Towards a synthetic view of axis specification mechanisms in vertebrates: insights from the dogfish

The genetic mechanisms, which control axis specification, apparently extensively diverge across vertebrates. In amphibians and teleosts, they are tightly linked to the establishment of an early dorso-ventral polarity. This polarity has no equivalent in amniotes, which unlike the former, retain a considerable plasticity for their site of axis formation until blastula stages and rely on signals secreted by extra-embryonic tissues for the establishment of their early rostro-caudal pattern. In order to better understand the links between these seemingly highly divergent mechanisms, we have used an evo-devo approach, aimed at reconstructing the gnathostome ancestral state and focussed on a chondrichthyan, the dogfish Scyliorhinus canicula. A detailed molecular characterization of the dogfish embryo at blastula and gastrula stages highlights striking similarities with all vertebrate model organisms including amniotes. It suggests the presence in the dogfish of territories homologous to the hypoblast and extra-embryonic ectoderm of the latter, which may therefore reflect the primitive condition of jawed vertebrates. In the ancestral state, these territories are specified at opposite sides of an early axis of bilateral symmetry, homologous to the dorso-ventral axis of amphibians and teleosts, and aligned with the later forming embryonic axis, from head to tail. Amniotes have diverged from this pattern through a posterior expansion of extra-embryonic ectoderm, resulting in an apparently radial symmetry at late blastula stages. These data delineate the broad outlines of the gnathostome ancestral pattern of axis specification and highlight an unexpected unity of mechanisms across jawed vertebrates. They illustrate the complementarity of comparative and genetic approaches for a comprehensive view of developmental mechanisms themselves.