Evolutionary convergence of sensory circuits in the pallium of amniotes

Neuroimaging and immunofluorescence of the Pseudopus apodus brain: unraveling its structural complexity

The present study provides an in-depth neuroanatomical characterization of the brain of Pseudopus apodus, combining magnetic resonance imaging (MRI) with histological analysis by immunofluorescence. In the telencephalon, the pallial regions showed distinct anatomical features, including a cortical structure, a dorsal ventricular ridge and the spherical nucleus, but prominent layering patterns, observable on histological slides, were not fully resolved by MRI. Subpallial structures, such as the nucleus accumbens and the basal ganglia, were delineated with histological clarity and further supported by MRI. In the hypothalamic and diencephalic regions, the dense and complex cellular composition made precise delineation of individual nuclei difficult by MRI, in contrast to the histological accuracy, however by MRI the identification of the major tracts running through these domains are clearly identifiable. Mesencephalic and rhombencephalic structures, including the optic tectum, isthmic nuclei, cerebellum, and reticular groups, were systematically described using a combination of histological and MRI techniques. In addition, immunofluorescence analysis of specific markers, such as Calretinin, ChAT, Isl1, Satb1, Serotonin and Tyrosine Hydroxylase, provided higher resolution of functional sub-regions, allowing precise identification of boundaries and facilitating comprehensive regional mapping, showing complex organizational arrangements, both in rostral regions, such as the dorsal ventricular crest, and in caudal regions, within the tegmental and posterior nuclei of the brain, including the ventral tegmental area, substantia nigra and raphe nuclei. These findings establish a robust neuroanatomical framework for Pseudopus apodus, contributing significantly to the understanding of reptile brain organization and providing valuable insights into the evolutionary adaptations underlying a limbless lizard neuroanatomy.

How do big brains evolve?

In both birds and mammals, variation in brain size predominantly reflects variation in mass or volume of the pallium (neocortex) and, to a lesser extent, of the cerebellum, suggesting convergent coevolution of brains and cognition. When brain measures are based on neuron counts, however, a surprisingly different picture emerges: The number of neurons in the cerebellum surpasses those in the pallium of all mammals (including humans and other primates) and in many but not all birds studied to date. In particular, parrots and corvids, clades known for cognitive abilities that match those of primates, have brains that contain more pallial than cerebellar neurons. Birds and mammals may thus have followed different evolutionary routes of pallial-cerebellar coordination behind enhanced cognitive complexity.

Enhancer-driven cell type comparison reveals similarities between the mammalian and bird pallium

Combinations of transcription factors govern the identity of cell types, which is reflected by genomic enhancer codes. We used deep learning to characterize these enhancer codes and devised three metrics to compare cell types in the telencephalon across amniotes. To this end, we generated single-cell multiome and spatially resolved transcriptomics data of the chicken telencephalon. Enhancer codes of orthologous nonneuronal and γ-aminobutyric acid-mediated (GABAergic) cell types show a high degree of similarity across amniotes, whereas excitatory neurons of the mammalian neocortex and avian pallium exhibit varying degrees of similarity. Enhancer codes of avian mesopallial neurons are most similar to those of mammalian deep-layer neurons. With this study, we present generally applicable deep learning approaches to characterize and compare cell types on the basis of genomic regulatory sequences.

Developmental origins and evolution of pallial cell types and structures in birds

Innovations in the pallium likely facilitated the evolution of advanced cognitive abilities in birds. We therefore scrutinized its cellular composition and evolution using cell type atlases from chicken, mouse, and nonavian reptiles. We found that the avian pallium shares most inhibitory neuron types with other amniotes. Whereas excitatory neuron types in amniote hippocampal regions show evolutionary conservation, those in other pallial regions have diverged. Neurons in the avian mesopallium display gene expression profiles akin to the mammalian claustrum and deep cortical layers, while certain nidopallial cell types resemble neurons in the piriform cortex. Lastly, we observed substantial gene expression convergence between the dorsally located hyperpallium and ventrally located nidopallium during late development, suggesting that topological location does not always dictate gene expression programs determining functional properties in the adult avian pallium.

Constrained roads to complex brains

Neural development and brain circuit evolution converged in birds and mammals.