Molecular basis for acquisition of de novo sensory circuit

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In the trunk of adult vertebrates, dorsal root ganglia (DRG) convey a variety of sensory modalities to the nervous system. However, during fish and amphibian development, intramedullary sensory neurons, Rohon-Beard (RB) cells, are responsible for juvenile mechanosensation and functionally substituted by DRG. In Xenopus RB cells, Six1 appears just before the switching. Forced expression of Six1 in Xenopus early embryo reduced the number of RB cells but induced premature extramedullary sensory neurogenesis. Genetic elimination of both Six1 and Six4 in mouse caused the emergence of RB-like cells within spinal cord. Since Six1 is expressed from the beginning of mouse DRG development, the heterochronic shift of the expressions of Six1 might be a key step in the evolution of vertebrate primary sensory system in the trunk.