Genesis versus epigenesis: the odd jobs of the Polycomb group of genes.

Cells need the products of the Polycomb Group of genes (PcG) to keep, through development, the memory of their genetic determination. The pleiotropic mutant phenotypes of PcG genes strikingly resemble morphological traits, considered as taxonomic characters with phylogenetic meaning, used in Dipteran classification. I argue that spatial modulation of the multimeric complexes formed by PcG products has played a role or escorted the genesis of species.