Divergent structural trajectories for pallial regionalization within reptiles reveal conserved evolutionary blueprints.

Abstract

The pallium is the evolutionary precursor of the neocortex and a central brain region underlying learning or social behavior, yet its histogenetic organization and cellular composition remain debated. Reptiles provide a key comparative framework to investigate pallial diversification and ancestral amniote traits. Here we analyzed the pallium of the turtle Trachemys scripta elegans and the gecko Paroedura picta using molecular markers, hybridization chain reaction, and 3D light-sheet fluorescence microscopy. We identify two subdivisions of the dorsal cortex defined by Satb1/Satb2 expression, consistent with patterns reported in other sauropsids. Turtles display a pronounced rostrocaudal pallial thickening, whereas in geckos this expansion is restricted rostrally, suggesting a more limited dorsal pallial enlargement in lepidosaurs. Conversely, geckos possess a nucleus sphericus absent in turtles that may correspond to the pallial amygdala described in chelonians. Other pallial territories, including medial and lateral cortical regions and the dorsal ventricular ridge, show conserved organizational features consistent with their presence in the last common ancestor of amniotes. Developmental analyses in turtles revealed apical and basal progenitors but no detectable eomes+ intermediate progenitors. These findings indicate that reptilian pallial diversification primarily reflects lineage-specific modulation of conserved neuronal transcriptional programs, providing insight into the evolutionary origins of the neocortex.