The Australian bearded dragon Pogona vitticeps. Credit: Max Planck Institute for Brain Research/ G. Laurent
Dragons and Brain Evolution
Nowadays, dragons are keeping Game of Thrones fans on their toes. According to recent research study conducted by Max Planck researchers on the brain of the Australian bearded dragon Pogona vitticeps, they are likewise using substantial insights into the development of vertebrate brains.
Early tetrapods (animals with 4 limbs) made the relocation from aquatic to terrestrial environments 320 million years ago, which resulted in the three main clades of vertebrates today: reptiles, birds (an offshoot of the reptilian tree), and mammals. Because of common ancestry, all tetrapod brains have a comparable basal architecture established during early development.
It is uncertain, nevertheless, how variations in this typical “Bauplan” added to clade-specific qualities. To address this issue, scientists at limit Planck Institute for Brain Research in Frankfurt produced a molecular atlas of the dragon brain and compared it to one from mice. Contrary to traditional opinion, which holds that a mammalian brain is a mix of an ancient “reptilian” brain and modern-day mammalian characteristics, their results indicate that both reptilian and mammalian brains have actually developed unique clade-specific neuron types and circuits from a common ancestral set.
To address this problem, researchers at the Max Planck Institute for Brain Research in Frankfurt developed a molecular atlas of the dragon brain and compared it to one from mice. Contrary to standard opinion, which holds that a mammalian brain is a mix of an ancient “reptilian” brain and modern mammalian traits, their results suggest that both mammalian and reptilian brains have actually developed distinct clade-specific nerve cell types and circuits from a typical ancestral set.
Max Planck researchers produced a cell-type atlas from the brain of a lizard. Computationally combination of this data with mouse transcriptomics exposed that several brain areas include mixtures of divergent and comparable nerve cells, suggesting ubiquitous nerve cell diversity in these brain regions. Their evolutionary diversity reflects changes in the developmental processes that produce them and may drive changes in the neural circuits they belong to”, states Professor Gilles Laurent, Director at the Max Planck Institute for Brain Research who led the new research study released in Science.
” For example, distinct brain areas do not work in seclusion, recommending that the development of interconnected areas, such as the thalamus and cortex, may in some way be correlated. A brain area in reptiles and mammals that obtained from a common ancestral structure might have progressed in such a way that it remains ancestral in one clade today, while it is “modern-day” in the other. On the other hand, it could be that both clades now contain a mix of common (ancient) and specific (unique) nerve cell types. These are the sorts of questions that our experiments tried to address”, Laurent adds.
While standard techniques to comparing developmental regions and projections in the brain do not have the needed resolution to reveal these resemblances and differences, Laurent and his group took a cellular transcriptomic method. Utilizing a strategy called single-cell RNA sequencing that identifies a big fraction of the RNA particles (transcriptomes) present in single cells, the scientists produced a cell-type atlas of the brain of the Australian bearded dragon Pogona vitticeps and compared it to existing mouse brain datasets.
Transcriptomic comparisons expose shared classes of neuron types.
” We profiled over 280,000 cells from the brain of Pogona and determined 233 unique types of neurons”, explains David Hain, a college student in the Laurent Lab and co-first author of the study. “Computational combination of our information with mouse data exposed that these nerve cells can be organized transcriptomically in common families, that most likely represent ancestral neuron types”, states Hain. In addition, he discovered that a lot of areas of the brain consist of a mix of common (ancient) and specific (novel) nerve cell types, as shown in the figure below.
College student Tatiana Gallego-Flores used histological strategies to map these cell types throughout the dragon brain and observed (to name a few) that nerve cells in the thalamus might be organized in two transcriptomic and anatomical domains, defined by their connectivity to other areas of the brain. Because these linked regions have actually had different fates in mammals and in reptiles, one of these areas being extremely divergent, comparing the thalamic transcriptomes of these 2 domains proved to be extremely interesting. Indeed, it revealed that transcriptomic divergence matched that of the target areas.
” This suggests that neuronal transcriptomic identity somewhat reflects, at least in part, the long-range connectivity of an area to its targets. Because we do not have the brains of ancient vertebrates, reconstructing the evolution of the brain over the previous half billion years will require connecting together very complicated molecular, developmental, anatomical, and functional data in such a way that is self-consistent. We live in very amazing times because this is becoming possible”, concludes Laurent.
Reference: “Molecular variety and development of neuron types in the amniote brain” by David Hain, Tatiana Gallego-Flores, Michaela Klinkmann, Angeles Macias, Elena Ciirdaeva, Anja Arends, Christina Thum, Georgi Tushev, Friedrich Kretschmer, Maria Antonietta Tosches and Gilles Laurent, 2 September 2022, Science.DOI: 10.1126/ science.abp8202.
Computationally integration of this data with mouse transcriptomics exposed that numerous brain locations contain mixtures of comparable and divergent nerve cells, recommending ubiquitous neuron diversity in these brain regions. Graduate trainee Tatiana Gallego-Flores utilized histological methods to map these cell types throughout the dragon brain and observed (amongst others) that nerve cells in the thalamus might be grouped in two physiological and transcriptomic domains, specified by their connectivity to other regions of the brain. Given that we do not have the brains of ancient vertebrates, rebuilding the advancement of the brain over the previous half billion years will require linking together extremely complicated molecular, developmental, anatomical, and functional data in a method that is self-consistent.