The evolution of complex nervous systems involved integrating gene regulatory networks that underlie different cell types. A promising approach to understanding this process is to compare the cell diversity and gene regulatory programs of early-branching, non-bilaterian animals such as sponges, cnidarians and placozoans. Writing in Cell in September 2023, Najle et al. characterize cell diversity and regulatory programs in Placozoa, a group of microscopic marine animals made of two cell layers, whose collective behaviour is controlled by small neuropeptides secreted by peptidergic cells with no cellular projections and synapses. Based on single-cell transcriptomes of four placozoans, the authors identified nine main somatic cell types as well as cells with intermediate expression profiles that might be converting to another cell type (a phenomenon also seen in sponges). Combining cell-type maps with genome-wide maps of regulatory elements, the researchers identified many conserved regulatory elements with highly conserved expression across the four placozoans; many new regulatory elements in old nodes; and only a few species-specific regulatory elements, which showed signatures of accelerated evolution and were associated with low expression conservation. Peptidergic cells, their secreted peptides and predicted receptors showed remarkable diversity, which suggests a complex peptidergic-cell signalling network. The 14 types of peptidergic cells identified in this study originated from a specific progenitor cell population with molecular signatures that are similar to those in neuronal progenitors in cnidarians and bilaterians. At a broader evolutionary scale, comparing cell-type transcriptomes across five species, the authors reconstructed the stepwise evolution of the neural transcriptome starting from peptidergic cell signalling in Placozoa, through the acquisition of key neuronal functions in the common ancestor to Cnidaria and Bilateria, to the evolution of specialized synapsis in the Bilateria ancestor. We highlight this paper because it is a great example of the power of recent technologies, such as single-cell RNA sequencing, applied to species with key positions in the tree of life to understand the evolution of complex systems.
Original reference: Cell 186, 4676–4693.e29 (2023)
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