SUMMARY Transition from smooth, lissencephalic brains to highly-folded, gyrencephalic structures is associated with neuronal expansion and breaks in neurogenic symmetry. Here we show that Neurog2 and Ascl1 proneural genes regulate cortical progenitor cell differentiation through cross-repressive interactions to sustain neurogenic continuity in a lissencephalic rodent brain. Using in vivo lineage tracing, we found that Neurog2 and Ascl1 expression defines a lineage continuum of four progenitor pools, with ‘double + progenitors’ displaying several unique features (least lineage-restricted, complex gene regulatory network, G 2 pausing). Strikingly, selective killing of double + progenitors using split-Cre; Rosa-DTA transgenics breaks neurogenic symmetry by locally disrupting Notch signaling, leading to cortical folding. Finally, consistent with NEUROG2 and ASCL1 driving discontinuous neurogenesis and folding in gyrencephalic species, their transcripts are modular in folded macaque cortices and pseudo-folded human cerebral organoids. Neurog2 / Ascl1 double + progenitors are thus Notch-ligand expressing ‘niche’ cells that control neurogenic periodicity to determine cortical gyrification. HIGHLIGHTS Neurog2 and Ascl1 expression defines four distinct transitional progenitor states Double + NPCs are transcriptionally complex and mark a lineage branch point Double + NPCs control neurogenic patterning and cortical folding via Notch signaling Neurog2 and Ascl1 expression is modular in folded and not lissencephalic cortices eTOC BLURB Emergence of a gyrencephalic cortex is associated with a break in neurogenic continuity across the cortical germinal zone. Han et al. identify a pool of unbiased neural progenitors at a lineage bifurcation point that co-express Neurog2 and Ascl1 and produce Notch ligands to control neurogenic periodicity and cortical folding.

Proneural genes define ground state rules to regulate neurogenic patterning and cortical folding