Clonal analysis of segmental and compartmental homoeotic transformations in polycomb mutants of Drosophila melanogaster.

We describe two new effects of Polycomb mutations on the determination of compartments in the wing of Drosophila. Ventral and posterior wing compartments are transformed partially to their dorsal and anterior counterparts. Although these new phenotypes are most strongly expressed in lethal pharate adults heteroallelic for Pc2 and a new allele PcT2, they are also found regularly but with low expressivity as dominant phenotypes of all the other Pc alleles we tested. Several different intersegmental homoeotic transformations caused by Polycomb have previously been described, leading to the hypothesis that Polycomb regulates the activity of certain selector genes normally active in specific segments. We now show that the degree of expression of the inter- and intrasegmental transformations are highly correlated in a range of different Pc genotypes, and that more than one determinative decision can be affected in a single compartment. This suggests that the wild-type Pc product may act as a general regulator of several different selector genes so as to influence both early embryonic and later determinative decisions in the imaginal discs. To test this idea we used clonal analysis to look at the effects of Pc on clonal restrictions at the dorsoventral and anterioposterior compartment boundaries, and its time of action with respect to each phenotype.