Brain expansion promoted by polycomb-mediated anterior enhancement of a neural stem cell proliferation program.

During central nervous system (CNS) development, genetic programs establish neural stem cells and drive both stem and daughter cell proliferation. However, the prominent anterior expansion of the CNS implies anterior-posterior (A-P) modulation of these programs. In Drosophila, a set of neural stem cell factors acts along the entire A-P axis to establish neural stem cells. Brain expansion results from enhanced stem and daughter cell proliferation, promoted by a Polycomb Group (PcG)->Homeobox (Hox) homeotic network. But how does PcG->Hox modulate neural-stem-cell-factor activity along the A-P axis? We find that the PcG->Hox network creates an A-P expression gradient of neural stem cell factors, thereby driving a gradient of proliferation. PcG mutants can be rescued by misexpression of the neural stem cell factors or by mutation of one single Hox gene. Hence, brain expansion results from anterior enhancement of core neural-stem-cell-factor expression, mediated by PcG repression of brain Hox expression.