Spatiotemporal regulation of nervous system development in the annelid .

BACKGROUND

How nervous systems evolved remains an unresolved question. Previous studies in vertebrates and arthropods revealed that homologous genes regulate important neurogenic processes such as cell proliferation and differentiation. However, the mechanisms through which such homologs regulate neurogenesis across different bilaterian clades are variable, making inferences about nervous system evolution difficult. A better understanding of neurogenesis in the third major bilaterian clade, Spiralia, would greatly contribute to our ability to deduce the ancestral mechanism of neurogenesis.

RESULTS

Using whole-mount in situ hybridization, we examined spatiotemporal gene expression for homologs of , , , -, , and in embryos and larvae of the spiralian annelid , which has a central nervous system (CNS) comprising a brain and ventral nerve cord. For all homologs examined, we found expression in the neuroectoderm and/or CNS during neurogenesis. Furthermore, the onset of expression and localization within the developing neural tissue for each of these genes indicates putative roles in separate phases of neurogenesis, e.g., in neural precursor cells (NPCs) versus in cells that have exited the cell cycle. -, -, and - are the earliest genes expressed in surface cells in the anterior and ventral neuroectoderm, while - expression initiates slightly later in surface neuroectoderm. - is expressed in single cells in neural and non-neural ectoderm, while - and - are localized to differentiating neural cells in the brain and ventral nerve cord.

CONCLUSIONS

These results suggest that the genes investigated in this article are involved in a neurogenic gene regulatory network in . We propose that Ct-SoxB1, Ct-SoxB, and Ct-Ngn are involved in maintaining NPCs in a proliferative state. Ct-Pros may function in division of NPCs, Ct-Ash1 may promote cell cycle exit and ingression of NPC daughter cells, and Ct-NeuroD and Ct-Msi may control neuronal differentiation. Our results support the idea of a common genetic toolkit driving neural development whose molecular architecture has been rearranged within and across clades during evolution. Future functional studies should help elucidate the role of these homologs during neurogenesis and identify which aspects of bilaterian neurogenesis may have been ancestral or were derived within Spiralia.