Neurogenesis in the spider Cupiennius salei

To uncover similarities and differences in neurogenesis in arthropod groups , we have studied the ventral neuroectoderm of the spider Cupiennius salei (Chelicerata, Aranea, Ctenidae). We found that invaginating cell groups aro se sequentially, at stereotyped positions in each hemisegment and in separa te waves, comparable with the generation of neuroblasts in Drosophila. Howe ver, we found no evidence for proliferating stem cells that would be compar able with the neuroblasts. Instead, the whole group of invaginating cells w as directly recruited to the nervous system. The invagination process is co mparable with Drosophila, with the cells attaining a bottle-shaped form wit h the nuclei moving inwards, while actin-rich cell processes remain initial ly connected to the surface of the epithelium. This general pattern is also found in another spider, Pholcus phalangioides, and appears thus to be con served at least among the Araneae. We have identified two basic helix-loop- helix encoding genes - CsASH1 and CsASH2 - that share sequence similarities with proneural genes from other species. Functional analysis of the genes by double-stranded RNA interference revealed that CsASH1 was required for t he formation of the invagination sites and the process of invagination itse lf, whereas CsASH2 seemed to be required for the differentiation of the cel ls into neurones. Our results suggest that the basic processes of neurogene sis, as well as proneural gene function is conserved among arthropods, apar t of the lack of neuroblast-like stem cells in spiders.