An Adaptive Threshold in Mammalian Neocortical Evolution
Figure 5
Distinct combinations of progenitor lineages are required to predict cortical neuron numbers for low- versus high-GI species.
(A) Schematics of the seven lineages used to construct neuron output in species. Note that n-bIP refers to the neurogenic subtype of basal intermediate progenitors, and that p-bIP can be either the proliferative subtype of basal intermediate progenitors or bRG undergoing symmetric proliferative division [5],[10]. (B) Plotted neuron output of the lineages in (A), beginning with two apical RG cells, over ten cell cycles. Series to the left of the seven curves summarize the neuron output of each lineage, where ni is the number of i divisions. A constant, c = 0.989, is incorporated into the series for lineage 5, allowing the series to converge on the true value of the lineage output as the number of divisions becomes increasingly numerous. (C) Ln-transformed plot of observed neuron counts as a function of neurogenic period for four species with a GI≤1.5 (open blue triangles) and six species with a GI>1.5 (open red circles). Predicted neuron counts were calculated using combinations of the lineages in (A), as specified in Table 2, that accurately fitted to the observed neuron counts either for mouse (closed gold symbols) or human (closed green symbols). Note that the mouse neurogenic program implicates only lineages 1–3, and the human neurogenic program only lineages 2–7. Errors bars represent 75% confidence intervals in cell-cycle length. See Figure S7 and Table S4 for observed and predicted data on a larger set of 17 species.