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

• Published in: BMC evolutionary biology Vol. 20; no. 1; p. 84
• Main Authors: Sur, A, Renfro, A, Bergmann, P J, Meyer, N P
• Format: Journal Article
• Language: English
• Published: England BioMed Central Ltd 14-07-2020; BioMed Central; BMC
• Subjects: Analysis; Animals; Annelida; Arthropods; Brain; Brain - cytology; Capitella; Capitella teleta; Cell cycle; Cell Cycle - genetics; Cell Division; Cell growth; Cell Proliferation - genetics; Central nervous system; Division; Embryos; Fluorescence; Gene expression; Gene Expression Regulation; Gene Regulatory Networks; Gene-regulatory network; Homology; Hybridization; Insects; Investigations; Kinetics; Labeling; Labelling; Models, Biological; Molecular modelling; Nervous system; Neural Plate - cytology; Neural precursor cells; Neural Stem Cells - cytology; Neural Stem Cells - metabolism; NeuroD protein; Neuroectoderm; Neurogenesis; Neurogenesis - genetics; Neurogenins; Neurons; Phylogeny; Polychaeta - cytology; Polychaeta - genetics; SOX Transcription Factors - metabolism; Spiralia; Thymidine; Ventral nerve cord; Vertebrates; Capitella teleta; Spiralia; Neurogenesis; Neural precursor cells; Annelida; Gene-regulatory network
• ISSN: 1471-2148; 1471-2148
• DOI: 10.1186/s12862-020-01636-1

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.
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.
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.