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Background  The notochord has organizer properties and is required for floor plate induction and dorsoventral patterning of the neural tube. This activity has been attributed to sonic hedgehog (shh) signaling, which originates in the notochord, forms a gradient, and autoinduces  shh  expression in the floor plate. However, reported data are inconsistent and the spatiotemporal development of the relevant  shh  expression domains has not been studied in detail. We therefore studied the expression dynamics of  shh  in rabbit, chicken and  Xenopus laevis  embryos (as well as  indian hedgehog  and  desert hedgehog  as possible alternative functional candidates in the chicken).    Results  Our analysis reveals a markedly divergent pattern within these vertebrates: whereas in the rabbit  shh  is first expressed in the notochord and its floor plate domain is then induced during subsequent somitogenesis stages, in the chick embryo  shh  is expressed in the prospective neuroectoderm prior to the notochord formation and, interestingly, prior to mesoderm immigration. Neither indian hedgehog nor desert hedgehog are expressed in these midline structures although mRNA of both genes was detected in other structures of the early chick embryo. In  X .  laevis ,  shh  is expressed at the beginning of gastrulation in a distinct area dorsal to the dorsal blastopore lip and adjacent to the prospective neuroectoderm, whereas the floor plate expresses  shh  at the end of gastrulation.    Conclusions  While  shh  expression patterns in rabbit and  X .  laevis  embryos are roughly compatible with the classical view of “ventral to dorsal induction” of the floor plate, the early  shh  expression in the chick floor plate challenges this model. Intriguingly, this alternative sequence of domain induction is related to the asymmetrical morphogenesis of the primitive node and other axial organs in the chick. Our results indicate that the floor plate in  X .  laevis  and chick embryos may be initially induced by planar interaction within the ectoderm or epiblast. Furthermore, we propose that the mode of the floor plate induction adapts to the variant topography of interacting tissues during gastrulation and notochord formation and thereby reveals evolutionary plasticity of early embryonic induction.    Electronic supplementary material  The online version of this article (10.1186/s13227-017-0090-x) contains supplementary material, which is available to authorized users.

Divergent axial morphogenesis and early shh expression in vertebrate prospective floor plate