The role of HOM-C genes in segmental transformations: reexamination of the Drosophila Sex combs reduced embryonic phenotype.

Homeotic genes in the Antennapedia Complex of Drosophila specify identity of the posterior head segments; the labial segment requires Sex combs reduced (Scr) for proper development, Deformed (Dfd) specifies maxillary and mandibular identity, and labial is necessary for intercalary segment identity. Although mutations in these genes cause homeotic transformations during imago development, the only obvious homeotic transformation during embryonic head development is found in Scr mutants, where a partial transformation of the labial segment to a more anterior, maxillary identity has been reported. This transformation is unusual because DFD protein does not accumulate in the labial cells of Scr mutants, although DFD is required for development of maxillary structures. Here, we present evidence that casts doubt on whether the labial to maxillary transformation actually exists in embryos lacking Scr. The observed morphological characteristics and gene expression patterns of various mutant embryos indicate a loss of segmental identity rather than a transformation.