School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, United Kingdom.
Proc Natl Acad Sci U S A. 2010 Dec 28;107(52):22576-81. doi: 10.1073/pnas.1008822108. Epub 2010 Dec 13.
One of the controversial debates on euarthropod relationships centers on the question as to whether insects, crustaceans, and myriapods (Mandibulata) share a common ancestor or whether myriapods group with the chelicerates (Myriochelata). The debate was stimulated recently by studies in chelicerates and myriapods that show that neural precursor groups (NPGs) segregate from the neuroectoderm generating the nervous system, whereas in insects and crustaceans the nervous tissue is produced by stem cells. Do the shared neural characters of myriapods and chelicerates represent derived characters that support the Myriochelata grouping? Or do they rather reflect the ancestral pattern? Analyses of neurogenesis in a group closely related to euarthropods, the onychophorans, show that, similar to insects and crustaceans, single neural precursors are formed in the neuroectoderm, potentially supporting the Myriochelata hypothesis. Here we show that the nature and the selection of onychophoran neural precursors are distinct from euarthropods. The onychophoran nervous system is generated by the massive irregular segregation of single neural precursors, contrasting with the limited number and stereotyped arrangement of NPGs/stem cells in euarthropods. Furthermore, neural genes do not show the spatiotemporal pattern that sets up the precise position of neural precursors as in euarthropods. We conclude that neurogenesis in onychophorans largely does not reflect the ancestral pattern of euarthropod neurogenesis, but shows a mixture of derived characters and ancestral characters that have been modified in the euarthropod lineage. Based on these data and additional evidence, we suggest an evolutionary sequence of arthropod neurogenesis that is in line with the Mandibulata hypothesis.
关于真节肢动物（Euarthropoda）的关系，有一个颇具争议的话题是昆虫、甲壳动物和多足动物（Mandibulata）是否拥有共同的祖先，或者多足动物是否与螯肢动物（Myriochelata）形成一个类群。最近对螯肢动物和多足动物的研究表明，神经前体细胞（NPGs）从产生神经系统的神经外胚层中分离出来，而在昆虫和甲壳动物中，神经组织是由干细胞产生的。多足动物和螯肢动物的共同神经特征是否代表支持 Myriochelata 分组的衍生特征？或者它们是否反映了祖先模式？对与真节肢动物密切相关的多毛类动物（onychophorans）的神经发生分析表明，与昆虫和甲壳动物相似，单个神经前体细胞在神经外胚层中形成，这可能支持 Myriochelata 假说。在这里，我们表明多毛类动物神经前体细胞的性质和选择与真节肢动物不同。多毛类动物的神经系统是由单个神经前体细胞的大量不规则分离产生的，与真节肢动物中有限数量和定型排列的 NPGs/干细胞形成鲜明对比。此外，神经基因没有表现出像真节肢动物那样建立神经前体细胞精确位置的时空模式。我们得出的结论是，多毛类动物的神经发生在很大程度上没有反映真节肢动物神经发生的祖先模式，而是显示了在真节肢动物谱系中经过修饰的衍生特征和祖先特征的混合。基于这些数据和其他证据，我们提出了一个与 Mandibulata 假说一致的节肢动物神经发生的进化序列。
