Journal Article

A pan-mammalian map of interhemispheric brain connections predates the evolution of the corpus callosum

Fenlon, Laura R., Kurniawan, Nyoman D., Morcom, Laura R., Paolino, Annalisa, Richards, Linda J., Suárez, Rodrigo, Kozulin, Peter

2018

Overview

The brain of mammals differs from that of all other vertebrates, in having a six-layered neocortex that is extensively interconnected within and between hemispheres. Interhemispheric connections are conveyed through the anterior commissure in egg-laying monotremes and marsupials, whereas eutherians evolved a separate commissural tract, the corpus callosum. Although the pattern of interhemispheric connectivity via the corpus callosum is broadly shared across eutherian species, it is not known whether this pattern arose as a consequence of callosal evolution or instead corresponds to a more ancient feature ofmammalian brain organization. Here we show that, despite cortical axons using an ancestral commissural route, monotremes and marsupials share features of interhemi-spheric connectivity with eutherians that likely predate the origin of the corpus callosum. Based on ex vivo magnetic resonance imaging and tractography, we found that connections through the anterior commissure in both fat-tailed dunnarts (Marsupialia) and duck-billed platypus (Monotremata) are spatially segregated according to cortical area topography. Moreover, cell-resolution retrograde and anterograde interhemispheric circuit mapping in dunnarts revealed several features shared with callosal circuits of eutherians. These include the layered organization of commissural neurons and terminals, a broad map of connections between similar (homotopic) regions of each hemisphere, and regions connected to different areas (heterotopic), including hyperconnected hubs along the medial and lateral borders of the cortex, such as the cingulate/motor cortex and claustrum/insula. We therefore propose that an interhemispheric connectome originated in early mammalian ancestors, predating the evolution of the corpus callosum. Because these features have been conserved throughout mammalian evolution, they likely represent key aspects of neocortical organization.

• Publisher: National Academy of Sciences
• Subject: Animals / Anterior commissure / Aquatic birds / Axons / Basal ganglia / Biological Evolution / Biological Sciences / Brain / Brain architecture / Brain mapping / Cerebral hemispheres / Connectome / Corpus callosum / Corpus Callosum - cytology / Corpus Callosum - diagnostic imaging / Corpus Callosum - physiology / Cortex (motor) / Datasets as Topic / Diffusion Tensor Imaging / Egg laying / Evolution / Female / Genomes / Hemispheres / Magnetic Resonance Imaging / Mammals / Mammals - physiology / Medical imaging / Motor ability / Neocortex - cytology / Neocortex - diagnostic imaging / Neocortex - physiology / Nervous system / Neural networks / Neural Pathways - physiology / Neuroimaging / Neuroscience / NMR / Nuclear magnetic resonance / Vertebrates