Evolution and aggregulates: role of the Polycomb-group genes of Drosophila.

The Polycomb-group (Pc-G) genes of Drosophila melanogaster are responsible for fixation of the repressed state for other selector genes, mostly homeotic but not exclusively. Different arguments suggest that they work cooperatively by specific recognition and structural modification of their targets. Their pleiotropic mutant phenotypes are strikingly similar to morphological traits used to classify Dipteran groups. I propose that the products of Pc-G genes can form a combinatorial multimeric regulatory system (the Aggregulates of F. Jacob) and that stable variations in the composition of this system have played an important role in Dipteran evolution.