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Cellular innovation driving nervous system evolution

Periodic Reporting for period 2 - NeuralCellTypeEvo (Cellular innovation driving nervous system evolution)

Reporting period: 2019-12-01 to 2021-05-31

The evolutionary emergence of nervous systems is yet enigmatic, but has interested society since Greek philosophers started to contemplate human origins. The NeuralCellTypeEvo project now investigates when and in what animal the first neuron emerged. With our research, we aim to understand the evolution of cells in animals. We trace how the multifunctional cells that were typical for early animals became distinct, by a step-wise process of cellular diversification involving division of labour and specialization, until the first neuron was born.
To gain insight into the evolution of neurons in animals, we have characterized all cells of the body in selected animals, which are especially interesting for tracing the evolutionary origin of neurons. This includes all genes that are differentially expressed between the cells, and also a description of cellular phenotypes by electron microscopy. In particular, we have characaterized all cell types of a sponge, which represents a group of animals that does not have a nervous system. Together with our collaborators, we found and characterized interesting new cell types expressing proteins that are known to play a role in nervous functions in other animals. For example, we found neuroid cells in sponge choanocyte chambers (Figure). Choanocytes are cells bearing long motile cilia that produce a water current. The neuroid cell communicates with the choanocytes via long cellular extensions. This first study has revealed that, even if sponges do not have a nervous system, they possess cells that share some properties with neurons and therefore may be related to cells from which the first neurons evolved. These findings shed new light on the evolution of the nervous system.
We are currently looking for similar cells by single-cell sequencing other marine animals with interesting properties and evolutionary positions. This way we attempt to interconnect protoneurons and true neurons in divergent species, to ultimately construct an evolutionary cell type that reveals the origin and subsequent diversification of neuronal types.
Neuroid cell (violet) in a sponge choanocyte chamber (blue). Credit: Jacob Musser, Klaske Schippers.