ABSTRACT
Neurons and sensory cells are key to the diversity of behaviours encountered in metazoans, and, consequently, how they arose has long been of interest. However, despite decades of study and a wealth of knowledge on neurons in select species, the neuronal cell type remains difficult to define. In this chapter, we discuss the reasons for this, focusing on the diversity of neuronal and sensory cell types in non-bilaterian animals. Single cell transcriptomes (scRNAseq) are able to identify a huge diversity of neuronal subtypes in bilaterians, but so far, this does not appear to be the case for non-bilaterians, such as cnidarians and ctenophores, nor in aneural animals, such as placozoans and sponges. Placozoans have a range of sensory cells that secrete and respond to peptides, and yet their cell types are not associated with transcriptomic markers of neurons in other animals. Sponges can sense and behave but lack neurons, and single cell transcriptome profiles show no indication of cell clusters with neuronal identity. ScRNAseq approaches stand to benefit from complementary detailed studies of cell-cell interactions, cell morphology, and physiology from non-model species, especially non-bilaterians. In general, avoiding ‘bilaterian’ bias will be key to identifying the cellular features which characterize connectivity and which could have led to the evolution of sensory-neural cell types in different taxa.
