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和神经系统进化提供了有价值的线索。对六足动物、甲壳动物、多足动物和螯肢动物的代表进行了以下研究：（1）干细胞样神经祖细胞的形成（神经母细胞）或小簇迁移神经元细胞，（2）每个半节内相应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，（4）这些过程背后的遗传网络。此外，早期轴突发生的数据提供了深入了解中枢神经系统轴突支架的建立和空间排列。然而，缺失的研究，在一些真节肢动物类群和外类群，如Onychophora和缓步动物，使重建祖先的神经发生，从而极性的真节肢动物的神经进化转化困难。由于它们在螯肢类或真节肢动物中的基础位置，海蜘蛛（Pycnogonida）对这项分析至关重要。然而，尽管最近取得了一些进展，大多数方面的pycnogonid神经发生仍然是有争议的，不够了解。为了减少这种知识的缺乏，神经前体的形成和轴突发育的密集藻将详细研究，结合不同的形态学方法和基因表达分析。它将被测试是否早期神经发生在海蜘蛛显示更多的相似性euchelicerates和多足类或onychophorans。所得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.