Polycomb complexes and the role of epigenetic memory in development

The availability of a many important developmental genes is determined by events that occurred at earlier developmental stages. In such genes, some kind of cellular memory encodes an epigenetic mark that is transmitted through many rounds of cell division and determines whether or how the gene will respond to the presence of activators. Polycomb mechanisms are perhaps the best known example of such epigenetic regulators. Polycomb complexes can bind to Polycomb Response Elements and establish a repressive chromatin state but they also leave an epigenetic mark on the affected genes such that the chromatin state will be recreated in the following cell cycle. Polycomb mechanisms and the nature and propagation of the epigenetic mark during development are reviewed with particular attention to the genetic and molecular evidence available from the fruit fly in which they were first discovered.