The second edition of the EMBO | EMBL Symposium: The Identity and Evolution of Cell Types brought together an increased number of researchers from this growing community. 315 scientists joined the virtual meeting and enjoyed four days of talks and poster presentations streamed live. A total of 72 posters were presented at the two live poster sessions out of which three were selected as the best posters by popular vote. Take a look at the winners and their work.
Molecular fingerprinting sea anemones and jellyfish: A transcriptomic approach to characterize Cnidarian cell types
Presenter: Alison Cole, University of Vienna, Austria
Animals typically consist of hundreds of different cell types, yet the evolutionary mechanisms underlying the emergence of new cell types are unclear. Cnidarians offer advantages to studies of metazoan cell type evolution, as they are the sister group to the Bilateria and yet comprise an extremely diverse set of lineages that exhibit variable life history strategies, life spans, regenerative properties, animal-defining cell types (ie. muscles and neurons), as well as clade-specific cell types (i.e. cnidocytes). Advances in single cell RNA sequencing have opened the frontiers for molecular profiling of cell types at a genome-wide scale. Application of these technologies for comparisons across species remains in its infancy, and is largely, but not exclusively, restricted to closely related species with well-defined orthologous gene sets. Here we present a large single cell dataset derived from the anthozoan polyp Nematostella vectensis (comprising both developmental and tissue-derived samples),the scyphozoan moon jelly (Aurelia aurita; comprising all life history stages as well as medusa tissue-derived samples), and the hydrozoan Clytia hemispherica (young medusa only). The same cell complement that is identifiable from species-specific genome-wide analyses is recoverable using only a set of 1:1:1 orthologous genes in all three species. Analyses of the reduced gene matrix combining all three species robustly identifies putatively homologous cell types amongst the neurosecretory derivatives, as well as cell populations with clear species-specific transcriptomic profiles. Interpretations of these data in the light of specific cell types will be discussedin order to demonstrate that the combination of unbiased single cell transcriptomes and gene-directed validations can permit the identification of novel and conserved cell types.
Stylophora pistillata cell atlas illuminates stony coral symbiosis, calcification and immunity
Presenter: Anamaria Elek, Centre for Genomic Regulation, Spain
Stony corals are colonial cnidarians that sustain the most biodiverse marine ecosystems on Earth: coral reefs. Life cycle of these animals involves a swimming larva that settles and metamorphoses into a sessile polyp, which in turn develops into the adult stage, depositing in the process a protein rich organic matrix and extracellular calcium carbonate crystals to form a stony skeleton. Despite their ecological importance, little is known about the cell types and molecular pathways that underpin the biology of reef-building corals. Using single-cell RNA sequencing, we have defined over 40 cell types across the three life stages of a stony coral Stylophora pistillata. Among others, we characterized previously unknown coral immune cells, endosymbiont alga-hosting cells, and calicoblasts responsible for calcium-carbonate skeleton formation in both settling polyp and the adult coral. Apart from these specialized coral cell types, we identified evolutionary conserved cell types by phylogenetic integration of our S. pistillata cell atlas with three other available cnidarian species. These evolutionary conservations include neuronal and gland cell types, cnidaria-specific cnidocytes, and others. Overall, this study reveals the molecular and cellular basis of stony coral biology, and addresses the evolution of cell type programs in three major cnidarian lineages separated by 500 million years of evolution.
Gene family evolution underlies cell type diversification in the hypothalamus of teleosts*
Presenter: Maxwell Shafer, Biozentrum, University of Basel, Switzerland
Hundreds of cell types form the vertebrate brain, but it is largely unknown how similar these cellular repertoires are between or within species, or how cell type diversity evolves. To examine cell type diversity across and within species, we performed single-cell RNA sequencing of ~130,000 hypothalamic cells from zebrafish (Danio rerio) and surface- and cave-morphs of Mexican tetra (Astyanax mexicanus). We found that over 75% of cell types were shared between zebrafish and Mexican tetra, which last shared a common ancestor over 150 million years ago. Orthologous cell types displayed differential paralogue expression that was generated by sub-functionalization after genome duplication. Expression of terminal effector genes, such as neuropeptides, was more conserved than the expression of their associated transcriptional regulators. Species-specific cell types were enriched for the expression of species-specific genes, and characterized by the neo-functionalization of members of recently expanded or contracted gene families. Within species comparisons revealed differences in immune repertoires and transcriptional changes in neuropeptidergic cell types associated with genomic differences between surface- and cave-morphs. The single-cell atlases presented here are a powerful resource to explore hypothalamic cell types, and reveal how gene family evolution and the neo- and sub-functionalization of paralogs contribute to cellular diversity.
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