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

Stephanie Schauer

Patricia Carreira

Ruchi Shukla

Daniel Gerhardt

Patricia Gerdes

Francisco Sanchez-Luque

Paola Nicoli

Michaela Kindlova

Serena Ghisletti+7 authors

Supplementary Material

^{Queensland Brain Institute, The University of Queensland, Brisbane, QLD 4070, Australia;}

^{Centre for Advanced Imaging, The University of Queensland, Brisbane, QLD 4070, Australia;}

^{School of Biomedical Sciences, The University of Queensland, Brisbane, QLD 4070, Australia}

^{To whom correspondence may be addressed. Email:}ua.ude.qu@zeraus.r or ua.ude.qu@sdrahcir.

Edited by Jon H. Kaas, Vanderbilt University, Nashville, TN, and approved August 1, 2018 (received for review May 14, 2018)

Author contributions: R.S., L.R.F., P.K., and L.J.R. designed research; R.S., A.P., L.R.F., L.R.M., P.K., and N.D.K. performed research; R.S., A.P., L.R.F., L.R.M., P.K., and N.D.K. analyzed data; and R.S., A.P., L.R.F., L.R.M., P.K., and L.J.R. wrote the paper.

Edited by Jon H. Kaas, Vanderbilt University, Nashville, TN, and approved August 1, 2018 (received for review May 14, 2018)

Published under the PNAS license.

Significance

The neocortex is a hallmark of mammalian evolution, and connections between both hemispheres integrate bilateral functions. In eutherians (e.g., rodents and humans), interhemispheric circuits course via the corpus callosum and share a similar connectome throughout species. Noneutherian mammals (i.e., monotremes and marsupials), however, did not evolve a corpus callosum; therefore, whether the eutherian connectome arose as consequence of callosal evolution or instead reflects ancient connectivity principles remains unknown. We studied monotreme and marsupial interhemispheric neocortical connectomes and compared these with eutherian datasets. This revealed interhemispheric connectivity features shared across mammals, with or without a corpus callosum, suggesting that an ancient connectome originated at least 80 million years before callosal evolution.

Keywords: diffusion tensor MRI, claustrum, cortical connectome, anterior commissure, corpus callosum

Significance

Abstract

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 of mammalian brain organization. Here we show that, despite cortical axons using an ancestral commissural route, monotremes and marsupials share features of interhemispheric 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.

Abstract

The vertebrate nervous system is organized into functional modules of spatially arranged neurons and fibers (1), where topographic maps shared between peripheral and central circuits mediate sensory-motor behaviors (2). Although such maps are abundant in the spinal cord, hindbrain, and midbrain, they are less evident in the telencephalic pallium of nonmammalian vertebrates, such as birds (3, 4). Mammals, however, evolved a highly topographic six-layered neocortex that recapitulates the peripheral sensory maps via point-to-point connections with subcortical regions (5 –7). Moreover, the left and right cortical hemispheres of mammals are heavily interconnected compared with fewer such connections in birds, in which sensory-motor and associative regions of the telencephalic pallium receive limited input from the contralateral hemisphere (8, 9) (Fig. 1). A second key evolutionary innovation was the origin of the corpus callosum exclusively in eutherian (placental) mammals (10 –12). Such an event likely involved rerouting neocortical axons medially to the dorsal region of the embryonic commissural plate, coupled with a process of midline tissue remodeling by embryonic astroglia (13), which is exclusively present in eutherians (14). The evolution of the corpus callosum as a distinct tract allowed a significant expansion of the number of interhemispheric neocortical connections in species with large brains (15). The corpus callosum carries fibers topographically arranged according to the position of their cell bodies (16 –18) and connects mostly similar (homotopic) but also different (heterotopic) regions in each hemisphere (Fig. 1B). However, although the map of callosal fibers in eutherians is well-established, and includes connectivity features that are highly conserved across species, such as the presence of bilateral hubs (19 –25), it remains unclear whether such features depend on the route taken by commissural axons or instead reflect more ancient organizational principles of neocortical connectivity.

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Fig. 1.

Evolution of cortical commissures. (A) Cladogram of birds and mammals showing the origin of the neocortex (NCx) and the corpus callosum (cc). (B) Coronal brain schematics. The anterior commissure (ac) of pigeons contains mostly unidirectional projections from the ventral arcopallium (Av) to contralateral homotopic and heterotopic targets, including the mesopallium (M), nidopallium (N), and hyperpallium intercalatum (HI). In eutherians, callosal axons are topographically segregated and connect mostly homotopic regions, whereas the ac carries axons from the piriform cortex (Pir) and lateral NCx. Homologous circuits in noneutherians are carried exclusively through the ac.

To address these questions, here we studied the main connectivity features of interhemispheric cortical circuits in noneutherian mammals and compared these with known eutherian connectomes. We found that the spatial segregation of interhemispheric axons across the anterior commissure in marsupials and monotremes resembles the arrangement of fibers across the callosal tract in eutherians, including the presence of point-to-point homotopic circuits. Furthermore, single cell-level circuit mapping via in vivo retrograde and anterograde tracer injections in marsupials revealed a highly conserved layer distribution of contralaterally projecting neurons, as well as homotopic, heterotopic, and hyperconnected circuits through the anterior commissure that strongly suggest an ancient precallosal origin.

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Acknowledgments

We thank the Queensland Nuclear Magnetic Resonance Network and the Australian National Imaging Facility for the operation of the scanners, the Queensland Brain Institute’s Advanced Microscopy Facility for histology imaging, and the University of Queensland Biological Resources and the Native Wildlife Teaching and Research Facility for all animal help. This work was supported by Australian Research Council Grants DP160103958 (to L.J.R. and R.S.) and DE160101394 (to R.S.), National Health and Medical Research Council Fellowships DP160103958 (to L.J.R. and R.S.) and DE160101394 (to R.S.), the Australian Postgraduate Award (to L.R.F. and L.R.M.), and the UQ-QBI Doctoral Scholarship award (to A.P.).

Acknowledgments

Footnotes

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1808262115/-/DCSupplemental.

Footnotes

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