Evolutionary History of Sensory Cell Types

Most animals have specialized sensory cell types for the detection of light, mechanical, and chemical stimuli. Whereas cnidarians have many multimodal cells, in bilaterians dedicated rhabdomeric and ciliary photosensory cell types have been described in addition to a mechanosensory cell type characterized by an apical cilium associated with a group of microvilli. Each of these bilaterian cell types appears to be specified by an evolutionarily stable core regulatory network (CoRN) of transcription factors, while downstream target genes are more variable, resulting in different lineage-specific adaptations. For example, judged from the recruitment of unrelated transmembrane proteins as chemoreceptors in different bilaterian lineages, chemosensory cells have most likely evolved many times independently from a conserved mechanosensory cell type. The CoRNs defining rhabdomeric photoreceptors share some components with the CoRNs of ciliary photoreceptors (e.g., Pax4/6, Otx) but others with mechanoreceptors (e.g., Atonal, POU4). This argues against an origin of these three cell types by a sequence of dichotomous “duplication and divergence” events from a common ancestral sensory cell type. Instead, it suggests that the evolution of rhabdomeric photoreceptors involved recombination and redeployment of core regulatory components from both ciliary photoreceptors and mechanoreceptors or the partly concerted evolution of these three cell types during an early phase of their evolutionary divergence from a common ancestral sensory cell type.