By Britt Bistis, Photo by Wenze Li
Brains exist in a wide variety of forms and functions - from the simple nerve nets of jellyfishes to the large and sophisticated brains of octopuses, birds, and primates. How do complex brains evolve? The Tosches lab published an exciting new paper in Science on the organization of the salamander brain, with important implications for the evolution of the brain in vertebrates.
Salamanders are amphibians well known in the scientific community for their unique ability to regenerate almost any part of their body, including the spinal cord and entire limbs. Until now, relatively little was known about their brains. The Tosches lab used powerful techniques, including single-cell transcriptomics and 3D brain imaging, to produce a high-resolution genetic and 3D molecular brain map of a salamander species commonly known as the Spanish ribbed newt. The researchers further leveraged their salamander single-cell data to address a critical question in evolutionary biology: do functional similarities between the mammalian neocortex and brain regions of other vertebrate species result from their common ancestry, or did they evolve independently due to common challenges shared across species?
This new atlas defines over 100 subtypes of neurons in the salamander forebrain, identified for their gene expression profiles at a single-cell resolution. Each neuron subtype is defined by a set of expressed genes that uniquely mark the cell type. By specifically binding fluorescently labeled molecules to the marker gene products, researchers in the Tosches lab were able to discover where each of these neuron subtypes are in the brain.
Using transcriptomics data from salamanders, lizards, turtles, and mice, the team was also able to perform a systematic and quantitative comparison of neuron types across species, reconstructing the "family tree" of these neurons. This showed that regions of the reptilian brain that perform functions similar to the mammalian neocortex are molecularly quite distinct, suggesting that this functional similarity evolved independently in both species. This work "clarifies the evolution of neuron types and brain regions associated with high cognitive functions in vertebrate brains," Dr. Tosches says, "[and] demonstrates the molecular uniqueness of certain types of neurons in the mammalian six-layered neocortex.”
“I am particularly proud of this paper,” she continued, “because these are our first results on our new research organism, the salamander Pleurodeles waltl, and are the groundwork for our ongoing and future research. " This paper is part of a special issue of Science on brain evolution, which includes another paper on evolution and the reptilian brain by Dr. Tosches and her former colleagues at the Max Planck Institute for Brain Research, and two other papers on the salamander brain.