Meeting Abstract
Neurons often are thought to have evolved from cells capable of sensing and communicating environmental signals to other cells. However, mechanistic understanding of sensory biology in early-branching metazoans is limited, and thus the basal conditions from which nervous systems may have originated remain poorly defined. Sponges are one of the earliest-branching extant animal groups and do not possess recognizable nervous systems. Currently it is debated whether the last common ancestor to all living animals was aneural, as in modern sponges, or possessed a nervous system, which was subsequently lost in the sponge lineage. Here we present recent analyses of the sensory system controlling larval settlement and metamorphosis in the demosponge Amphimedon queenslandica. We show evidence that an epithelial cell type enriched in the anterior third of the larva, referred to as the flask cells, constitutes the chemosensory organ for sensing metamorphic cues associated with the settlement substrate. At metamorphosis, these cells undergo metamorphic signal-dependent cellular transformation into a stem cell type—the archeocytes. Given that the regulation of the initiation of metamorphosis by larval chemosensory neurons is a conserved feature of Eumetazoa, we propose that a larval sensory system to detect and respond to inductive environmental signals was present in the last common ancestor of Eumetazoa and sponges. However, the eumetazoan and sponge chemosensory organs strikingly differ in their developmental potential; eumetazoan larval chemosensory organs and nervous systems are thought to be terminally differentiated, while the sponges’ are not. The terminal differentiation mechanisms may thus be critical for the emergence, or the maintenance, of stably interconnected networks of neurons in evolution.
