The architecture of neocortical projection neurons is subject of a complex gene control. Here we demonstrated that Foxg1, a transcription factor gene which patterns the early rostral brain and sets the pace of telencephalic neuronogenesis, specifically stimulates dendrite elongation. This phenomenon occurs in vivo like in vitro, and it is detectable even upon moderate changes of Foxg1 expression levels. We found that Foxg1 acts by stimulating Hes1, which in turn upregulates pCreb1, a well-known pro-dendritogenic effector, and downregulates Syt and Ndr1, namely two established antagonizers of dendrite elongation. Moreover, Foxg1-driven pCreb1 upregulation requires PKA and AKT, and correlates with reduced PP1 and PP2A phosphatase activity. These findings contribute to clarify normal neurodevelopmental and activity-related regulation of neuritogenesis. They further suggest that an abnormal sizing of the dendritic tree of neocortical projection neurons may occur in West and Rett syndrome patients with anomalous FOXG1 allele dosages and contribute to their neurolopathological profiles.

Cytogenetics has historically played a key role in research on squirrel monkey (genus Saimiri) evolutionary biology. Squirrel monkeys have a diploid number of 2n = 44, but vary in fundamental number (FN). Apparently, differences in FN have phylogenetic implications and are correlated with geographic regions. A number of hypothetical mechanisms were proposed to explain difference in FN: translocations, heterochromatin, or, most commonly, pericentric inversions. Recently, an additional mechanism, centromere repositioning, was discovered, which can alter chromosome morphology and FN. Here, we used chromosome banding, chromosome painting, and BAC-FISH to test these hypotheses. We demonstrate that centromere repositioning on chromosomes 5 and 15 is the mechanism that accounts for differences in FN. Current phylogenomic trees of platyrrhines provide a temporal framework for evolutionary new centromeres (ENC) in Saimiri. The X-chromosome ENC could be up to 15 million years (my) old that on chromosome 5 as recent as 0.3 my. The chromosome 15 ENC is intermediate, as young as 2.24 my. All ENC have abundant satellite DNAs indicating that the maturation process was fairly rapid. Callithrix jacchus was used as an outgroup for the BAC-FISH data analysis. Comparison with scaffolds from the S. boliviensis genome revealed an error in the last marmoset genome release. Future research including at the sequence level will provide better understanding of chromosome evolution in Saimiri and other platyrrhines. Probably other cases of differences in chromosome morphology and FN, both within and between taxa, will be shown to be due to centromere repositioning and not pericentric inversions.