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

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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.