Cell type profiling in salamanders identifies innovations in vertebrate forebrain evolution

Abstract

The evolution of advanced cognition in vertebrates is associated with two independent innovations in the forebrain: the six-layered neocortex in mammals and the dorsal ventricular ridge (DVR) in sauropsids (reptiles and birds). How these novelties arose in vertebrate ancestors remains unclear. To reconstruct forebrain evolution in tetrapods, we built a cell type atlas of the telencephalon of the salamander Pleurodeles waltl. Our molecular, developmental, and connectivity data indicate that parts of the sauropsid DVR trace back to tetrapod ancestors. In contrast, the salamander dorsal pallium is devoid of cellular and molecular characteristics of the mammalian neocortex, yet shares similarities with entorhinal cortex and subiculum. Our findings chart the series of innovations that resulted in the emergence of the sauropsid DVR, and the mammalian six-layered neocortex.

Glutamatergic neuron types in the amygdala of the urodele amphibian Pleurodeles waltl

“…In a recent scRNAseq study (Woych et al 2022), we identified 114 clusters of neurons sampled from the adult telencephalon of the urodele amphibian Pleurodeles waltl . Of these, 41 clusters were annotated as telencephalic glutamatergic neurons, on the basis of the expression of marker genes including the transcription factors Foxg1, Emx1, Tbr1 , and Neurod2 , the glutamate transporters Slc17a6 and Slc17a7 , and for the lack of expression of diencephalic markers (Woych et al 2022). After staining brain sections or whole-mount brain preparations for cluster-specific marker genes, we assigned a regional identity to each of these clusters.…”

“…The dataset was generated from single-cell RNA sequencing of the Pleurodeles waltl brain as described in (Woych et al 2022). For in-depth analysis of the amygdala, the data were subsetted, and heterogeneous clusters (TEGLU25, 35 and 38) were subclustered based on differential gene expression.…”

“…Tissue preparation, in situ hybridization, immunochemistry, and imaging of brain sections were performed as described in (Woych et al 2022) and summarized in brief below. Adult animals were anesthetized by submersion in 0.2% MS-222, perfused with PBS and 4% PFA, and decapitated.…”

“…However, it is important to highlight that these terms have not been updated to follow the changing tetrapartite model for the pallium. For example, new data suggests the homology of the urodele lateral pallium with bowl cells in the reptilian lateral cortex and with semilunar cells in the mammalian piriform cortex; therefore in amphibians the term “lateral pallium” refers to neurons homologous to neurons in the amniote ventral pallium, and not in the amniote lateral pallium (Wullimann 2017; Tosches 2021; Lust et al 2022; Woych et al 2022).…”

“…To address these questions, here we characterize glutamatergic neuron types in the amygdala of the urodele amphibian Pleurodeles waltl . In a recent study, we used scRNAseq to profile the entire telencephalon of Pleurodeles ( Woych et al . 2022 ).…”