The gene headcase is expressed in all proliferating imaginal cell lineages. This gene is induced during the prepupal ecdysone pulse but does not substantially change expression levels during the late larval ecdysone pulse. Imaginal tissues in headcase null mutants appear normal in size and shape but fail to differentiate normally. These mutants are invariably pupal lethal and show pleiotropic effects in adult tissues. The predominant headcase loss of function phenotype is defective head development.
Mutants can display deletion of the head capsule, leaving only a protruding proboscis.
These data demonstrate that factors required for cellular differentiation during metamorphosis are present in the data set. There are 29 other EST sequences encoding novel genes that display a greater than threefold induction from PF to 12 hours APF but do not display a threefold or greater change in expression level during the late larval ecdysone pulse. In order to assess the possible role of the headcase gene in imaginal tissue development, deletions removing part or all of the mapped hdc transcription unit were generated by imprecise P-element excision. All deletions cause pupal lethality when homozygous and heterallelic combinations show similar mutant phenotypes, suggesting that all alleles are functionally null. Although hdc mutants die as pupae, all larval imaginal discs are present, and their size and shape appear normal.