Stepwise emergence of the neuronal gene expression program in early animal evolution

Area of Science:

• Evolutionary biology
• Developmental biology
• Genomics

Background:

• The evolution of cell types, including neurons, is a key event in early animal history.
• Non-bilaterian animals like placozoans, lacking neurons but exhibiting coordinated behaviors, offer insights into early cell signaling.
• Peptidergic cells in placozoans are potential candidates for studying early neuronal development.

Purpose of the Study:

• To investigate the neuronal affinities of peptidergic cells in placozoans.
• To understand the evolutionary origins of neuronal gene modules.
• To explore cell type conservation and homeostasis in early animals.

Main Methods:

• Phylogenetic analysis
• Chromatin profiling
• Comparative single-cell genomics across four placozoan species

Main Results:

• Identified conserved cell type expression programs in placozoans, including transdifferentiating and cycling cells.
• Discovered fourteen peptidergic cell types expressing neuronal-associated components, derived from progenitor cells with neurogenesis signatures.
• Observed a lack of this conserved expression in earlier-branching animals like sponges and ctenophores.

Conclusions:

• Key neuronal developmental and effector gene modules evolved prior to the emergence of cnidarian/bilaterian neurons.
• These modules likely functioned within the context of paracrine cell signaling.
• Placozoans provide a model for understanding the pre-neuronal evolution of complex cellular functions.