A genetic and developmental analysis of mutations in labial, a gene necessary for proper head formation in Drosophila melanogaster.

We have undertaken a developmental genetic analysis of labial (lab), the most proximal gene in the Antennapedia complex (ANT-C) of Drosophila melanogaster. The terminal phenotype of mutant embryos was examined in cuticle preparations, in thin sections, and by scanning electron microscopy. These preparations revealed a failure of head involution and the loss or disruption of several head structures, including the salivary glands and the H-piece and ventral arm of the cephalopharyngeal apparatus. Although these structures are presumed to derive from the gnathocephalic segments, we argue that the observed defects are likely to be a secondary consequence of a failure of head involution. A function for lab in the development of the adult head was inferred from the phenotype of animals bearing hypomorphic alleles and from clones of lab- tissue generated by mitotic recombination. Two aspects of the mutant phenotype were manifested. Ventrally, a deletion and/or disruption of tissue occurred in the maxillary palp and vibrissae regions. Dorsally, the posterior head appeared to be transformed to a thoracic-like identity. Mutations in lab, like those in the Deformed and proboscipedia loci of the ANT-C, reveal a homoeotic phenotype only in the adult stage of the life cycle.