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Background  Diverse architectures of nervous systems (NSs) such as a plexus in cnidarians or a more centralized nervous system (CNS) in insects and vertebrates are present across Metazoa, but it is unclear what selection pressures drove evolution and diversification of NSs. One underlying aspect of this diversity lies in the cellular and molecular mechanisms driving neurogenesis, i.e. generation of neurons from neural precursor cells (NPCs). In cnidarians, vertebrates, and arthropods, homologs of SoxB and bHLH proneural genes control different steps of neurogenesis, suggesting that some neurogenic mechanisms may be conserved. However, data are lacking for spiralian taxa.    Results  To that end, we characterized NPCs and their daughters at different stages of neurogenesis in the spiralian annelid  Capitella teleta . We assessed cellular division patterns in the neuroectoderm using static and pulse-chase labeling with thymidine analogs (EdU and BrdU), which enabled identification of NPCs that underwent multiple rounds of division. Actively-dividing brain NPCs were found to be apically-localized, whereas actively-dividing NPCs for the ventral nerve cord (VNC) were found apically, basally, and closer to the ventral midline. We used lineage tracing to characterize the changing boundary of the trunk neuroectoderm. Finally, to start to generate a genetic hierarchy, we performed double-fluorescent in-situ hybridization (FISH) and single-FISH plus EdU labeling for neurogenic gene homologs. In the brain and VNC,  Ct-soxB1  and  Ct-neurogenin  were expressed in a large proportion of apically-localized, EdU +  NPCs. In contrast,  Ct-ash1  was expressed in a small subset of apically-localized, EdU +  NPCs and subsurface, EdU −  cells, but not in  Ct-neuroD  +  or  Ct-elav1  +  cells, which also were subsurface.    Conclusions  Our data suggest a putative genetic hierarchy with  Ct-soxB1  and  Ct-neurogenin  at the top, followed by  Ct-ash1 , then  Ct-neuroD , and finally  Ct-elav1 . Comparison of our data with that from  Platynereis dumerilii  revealed expression of  neurogenin  homologs in proliferating NPCs in annelids, which appears different than the expression of vertebrate  neurogenin  homologs in cells that are exiting the cell cycle. Furthermore, differences between neurogenesis in the head versus trunk of  C. teleta  suggest that these two tissues may be independent developmental modules, possibly with differing evolutionary trajectories.

Investigating cellular and molecular mechanisms of neurogenesis in Capitella teleta sheds light on the ancestor of Annelida