Molecular basis for acquisition of de novo sensory circuit

在成年脊椎动物的躯干中，背根神经节（DRG）向神经系统传递各种感觉方式。然而，在鱼类和两栖类动物的发展，髓内感觉神经元，Rohon-Beard（RB）细胞，负责少年机械感觉和功能的DRG取代。在非洲爪蟾RB细胞中，Six 1恰好出现在转换之前。Six 1在非洲爪蟾早期胚胎中的强制表达减少了RB细胞的数量，但诱导了过早的髓外感觉神经发生。小鼠中Six 1和Six 4的基因消除引起脊髓内RB样细胞的出现。由于Six 1在小鼠背根神经节发育初期就开始表达，因此Six 1表达的异时性变化可能是脊椎动物躯干初级感觉系统进化的关键步骤。

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.