Meeting Abstract
A key question concerning animal evolution is how centralized nervous systems (CNSs) evolved and contributed to organismal diversity. In many animals with a CNS, a region of ectoderm receives extrinsic signals instructing it to become neural during the process of dorsal-ventral (D-V) axis specification. In vertebrates and insects, both processes rely in part on inhibition of BMP signaling by secreted antagonists, leading some to infer that neural specification is homologous within Bilateria. However, the little data we have from spiralians suggest that BMPs may not have played an ancestral role in neural specification. We studied to what extent BMP signaling is involved in neural fate and D-V axis specification in the spiralian annelid Capitella teleta. We analyzed expression of BMP ligands and their antagonists and found localization to specific quadrants during cleavage stages and to specific tissues after gastrulation. We manipulated BMP signaling during early-cleavage and gastrulation stages by soaking in recombinant BMP4 protein or in the inhibitor dorsomorphin. BMP4 protein did not disrupt D-V axis formation or block ventral nerve cord formation. Instead, BMP4 dramatically affected brain formation, causing a third brain lobe and eye to form. Treatment with dorsomorphin, which should block signaling downstream of BMPs, did not disrupt D-V axis formation and instead resulted in a loss or decrease of tissue in the ventral nerve cord. These results suggest that BMP signaling does not limit the domain of neural ectoderm in C. teleta as it does in insects and vertebrates. Our results are in agreement with data from other spiralians, indicating that nervous system evolution may have been more complicated than a single centralization event at the base of Bilateria.
