Single-cell transcriptomics finds evolutionary innovations in the brain of reptiles and amphibians

Across four studies, evolutionary innovations in reptile and amphibian brains are revealed via comparative single-cell transcriptomics. While vertebrate brain evolution has traditionally focused on similarities in brain regions across disparate species, this new research highlights the role of cell type evolution in vertebrate brain innovations. In recent years, hundreds of distinct cell types have been identified in specialized brain regions in mice. However, how such a diversity of cell types and regions evolved remains unknown. Here, in four studies, researchers are using single-cell and spatial transcriptomics to investigate the evolution of brain-scale cell types in reptiles and amphibians to better understand the evolutionary roots of this diversity.

In the first study, David Hain and his colleagues used single-cell transcriptomics to create a whole-brain cell atlas of the bearded dragon lizard and compared it to that of the mouse. Hain et al. found that cells from broadly defined brain regions in both species match each other, suggesting deeply conserved region-specific gene expression signatures. However, when mapped at a higher resolution, the authors observed very dissimilar cell types across species in nearly every division of the brain. The existence of both conserved and novel cell types in conserved brain regions indicates that brain cell types are evolutionarily plastic and capable of independently developing new and innovative expression signatures and functions.

Three other studies expand on these findings, focusing on the amphibian forebrain – the part of the mammalian brain that contains the six-layered neocortex, which amphibians lack. Jamie Woych and his colleagues have assembled an atlas of cell types in this region to chart the evolutionary innovations that set it apart from other vertebrate brains. Katharina Lust and colleagues and Xiaoyu Wei and colleagues present single-cell analyzes of the axolotl forebrain, with a particular focus on understanding why the brain of this animal is so much more capable of regeneration than the brain of mammals. “These studies highlight the potential for applying powerful transcriptomic methods that are typically reserved for mice to non-standard models,” write Dylan Faltine-Gonzalez and Justus Kebschull in a related perspective. “Each of the papers produced massive single-cell and often multimodal datasets and extracted publicly available data, showing the importance of data sharing and the power of accumulating single-cell data from many species for evolutionary comparisons.”

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