DAMEN, W.G.M.; Institute for Genetics, University of Cologne, 50931 Cologne, Gemany: Terminal addition in arthropods: The molecular basis of segmentation in spiders and millipedes

There is an ongoing discussion whether segmentation in different phyla has a common origin sharing a common genetic program. We study the evolution of the segmentation process in the arthropod phylum using the spider Cupiennius salei and the millipede Glomeris marginata as models. The analysis of ‘classical’ segmentation genes (segment polarity genes, pair rule genes, gap genes) in the spider and millipede shows that there are differences as well as similarities compared to insects. Furthermore, in the spider Notch-signaling plays a key role in segmentation, similar as Notch-signaling does in vertebrate somitogenesis. Segments do not form properly in Notch and Delta RNAi embryos, while in Su(H) and presenilin RNAi embryos posterior segments do not form at all. Taken together, there are a number of similarities, but also a number of differences in the segmentation process of the spider, the millipede, and insects. The involvement of Notch-signaling and the dynamic expression of pair rule gene orthologues show similarities to vertebrate somitogenesis where Notch-signaling and hairy-type bHLH genes are part of the somitogenesis oscillator that brings about the somites. This result forms an important argument in favor of a common origin of segmentation in arthropods and vertebrates.