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 ident ity of the posterior head segments; the labial segment requires Sex co mbs reduced (Scr) for proper development, Deformed (Dfd) specifies max illary and mandibular identity, and labial is necessary for intercalar y segment identity. Although mutations in these genes cause homeotic t ransformations during image development, the only obvious homeotic tra nsformation during embryonic head development is found in Scr mutants, where a partial transformation of the labial segment to a more anteri or, maxillary identity has been reported. This transformation is unusu al because DFD protein does not accumulate in the labial cells of Scr mutants, although DFD is required for development of maxillary structu res. Here, we present evidence that casts doubt on whether the labial to maxillary transformation actually exists in embryos lacking Scr. Th e observed morphological characteristics and gene expression patterns of various mutant embryos indicate a loss of segmental identity rather than a transformation. (C) 1996 Academic Press, Inc.