The evolutionary path of reptile and amphibian brains

In recent years, hundreds of distinctive cell types in specialized brain regions have been identified in mice. However, a great lack of knowledge has been dragging on how such a diversity of cell types and regions evolved.

Now, in four recent studies, researchers have turned to single-cell and spatial transcriptomics to investigate brain-scale cell-type evolution in reptiles and amphibians to better understand the evolutionary roots of such diversity.

In the first study, David Hain’s team at the Max Planck Institute for Brain Research in Germany used single-cell transcriptomics to create a cell atlas of the entire bearded dragon lizard brain and compared it to that of the mouse.

Hain and colleagues found that cells from broadly defined brain regions in both species correspond to each other, suggesting the profound conservation of region-specific gene expression signatures. However, when mapped at higher resolution, the study authors observed very different cell types between species in almost all brain divisions.

The existence of conserved and novel cell types within conserved brain regions indicates that brain cell types are evolutionarily plastic and capable of independently developing novel and innovative expression features and functions.

Snakes are among the most feared reptiles. (Photo: Zarina Sheikh, USGS Western Ecological Research Center)

Three other studies expand on these findings, focusing on the amphibian telencephalon, the part of the brain that in mammals contains the six layers of the neocortex, which amphibians lack.

Jamie Woych’s team, from Columbia University in New York, United States, compiled a cell-type atlas of this region to track the evolutionary innovations that differentiate it from other vertebrate brains.

The team of Katharina Lust, from the Vienna Biocenter (VBC) in Austria, and Xiaoyu Wei, from the Beijing Institute of Genomics in China, present single-cell analyzes of the axolotl telencephalon, paying particular attention to understanding why the cerebrum of this animal has a regenerative capacity much greater than that of the mammalian brain.

All four studies have been published in the academic journal Science. The one by David Hain’s team is entitled “Molecular diversity and evolution of neuron types in the amniote brain”. That of Jamie Woych’s team, “Cell-type profiling in salamanders identifies innovations in vertebrate forebrain ecolution”. That of Katharina Lust’s team, “Single-cell analyzes of axolotl telencephalon organization, neurogenesis, and regeneration”. And that of Xiaoyu Wei’s team, “Single-cell Stereo-seq reveals induced progenitor cells involved in axolotl brain regeneration”. (Source: AAAS)