfd functions as a homeotic selector gene in the determination of posterior head segments.

The homeobox gene family of Drosophila appears to control a variety of position‐specific patterning decisions during embryonic and imaginal development. Most of these patterning decisions determine groups of cells on the anterior‐posterior axis of the Drosophila germ band. We have isolated a novel homeobox gene from Drosophila, designated H2.0. H2.0 has the most diverged homeobox so far characterized in metazoa, and, in contrast to all previously isolated homeobox genes, H2.0 exhibits a tissue‐specific pattern of expression. The cells that accumulate transcripts for this novel gene correspond to the visceral musculature and its anlagen.

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