Homoeosis in Drosophila. II. a Genetic Analysis of Polycomb.

Three dominant mutant alleles of the Polycomb locus of Drosophila melanogaster are associated with homoeotic transformations of meso- and metathoracic to prothoracic legs, a homoeotic transformation of antennae to legs, and abnormalities of wings and some thoracic bristles. Puro and Nygrén (1975) localized Polycomb in the proximal left arm of chromosome 3 within salivary gland chromosome interval 77E,F-80. In the present study, the location and dosage relationships of this locus were examined, using translocation-generated segmental aneuploidy. The results indicate that Polycomb lies within interval 78C,D-79D, and that the locus is haplo-insufficient. Males hypoploid for this interval show meso- and metathoracic leg transformations, and both males and females show wing abnormalities. In addition, the legs of hypoploids of both sexes are shorter than those of wild-type flies, and show aberrancies of segmentation, chaetal number and distribution, and other morphological characteristics. Hypoploid flies do not express a homoeotic antennal-leg transformation, but the deficiency is associated with a Minute phenotype that is known to suppress this transformation in Polycomb flies; thus it cannot be ascertained whether the antennal-leg transformation is a haplo-insufficient phenotype. It is suggested that the expression of non-homoeotic pleiotropic effects provides a criterion for identifying homoeotic mutations that do not function directly in the establishment of determined states, but rather cause homoeosis indirectly. Polycomb is interpreted in this fashion, and it is suggested that the mutant syndrome may result from localized cell death.