Deep deuterostome origins of vertebrate brain signalling centres

Neuroectodermal signalling centres induce and pattern many novel vertebrate brain structures but are absent, or divergent, in invertebrate chordates. This has led to the hypothesis that signalling centre genetic programs were first assembled in stem vertebrates, which potentially drove morphological innovations. However, this scenario presumes that extant cephalochordates accurately represent ancestral chordate characters, which has not been tested using close chordate outgroups. Here, we report that genetic programs homologous to three vertebrate signalling centres; the anterior neural ridge, zona limitans intrathalamica, and isthmic organizer are present in the hemichordate Saccoglossus kowalevskii. Fgf8/17/18, sfrp1/5, hh , and wnt1 are expressed in vertebrate-like arrangements in hemichordate ectoderm, and homologous genetic mechanisms regulate ectodermal patterning in both animals. We propose these genetic programs were components of an unexpectedly complex, ancient genetic regulatory scaffold for deuterostome body patterning that degenerated in amphioxus and ascidians, but was retained to pattern divergent structures in hemichordates and vertebrates.