Vasopressin-oxytocin-type signaling is ancient and has a conserved water homeostasis role in euryhaline marine planarians

Vasopressin/oxytocin (VP/OT)-related peptides are essential for mammalian antidiuresis, sociosexual behavior, and reproduction. However, the evolutionary origin of this peptide system is still uncertain. Here, we identify orthologous genes to those for VP/OT in Platyhelminthes, intertidal planarians that have a simple bilaterian body structure but lack a coelom and body-fluid circulatory system. We report a comprehensive characterization of the neuropeptide derived from this VP/OT-type gene, identifying its functional receptor, and name it the platytocin system. Our experiments with these euryhaline planarians, living where environmental salinities fluctuate due to evaporation and rainfall, suggest that platytocin functions as an antidiuretic hormoneand also organizes diverse actions including reproduction and chemosensory-associated behavior. We propose that bilaterians acquired physiological adaptations to amphibious lives by such regulation of the body fluids. This neuropeptide-secreting system clearly became indispensable for life even without the development of a vascular circulatory system or relevant synapses.

Automated summary

VP/OT-related peptides play essential roles in antidiuresis, sociosexual behavior, and reproduction in mammals. This study identifies orthologous genes to VP/OT in planarians and characterizes the neuropeptide derived from this gene, naming it the platytocin system. The experiments suggest that platytocin functions as an antidiuretic hormone and also plays a role in reproduction and chemosensory behavior. The findings provide insights into the physiological adaptations of bilaterians to amphibious lives.