Meeting Abstract
Neural development often starts with a region of ectoderm receiving extrinsic signals instructing it to become neural. Our understanding of nervous system development largely comes from studies within Deuterostomia and Ecdysozoa, and less so from the third major bilaterian clade, Spiralia. Studying how nervous systems develop in different animal groups will help us reconstruct how they evolved. To investigate whether intrinsic or extrinsic signals are involved in early neural specification in a spiralian, we isolated embryonic blastomeres from the annelid Capitella teleta. After six days, daughters of isolated 1st-quartet micromeres, fated to form the episphere including the brain, expressed the pan-neuronal gene Ct-elav1, indicating a possible role for lineage-specific neural determinants in brain formation. Isolated 2d micromeres, fated to form the trunk ectoderm and ventral nerve cord (VNC) did not survive. However, when the 2d micromere was isolated with other micromeres, larvae developed an elongated trunk and episphere, with Ct-elav1 expressed exclusively within the episphere. These results show that 2d requires extrinsic signals from the 2nd-quartet macromeres or their descendants, which generate the mesoderm and endoderm, to specify the VNC. We propose that autonomous specification of anterior neural ectoderm evolved in spiralians, enabling the swimming larvae to respond quickly to environmental cues. In contrast, multiple signaling pathways could have been co-opted to conditionally specify the VNC. Future experiments will examine the transcriptomic profile of isolated blastomeres to identify putative genes involved in neural specification in C. teleta. This will ultimately provide insight into how neural specification evolved within Bilateria.
