No abstract

Sponges and evolutionary origins Sponges represent our distant animal relatives. They do not have a nervous system but do have a simple body for filter feeding. Surveying the cell types in the freshwater sponge Spongilla lacustris , Musser et al . found that many genes important in synaptic communication are expressed in cells of the small digestive chambers. They found secretory machinery characteristic of the presynapse in small multipolar cells contacting all other cells and also the receptive apparatus of the postsynapse in the choanocytes that generate water flow and digest microbial food. These results suggest that the first directed communication in animals may have evolved to regulate feeding, serving as a starting point on the long path toward nervous system evolution. —BAP

Highlights Solving nervous system evolution requires cross-species comparison of neuronal types. Neuronal types are commonly defined by their specific structure and function. We provide an operational definition of cell types that allows evolutionary comparison. The identity of neuronal types is best reflected by specifying transcription factors. Families of related neuronal types are conserved across large evolutionary distances.