Heterogeneous Fates of Simultaneously-Born Neurons During Early Corticogenesis

13 Neocortical excitatory neurons belong to diverse cell types, which can be distinguished by their 14 dates of birth, laminar location, connectivity and molecular identities. During embryogenesis, apical 15 progenitors (APs) located in the ventricular zone first give birth to deep-layer neurons, and next to 16 superficial-layer neurons. While the overall sequential construction of neocortical layers is well17 established, whether multiple neuron types are produced by APs at single time points of corticogenesis 18 is unknown. To address this question, here we used FlashTag to fate-map simultaneously-born (i.e. 19 isochronic) cohorts of neurons at successive stages of corticogenesis. We reveal that early in 20 corticogenesis, isochronic neurons differentiate into heterogeneous laminar, hodological and molecular 21 cell types. Later on, instead, simultaneously-born neurons have more homogeneous fates. Using single22 cell gene expression analyses, we identify an early postmitotic surge in the molecular heterogeneity of 23 nascent neurons during which some early-born neurons initiate and partially execute late-born neuron 24 transcriptional programs. Together, these findings suggest that as corticogenesis unfolds, mechanisms 25 allowing increased homogeneity in neuronal output are progressively implemented, resulting in 26 progressively more predictable neuronal identities. 27