Search for: All records

Over the course of more than half a billion years of independent evolution, cnidarians (e.g. sea anemones, corals and jellyfishes) have evolved diverse, multicellular, mechanosensory structures ranging from tentacles of hydroids to gravity-sensors of moon jellyfish. The ectodermal epithelium of mechanosensory structures houses the mechanosensory neuron – known as the concentric hair cell – characterized by an apical mechanosensory apparatus consisting of a single cilium surrounded by one or multiple rings of microvilli/stereovilli. While distinct concentric hair cell types are known to occur within life-cycle-stage-specific structures such as the sea anemone tentacles, it is unclear whether diverse concentric hair cell types exist across life cycle phases of any cnidarian. Here we report evidence from the hydrozoan Cladonema pacificum that concentric hair cells of sedentary polyps are distinct from those of free-swimming medusae. By carrying out touch assays, we demonstrate that polyps and medusae exhibit distinct mechanosensory behaviors. Moreover, we find that concentric hair cells in the ectodermal epithelium of touch-sensitive regions in polyps differ from those in medusae in the morphology of apical sensory apparatuses. Furthermore, polyp-type concentric hair cells are not retained in the ectoderm of medusa buds, and medusa-type concentric hair cells begin to form de novo during medusa formation. Taken together, these findings suggest that distinct mechanosensitive behaviors of polyps and medusae are mediated by morphologically different sets of mechanosensory neurons that develop via life-cycle-stage-specific mechanisms. We propose that cell type diversification of mechanosensory neurons occurred not only within a given life cycle phase but across life cycle phases in cnidarian evolution. 

Abstract The third iteration of the Cnidarian Model Systems Meeting (Cnidofest) was held August 14–17th, 2024 at Lehigh University in Bethlehem, PA. The meeting featured presentations from laboratories representing 11 countries, covering a broad range of topics related to cnidarian species. The research highlighted diverse topics, with sessions focused on regeneration, evo-devo, genomics, symbiosis, cell biology, physiology, neurobiology, and development. A notable shift at this meeting was the extent to which established cnidarian model systems have caught up with the classical laboratory models such asDrosophilaand vertebrates, with modern genomic, genetic, and molecular tools now routinely applied. In addition, more cnidarian systems are now being developed for functional studies by the community, enhancing our ability to gain fundamental insights into animal biology that are otherwise difficult in the complex bilaterian model systems. Together, the integration of cnidarian and bilaterian model systems provides researchers with a broader toolkit for selecting animal models best suited to address their specific biological questions. 

Although specialized mechanosensory cells are found across animal phylogeny, early evolutionary histories of mechanoreceptor development remain enigmatic. Cnidaria (e.g. sea anemones and jellyfishes) is the sister group to well-studied Bilateria (e.g. flies and vertebrates), and has two mechanosensory cell types - a lineage-specific sensory-effector known as the cnidocyte, and a classical mechanosensory neuron referred to as the hair cell. While developmental genetics of cnidocytes is increasingly understood, genes essential for cnidarian hair cell development are unknown. Here we show that the class IV POU homeodomain transcription factor (POU-IV) - an indispensable regulator of mechanosensory cell differentiation in Bilateria and cnidocyte differentiation in Cnidaria - controls hair cell development in the sea anemone cnidarian Nematostella vectensis. N. vectensis POU-IV is postmitotically expressed in tentacular hair cells, and is necessary for development of the apical mechanosensory apparatus, but not of neurites, in hair cells. Moreover, it binds to deeply conserved DNA recognition elements, and turns on a unique set of effector genes - including the transmembrane-receptor-encoding gene polycystin 1 - specifically in hair cells. Our results suggest that POU-IV directs differentiation of cnidarian hair cells and cnidocytes via distinct gene regulatory mechanisms, and support an evolutionarily ancient role for POU-IV in defining the mature state of mechanosensory neurons.