Cytogenetic Analysis of Chromosome 3 in DROSOPHILA MELANOGASTER: Isolation and Characterization of Four New Alleles of the Proboscipedia (pb) Locus.

Previous studies on proximal 3R have cytologically localized the dominant homeotic loci Antennapedia (Antp), Multiple Sex Comb (Msc), Nasobemia (Ns), and Extra Sex Comb (Scx). In this study we set out to find the site of the proboscipedia (pb) locus. In order to accomplish this, four new alleles of this homeotic gene were induced with gamma rays. Genetic and cytogenetic analyses have shown that the pb locus resides in polytene chromosome bands 84A1-6, immediately adjacent to the Antp gene complex in 84B1-2. An analysis of the morphology of the proboscis and the dose relationships of the four new alleles have shown that this homeosis is unusual in at least two respects. First, the two different developmental fates realized in the proboscis at 18 degrees (labial palps --> arista) and 29 degrees (labial palps --> leg) under the influence of pb(1) grown at 18 degrees , while the remaining three are like pb(1) at 29 degrees . Dosage studies reveal that this difference reflects a hypomorphic vs. amorphic condition. Second, like the original, these new alleles produce a prothoracic rather than a mesothoracic leg in the proboscis. Both of these results indicate that pb is unique among the homeotics, and as such it may offer some new insights into developmental processes.