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Abstract
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.