Evolutionary insight: In the brains of reptiles and amphibians
Complex creatures consist of trillions and trillions of cells, and few of these cells appear and act identically to each other. In fact, the small structures that make up an animal’s tissues are specialized. They come in all sorts of shapes and sizes, contain all sorts of structures, and perform all sorts of functions.
This diversity of cells did not appear overnight. A series of four new studies Posted in Science uses the genetic expressions of different cell types to better demonstrate their development over time, particularly inside the brain of reptiles and amphibians.
Cells in evolution
Scientists have long understood that different types of cells exist throughout the body are differentiated by different gene expressions. But only recent research has begun to reveal the full extent of this diversity. In recent years, for example, studies showed that hundreds of cell types exist even in small sections of the adult mouse brain, one of the most common model organisms in all scientific research.
But despite these advances in discerning the extent of cell diversity, the process by which this diversity develops remains elusive. By studying gene expression inside these small structures, scientists have developed a better understanding of the evolutionary processes behind cell diversification in reptiles and amphibians, a pair of unusual scientific models.
Express the difference
In the first of four studies, a team analyzed the gene expressions of different cell types in a bearded dragonbrain using a method called comparative single-cell transcriptomics. They then used their analysis to create a map, called a cell type atlas, of the different types of cells in the lizard’s brain, which is common to Australia and covered in clusters of spiny scales.
The team compared the cell type atlas of the bearded dragon’s brain to that of the mouse brain and found that cell types in major brain regions matched each other. They classified these cells as “conserved”, meaning that their expression remains the same over time and across species due to natural selection.
That said, by comparing cell type atlases from the bearded dragon and the mouse more closely, the team found several distinct cell types between the two animals in more specific brain regions. This coexistence of conserved and distinct cell types, the scientists say, indicates that the cells in these areas are plastic, capable of changing and evolving over time.
According to the scientists, the three additional studies only reinforced the initial results. Again using single-cell transcriptomics, the teams assembled cell-type maps of the forebrain area of the amphibian brain, specifically that of the axolotls, an aquatic frilled salamander from Mexico. They then used these maps to isolate cells unique to amphibians and axolotls, paying particular attention to cells involved in brain regeneration after injury. The results once again revealed the ability of brain cells to evolve.
“These studies highlight the potential for applying powerful transcriptomic methods usually reserved for mice to non-standard models,” conclude Lehigh University researchers Dylan Faltine-Gonzalez and Justus Kebschull in a related point of viewaccording A press release. “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.”