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 arose sequentially, at stereotyped positions in each hemisegment and in separate waves, comparable with the generation of neuroblasts in Drosophila. However, we found no evidence for proliferating stem cells that would be comparable with the neuroblasts. Instead, the whole group of invaginating cells was directly recruited to the nervous system. The invagination process is comparable with Drosophila, with the cells attaining a bottle-shaped form with the nuclei moving inwards, while actin-rich cell processes remain initially connected to the surface of the epithelium. This general pattern is also found in another spider, Pholcus phalangioides, and appears thus to be conserved 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 the formation of the invagination sites and the process of invagination itself, whereas CsASH2 seemed to be required for the differentiation of the cells into neurones. Our results suggest that the basic processes of neurogenesis, as well as proneural gene function is conserved among arthropods, apart of the lack of neuroblast-like stem cells in spiders.