Marine flatworms as windows into the evolution of bilaterian neuropeptide signalling

The evolutionary origin(s) of neuropeptide signalling is still largely unknown. It has been hypothesised that signal transmission via neuropeptides played a crucial role in the regulation of nervous systems in the bilaterian ancestor. Identification, expression and functional analyses of neuropeptides and their receptors across different taxonomic groups are important for better understanding the origin(s) of bilaterian neuropeptide signalling. An example is flatworms belonging to the Platyhelminthes (spiralian protostomes) and Xenacoelomorpha (sister to all Bilateria or sister to Ambulacraria). Despite their simple morphology (i.e., a body plan lacking a coelom and circulatory system), they often possess brain-like central nervous systems and a number of bilaterian-conserved neuropeptides. This feature makes them useful models for analyses of neuropeptides and their links to nervous systems. Although previous studies have revealed orthologous relationships between vertebrate neuropeptides and those found in both marine Platyhelminthes and Xenacoelomorpha, our work suggests possible ancestral functions of vasopressin/oxytocin (VP/OT) peptides (platytocin) in these groups.