Neuroectodermal signalling centres induce and pattern many novel vertebrate brain structures but are absent, or divergent, in invertebrate chordates. This has led to the idea that signalling-centre genetic programs were first assembled in stem vertebrates and potentially drove morphological innovations of the brain. 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 (a single gene homologous to vertebrate Fgf8, Fgf17 and Fgf18), 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 that 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.
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- Supplementary Information (2.3M)
This file contains Supplementary Figures 1-4 and Supplementary Tables 1-2. Please note that Supplementary Figure 1 shows that fz5/8 siRNA affects proboscis patterning specifically and that Supplementary Figure 2 shows Ptch expression in wild-type S. kowalevskii embryos and spectrum of phenotypes after hh siRNA injection. Ptch expression indicates that hh can signal to numerous body regions. Hh siRNA injection causes pleiotropic effects on AP and DV patterning.