I am interested in the evolution of the six-layered mammalian neocortex. A central question in evolutionary neurobiology involves identifying the homolog of the neocortex in birds and reptiles. Birds completely lack a cortex in the dorsal telencephalon; however, based on similarities in neuronal connectivity, it has been hypothesized that specific nuclei within the avian telencephalon are homologous to specific layers of the mammalian neocortex. This layers-as-nuclei hypothesis, or the Karten hypothesis, implies homology of cell types rather than of brain structures.

Recently, genes with layer-specific expression patterns have been identified in the rodent. Our lab used these ‘layer markers’ to test the Karten hypothesis. For example, we have found that layer 4 markers are expressed in the expected chick ‘layer 4’ nuclei, and that the layer 5 markers are expressed in the expected chick ‘layer 5’ nuclei, providing strong molecular evidence to support the Karten hypothesis.

          I am currently working on several aspects of this project. First, I am examining gene expression of candidate layer markers in the ferret, a carnivore with a large, gyrencephalic brain. To date, I have found substantial differences between ferret and mouse gene expression patterns, suggesting variability in the cell-type composition of the neocortical column across mammals. Second, I am looking at the expression of these genes in the zebrafish, a type of teleost fish. This addresses the question of how conserved these molecularly defined cell types are across vertebrates, and when they might have evolved.