Nervous system development in sea spiders (Arthropoda, Pycnogonida): combining morphology and gene expression to reconstruct the euarthropod ground pattern

神经发生的比较研究为真节肢动物的系统发育关系和神经系统进化提供了有价值的线索。六足类、甲壳类、多足类和螯足类的代表动物已经在以下方面进行了研究：(1)干细胞样神经祖细胞（神经母细胞）或小簇迁移的神经细胞的形成，(2)在每个半节内各自神经前体的定型排列，(3)外胚层细胞增殖和迁移之间的关联，以及(4)这些过程背后的遗传网络。此外，早期轴突发生的数据为中枢神经系统轴突支架的建立和空间排列提供了见解。然而，由于缺乏对某些真节肢动物类群和外类群（如爪虫和缓步动物）的研究，使得对原始神经发生的重建变得困难，从而使真节肢动物神经发生进化转变的极性变得困难。由于其在螯足类或真节肢动物中的基础位置，海蜘蛛（Pycnogonida）对这一分析至关重要。然而，尽管最近取得了一些进展，但关于碧诺索细胞神经发生的大多数方面仍然存在争议和不足。为了减少这种知识的缺乏，我们将结合不同的形态学方法和基因表达分析，详细研究海蛾的神经前体形成和轴突发生。研究人员将测试海蜘蛛的早期神经发生是否与拟合足类和多足类动物或爪足类动物更相似。获得的数据将有助于重建真节肢动物神经发生的进化变化。

Comparative studies of neurogenesis provide valuable clues for phylogenetic relationships and ner-vous system evolution of euarthropods. Representatives of hexapods, crustaceans, myriapods, and chelicerates have been studied with respect to (1) the formation of stem-cell like neural progenitor cells (neuroblasts) or small clusters of immigrating neuronal cells, (2) a stereotyped arrangement of the respective neural precursors within each hemisegment, (3) an association between ectodermal cell proliferation and immigration, and (4) the genetic 2 network underlying these processes. Moreover, data on early axonogenesis provide insights into the establishment and spatial arrangement of the axonal scaffold in central nervous system. However, missing studies in some euarthropod taxa and outgroups such as Onychophora and Tardigrada render the reconstruction of ancestral neurogenesis and thus the polarity of neurogenic evolutionary transformations of euarthropods difficult. Owing to their basal position either in chelicerates or euarthropods in general, sea spiders (Pycnogonida) are critical for this analysis. However, despite some recent progress most aspects of pycnogonid neuro-genesis are still controversial and insufficiently known. To diminish this lack of knowledge, neural precursor formation and axonogenesis of pycnogonids will be studied in detail, combining different morphological approaches and gene expression analyses. It will be tested whether early neurogenesis in sea spiders shows more similarities to euchelicerates and myriapods or to onychophorans. Obtai-ned data will contribute to the reconstruction of evolutionary changes in euarthropod neurogenesis.