Origin of the neuro‐sensory system: new and expected insights from sponges

Abstract

The capacity of all cells to respond to stimuli implies the conduction of information at least over short distances. In multicellular organisms, more complex systems of integration and coordination of activities are necessary. In most animals, the processing of information is performed by a nervous system. Among the most basal taxa, sponges are nerveless so that it is traditionally assumed that the integrated neuro-sensory system originated only once in Eumetazoa, a hypothesis not in agreement with some recent phylogenomic studies. The aim of this review is to show that recent data on sponges might provide clues for understanding the origin of this complex system. First, sponges are able to react to external stimuli, and some of them display spontaneous movement activities. These coordinated behaviors involve nervous system-like mechanisms, such as action potentials and/or neurotransmitters. Second, genomic analyses show that sponges possess genes orthologous to those involved in the patterning or functioning of the neuro-sensory system in Eumetazoa. Finally, some of these genes are expressed in specific cells (flask cells, choanocytes). Together with ultrastructural data, this gives rise to challenging hypotheses concerning cell types that might play neuro-sensory-like roles in sponges.

The Notch pathway in the annelidPlatynereis: insights into chaetogenesis and neurogenesis processes

“…Interestingly, some of the Delta-like genes are expressed in larval cells interpreted as sensory cells. Some of these cells might be derived from ancestral metazoan sensory cells that were the starting point for the first neurons [74,75]. …”

Vertebrate Cranial Placodes as Evolutionary Innovations—The Ancestor's Tale

“…Unless sponges have secondarily lost neurons (see above), this suggests that sensory cells in the eumetazoans may have evolved by co-option and diversification of several molecular components expressed in different sponge cells. The choanocytes of sponges, which bear cilia surrounded by a microvillar collar, resemble both the unicellular choanoflagellates, the closest unicellular relatives of metazoans, and the ciliated sensory cells found in eumetazoans and have been proposed to be potential evolutionary precursor of sensory cells, although there is currently little molecular evidence to support this (Fritzsch et al, 2007;Jacobs et al, 2007;Renard et al, 2009). …”

Structure, Distribution, and Function of Neuronal/Synaptic Spinules and Related Invaginating Projections

“…Simple metazoans called placozoans have fiber cells that superficially resemble neurons and extend filopodia-sized processes that make close contacts with other cells (Jorgensen 2014; Smith et al 2014). Sponges, another very simple metazoan group, may have cells like this (Pavans de Ceccatty 1966; Renard et al 2009; Nickel 2010). In the sponge, Tethya lyncurium , these elongate cells form partially invaginated contacts with at least two other kinds of cells, i.e., an enlarged end (“bouton”) of the filopodial process fits into a shallow pocket on the receiving cell (Fig.…”