Amazing stuff!
"Free-living marine placozoans are considered the simplest animals on Earth. Their bodies resemble a disk and consist of six to nine cell types. They move by ciliary beating, consume food particles through engulfment, and reproduce by fission. However, ... have shown that these tiny, unassuming creatures may hold the secret to the evolution of the complex nervous systems of higher animals. Placozoans do not have neurons; instead, the collective behavior of their cells is controlled by neuropeptides secreted by peptidergic cells. Detailed investigation of these cells revealed unexpected diversity in cell types and functional specificities. More surprisingly, these cells expressed many of the genes found in evolved nervous systems and differentiate from stem cells through a pathway that resembles neurogenesis in higher animals."
From the highlights and abstract:
"Highlights
• Comparative single-cell genomics reveals cell type diversity in the phylum Placozoa
• Fourteen placozoan peptidergic cell types expressing neuronal genes
• Post-translationally modified neuropeptides define elaborate cell signaling network
• Peptidergic progenitors with neurogenesis-like differentiation from epithelial cells
Summary
The assembly of the neuronal and other major cell type programs occurred early in animal evolution. We can reconstruct this process by studying non-bilaterians like placozoans. These small disc-shaped animals not only have nine morphologically described cell types and no neurons but also show coordinated behaviors triggered by peptide-secreting cells. We investigated possible neuronal affinities of these peptidergic cells using phylogenetics, chromatin profiling, and comparative single-cell genomics in four placozoans. We found conserved cell type expression programs across placozoans, including populations of transdifferentiating and cycling cells, suggestive of active cell type homeostasis. We also uncovered fourteen peptidergic cell types expressing neuronal-associated components like the pre-synaptic scaffold that derive from progenitor cells with neurogenesis signatures. In contrast, earlier-branching animals like sponges and ctenophores lacked this conserved expression. Our findings indicate that key neuronal developmental and effector gene modules evolved before the advent of cnidarian/bilaterian neurons in the context of paracrine cell signaling."
Graphical abstract
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