Conserved cell types in the early embryonic brain across vertebrates
Laboratory of Neural Stem Cells and Neurogenesis, ACHUCARRO
Organogenesis is the most evolutionarily conserved developmental stage when comparing morphology and whole-body transcriptome across vertebrates. During this phylotypic stage, body axes are defined and cell types are given their spatial identity. Due to its physiological importance, these segmentation and patterning genes (e.g. HOXs) are thought to be evolutionary conserved. However, across vertebrates, there is yet no evidence of transcriptomic conservation in essential organs as the brain, nor identification of homologue neural cell types in its initial neurodevelopmental bauplan. Thus, by performing single cell RNAseq and in situ hybridization assays, we obtained cellular and molecular atlases of the early developing brains of five vertebrate species: chicken, gecko, mice, zebrafish, and human. These single cell atlases allowed us to identify equivalent neuroanatomical identities that naturally segment different vertebrate early brains and the genes that pattern these regions. Secondly, to unbiasedly evaluate the transcriptional conservation of these cell types across species, we performed three complementary methods: correlation of gene specificity indexes, datasets integration ("RPCA") and label transference. All approaches proved a high transcriptional conservation of equivalent morphogenic organizers and neuromeric identities among these vertebrate species, specially at transcription factor level. These results confirm the existence of a common phylotypic brain as well as the conservation of homologue neural cell types mastering its underlying bauplan. Therefore, this bauplan conservation is essential to stablish the foundations for assembling vertebrate brain structures, but also it sets the diversity boundaries within which these brains were allowed to evolve. Such an important constrain that early embryonic vertebrate brain has barely changed despite 500 million years of evolution.