无脊椎动物脊索动物,比如Ciona,可以深入了解脊索门的进化。无脊椎动物脊索和脊椎动物之间共有的解剖特征可以作为它们存在于共同脊索祖先中的证据。尽管CionaCNS具有〜180个神经元,但Ciona幼虫和脊椎动物的中枢神经系统具有相似的解剖结构。然而,CionaCNS与脊椎动物之间的保守深度尚未解决。Ciona尾部中枢神经系统,虽然看起来像脊髓,迄今为止被认为缺乏运动神经元,质疑它与脊椎动物脊髓的同源性。我们在这里展示了Ciona幼虫尾部中枢神经系统,事实上,沿着它的长度有功能性运动神经元,指向脊索底部存在脊髓样结构。我们进一步扩展了对共享中枢神经系统解剖结构的分析,以探索Ciona运动神经节,它被认为是脊椎动物后脑的同源物,脊髓,或者两者兼而有之。我们发现背侧运动神经节中的一组神经元共享解剖位置,发育途径,神经电路架构,和脊椎动物小脑的基因表达。然而,功能上,Ciona集群似乎与脊椎动物的小脑样结构更相似,就接收和处理直接感官输入而言。这些发现与早期的推测一致,即小脑是从小脑样结构进化而来的,并表明后者的结构存在于普通脊索祖先的背侧后脑中。
Invertebrate chordates, such as the tunicate
Ciona, can offer insight into the evolution of the chordate phylum. Anatomical features that are shared between invertebrate chordates and vertebrates may be taken as evidence of their presence in a common chordate ancestor. The central nervous systems of
Ciona larvae and vertebrates share a similar anatomy despite the
Ciona CNS having ~180 neurons. However, the depth of conservation between the
Ciona CNS and those in vertebrates is not resolved. The
Ciona caudal CNS, while appearing spinal cord-like, has hitherto been thought to lack motor neurons, bringing into question its homology with the vertebrate spinal cord. We show here that the
Ciona larval caudal CNS does, in fact, have functional motor neurons along its length, pointing to the presence of a spinal cord-like structure at the base of the chordates. We extend our analysis of shared CNS anatomy further to explore the Ciona \"motor ganglion\", which has been proposed to be a homolog of the vertebrate hindbrain, spinal cord, or both. We find that a cluster of neurons in the dorsal motor ganglion shares anatomical location, developmental pathway, neural circuit architecture, and gene expression with the vertebrate cerebellum. However, functionally, the Ciona cluster appears to have more in common with vertebrate cerebellum-like structures, insofar as it receives and processes direct sensory input. These findings are consistent with earlier speculation that the cerebellum evolved from a cerebellum-like structure, and suggest that the latter structure was present in the dorsal hindbrain of a common chordate ancestor.