Unique Transcriptomic Cell Types of the Granular Retrosplenial Cortex Are Preserved across Mice and Rats despite Dramatic Changes in Key Marker Genes.

The granular retrosplenial cortex (RSG) supports memory, orientation, and fear processing. The mouse RSG contains several cell types that are remarkably distinct from those found in other cortical regions, including low rheobase neurons that dominate Layer 2/3 (L2/3 LR) and similarly exclusive pyramidal cells in Layer 5a (L5a RSG). While the functions of the RSG are extensively studied in both mice and rats, it remains unknown if the transcriptomically unique cell types of the mouse RSG are evolutionarily conserved in rats. Here, we show that mouse and rat RSG contain the same unique cell types, with L2/3 LR and L5a RSG cell types together representing >50% of all RSG neurons in each species. This preservation of cell types in male and female rats happens despite dramatic changes in key cell-type-specific marker genes, with the Scnn1a expression that selectively tags mouse L5a RSG neurons completely absent in rats. Important for Cre-driver line development, we identify alternative, cross-species genes that can be used to selectively target the cell types of the RSG in both mice and rats. Our results show that the unique cell types of the RSG are conserved across millions of years of evolution and emphasize stark species-specific differences in marker genes that need to be considered when making cell-type-specific knock-in lines across species.